SUCCESS AND FAILURE OF CYZENIS ALBICANS IN CONTROLLING
ITS HOST THE WINTER MOTH
by
JENS ROLAND
B.Sc. (Hons), University of Alberta, Edmonton, 1976
M.Sc, University of British Columbia, Vancouver, 1981
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
in
THE FACULTY OF GRADUATE STUDIES
(Department of Zoology and Institute of Animal Resource Ecology)
We accept this thesis as conforming
to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
July 1986
(c) Jens Roland, 1986
HJTV In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of Io^0^
The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3
Date Jul* 14' 1986- ii
ABSTRACT
Situations in which the parasitic fly Cyzenis albicans appears to control numbers of its host the winter moth, were compared with situations where it does not. Comparisons were made by field observation and experimentation during the first
four years of winter moth decline following introduction of
C. albicans and Agirypon f laveolatum in Victoria, British
Columbia, Canada.
Successful control of winter moth on oaks was compared with failure on apple trees. Phenology of winter moth development differs on the two tree species, giving rise to potential asynchrony between host and parasite on apple trees.
Difference in the degree of asynchrony on the two tree species does not, however, result in a difference in level of parasitism of winter moth. Aggregative attack by Cyzenis, in response to host density, occurs when winter moth are on oaks, but is absent when winter moth are on apple. Difference in pattern of attack did not result in difference in mortality from parasitism at high density but did so at low density.
The manipulation of predatory beetle abundance by as much as 100-fold did not affect the magnitude of mortality of winter moth, and affected mortality of the parasitoid only slightly.
Large, density-dependent mortality of Cyzenis in the soil did not preclude their increase to high density, and hence would not affect the impact of Cyzenis on winter moth densities. The magnitude and degree of density-dependence of winter moth pupal mortality in the soil (regardless of beetle abundance) suggests that mortality of this stage, rather than parasitism of larvae iii
by introduced parasites, has caused population decline in both
British Columbia and Nova Scotia. The factor causing most mortality in the soil in British Columbia appears to be pathogens. Suggested mechanism for the increase in soil mortality during the four years of study are: (1) introduction of and/or increased pathogen transmission via Cyzeni s or (2) activation of latent infection by density-induced stress of the host.
This study implicates factors other than parasitism by introduced parasitoids in the suppression of winter moth numbers. Correlation between degree of aggregation in response to host density and degree of successful control, however, supports the claim that parasitoids can stabilize host populations at low density. iv
TABLE OF CONTENTS
ABSTRACT . i i
LIST OF TABLES/. . '....viii
LIST OF FIGURES ...... x
ACKNOWLEDGEMENTS .• -xi ii
CHAPTER ONE: GENERAL INTRODUCTION . . ...1
CHAPTER TWO: PLANT PHENOLOGY AND HOST-PARASITOID
SYNCHRONY 8
Introduction ...... 8
Methods .12
Study sites ...12
Time scale .' 15
Population phenology ...18
Parasitism •• 2 0
Control of phenology ....2 1
Effect of plant species on winter moth development ...21
Effect of plant species on winter moth survival 22
Results . 23
Population phenology ...... 2 3
Pa ra s i t i sm 23
Timing of attack ..23
Levels of parasitism .27
Control of spring emergence 34
Effect of plant species on winter moth development ...34
Effect of plant species on winter moth survival ...... 42
Close vs Spartan apple varieties 46
Discussion ... .46 CHAPTER THREE: EFFECT OF HOST PLANT SPECIES ON LEVEL AND
PATTERN OF CYZENIS OVIPOSITION ...52.
I nt roduct ion 52
Methods .54
Cage experiments 54
Field studies ...55
Host abundance 55
Parasitoid abundance ...56
Oviposition and parasitism 57
Oak-leaf extract on apple trees 59
Results 60
Cage experiments 60
Oviposition in the field 60
Egg numbers ..60
Oviposition in response to defoliation 62
Oviposition in response to host density 67
Statistical aggregation of Cyzenis eggs 72
Aggregation on foliage 72
Aggregation of attacks among hosts .75.
Host mortality from parasitism 78
Percent parasitism vs defoliation. ....78-
Percent parasitism vs host density 78
Oak-leaf extract on apple trees ...86
Di scussion . . ".. 89
Parasitoid behaviour 89
Host mortality ...92
CHAPTER FOUR: MANIPULATION OF MORTALITY IN THE SOIL .....100
Introduction . ... 100 vi
Methods 103
Manipulation and measurement of beetle abundance ....102
Experimental design 1983-84 ...... 102
Experimental design 1984-8.5 103
Beetle trapping in the oakwood ...106
Host and parasitoid abundance and survival ...... 109
Effect of diazinon 111
Fate of pupae ...... 114
Forced pupation 114
Presence/absence of beetles 115
Predation cage experiments 116
Results .1 1 7
Beetle abundance 117
Total numbers 117
Functional response 121
Host and parasitoid mortality 126
Effect of diazinon 126
Effect of beetles 127
Effect of prey type 134
Effect of habitat type 134
Production of Cyzenis relative to its host 141
Fates of pupae 141
Fate in the presence/absence of beetles ...141
Effect of density 148
Interaction of predation and parasitism ...150
Predation over time ...150
Predation cage experiments 153
Discussion 1 53 vii
CHAPTER FIVE: EFFECT OF FLY EXCLUSION ON SOIL MORTALITY
AND GENERATION MORTALITY 1 70
Introduct ion . . . . 1 70
Methods . ... . 171
Fly exclusion ..171
Density and survival estimates ....172
Results . 173
Apple orchard, 1984 ...... 173
Apple orchard, 1985 .176
Oakwood, 1985 ...... 176
Discussion •„..... 1 78
CHAPTER SIX: EFFECT OF PARASITISM IN POPULATION MODELS ..181
I n t r oduc t i on 1 8 1
Methods .... 182
Model I 182
Model II . . . . 190
Results ... . . 191
Model I ...... 191
Model II .....194
Discussion .1 94
CHAPTER SEVEN: GENERAL CONCLUSIONS .198"
LITERATURE CITED 20 1
APPENDICES 209 v i i i
LIST OF TABLES
Table I. Difference in winter moth phenology on apple and oak 26
Table II. Effect of phenology on percent parasitism in Victoria 26
Table III. Effect of phenology on parasitism at five sites on Saanich Peninsula ...28
Table IV. Density of Cyzenis in the apple orchard and oak wood 28
Table V. Winter moth pupal weights and density of first-instar larvae on apple and oak .41
Table VI. Winter moth mortality from hatch to pupation on apple and oak .. 4 5
Table VII. Mortality from parasitism in years of good and poor host-parasitoid synchrony at Wytham Wood, England 49
Table VIII. Number of Cyzenis eggs oviposited on apple and oak foliage in cage experiments ...61
Table IX. Number of Cyzenis eggs oviposited on apple and oak in 1984 as a function of level of def ol iat ion . . . '... 65
Table X. Number of Cyzenis eggs oviposited on apple and oak in 1985 as a function of level of defoliation ..66
Table XI. Number of Cyzenis eggs oviposited on apple and oak in 1984 as a function of host density ..68
Table XII. Number of Cyzenis eggs oviposited on apple and oak in 1985 as a function of host density ..69
Table XIII. Distribution of Cyzenis attacks among winter moth larvae from oak and apple trees 76
Table XIV. Aggregation of Cyzenis eggs in apple and oak foliage, and levels of parasitism at five sites on Saanich Peninsula .....77
Table XV. Regression statistics for percent parasitism as a function of defoliation and host density 81
Table XVI. Percent parasitism of winter moth on apple and oak in Victoria ....85 ix
Table XVII. Population estimates of predatory beetles ....120
Table XVIII. Anova table for the effect of beetle manipulation on winter moth mortality (k5) ..128
Table XIX. Anova table for the effect of beetle manipulation on Cyzenis mortality 129
Table XX. Anova table for the effect of prey type (host vs parasitoid) on mortality in the soil, in the orchard (k5a vs kcyz) 135
Table XXI. Anova table for the effect of prey type (host v_s parasitoid) on mortality in the soil, in the orchard (k5 vs kcyz) ....136
Table XXII. Anova table for the effect of prey type (host vs parasitoid) on mortality in the soil in the oakwood 137
Table XXIII. Anova table for the effect of habitat type (orchard vs oakwood) on winter moth mortality in the soil ....140
Table XXIV. Anova table for the effect of habitat type (orchard y_s oakwood) on Cyzenis mortality in the soi 1 142
Table XXV. Fate of pupae in the soil in response to manipulation of density 147
Table XXVI. Interaction between parasitism and predation on winter moth pupae in the field .149
Table XXVII. Fate of pupae in beetle removal and control plots 149
Table XXVIII. Predation on parasitized and unparasitized. pupae in cage experiments 154
Table XXIX. Predation on infected and uninfected pupae in cage experiments 155
Table XXX. Mortality of winter moth on: fly-exclusion trees, bird-exclusion trees, and control trees ...177 LIST OF FIGURES
Figure 1. Winter moth population density, 1983-85 4
Figure 2. Schematic pattern of phenology of host
pupation and parasitoid oviposition 10
Figure 3. Map of Saanich Peninsula with study sites 13
Figure 4. Winter moth development rate as a function of temperature ....16 Figure 5. Observed phenology of winter moth development on apple and oak, and timing of parasitoid oviposition 24
Figure 6. Timing of pupation of parasitized and unparasitized winter moth pupae .....30
Figure 7. Phenology of winter moth development on apple varieties Close and Spartan .. . ..32
Figure 8. Winter moth hatch and Cyzenis emergence in response to increased ambient temperture 35
Figure 9. Weight gain of winter moth larvae fed apple and oak foliage 37
Figure 10. Weight of winter moth pupae as a function of larval density ....39
Figure 11. Intrageneration mortality of winter moth larvae from first-instar to pupation on apple and oak ..43
Figure 12. Regression of number of Cyzenis eggs oviposited vs level of defoliation on apple and oak foliage 63
Figure 13. Regression of number of Cyzenis eggs oviposited vs host density on apple and oak trees . . . . . 70
Figure 14. Slope of regressions of Cyzenis eggs vs host density plotted against mean host density in orchard and oakwood, 1983-85 ...73
Figure 15. Percent parasitism of winter moth as a function of level of defoliation of apple and oak foliage .79
Figure 16. Percent parasitism of winter moth as a function of winter moth density 82 xi
Figure 17. Efficiency of Cyzenis attacking winter moth on apple and oak trees 87
Figure 18. Percent parasitism of winter moth larvae on trees sprayed with oak-foliage extract and on control trees 90
Figure 19. Superparasitism of winter moth larvae as a function of absolute parasitism on apple and oak trees . . 96
Figure 20. Experimental design for the manipulation of predatory beetles, 1983-84 ..104
Figure 21. Experimental design for the manipulation of predatory beetles, 1984-85 ..107
Figure 22. Experimental design for the effect of diazinon on beetle numbers and on host and parasitoid mortality ..112
Figure 23. Beetle abundance in the removal and control plots 1 18
Figure 24. Beetle abundance in the diazinon and control
plots .122
Figure 25. Beetle abundance in the orchard and oakwood ...124
Figure 26. Mortality of winter moth and Cyzenis in the beetle manipulation experiment, 1983-84 130 Figure 27. Mortality of winter moth and Cyzen i s in the beetle manipulation experiment, 1984-85 132
Figure 28. Mortality of winter moth and Cyzenis in the oakwood 138
Figure 29. Ratio of number of Cyzenis per winter moth surviving in the presence and absence of beetle predators .143
Figure 30. Ratio of number of Cyzenis per winter moth surviving in the presence of beetle predators in the orchard and oakwood . 145
Figure 31. Depth of pupation for parasitized, unparasitized, and depredated winter moth pupae .151
Figure 32. Re-analysis of Embree's data on contribution by various factors to winter moth generation mortality in Nova Scotia 159 xi i
Figure 33. Intra-generation mortality due to parasitism and mortality in the soil in the orchard and oakwood, 1983-1985 . ,. 166
Figure 34. Inter-generation mortality due to parasitism and of pupae in the soil for the orchard and oakwood, 1983-1985 168
Figure 35. Mortality of winter moth on: fly-exclusion trees, bird-exclusion trees, and control trees 174
Figure 36. Density-dependant survival functions used in model I for the apple orchard 183
Figure 37. Density-dependant survival functions used in model I for the oakwood 185
Figure 38. Fecundity of female winter moth as a
function of larval density .188
Figure 39. Results from model I 192
Figure 40. Results from model II 195 xiii
ACKNOWLEDGEMENTS
Two people have made major contributions to this project:
Judy Myers has provided an excellent environment, both intellectual and practical, for me to pursue this project. Her continual support, encouragement and enthusiasm, provided a very special opportunity for me; Angela Smith assisted with all aspects of field work through the entire project. Much of the work would not have been possible without her enthusiastic and high quality assistance.
Members of my research committee: D. H. Chitty, D. R.
Gillespie, W. E. Neill, R. A. Ring, G. G. E. Scudder and A. R.
E. Sinclair, provided helpful comments and suggestions in the early stages of the research, and in the final drafts of the thesis. Drs. Gillespie (Agriculture Canada) and Ring
(University of Victoria), were very helpful in providing logistic support. I also thank the Saanich Parks Board for permission to conduct research in Mt. Tolmie Park, and the
University of Victoria for use of the apple orchard.
C. J. Krebs and D. Ludwig provided helpful advice on sampling design and on data analysis. W. Lazorko and A. Smetana verified species of carabid and staphylinid beetles, respectively. Discussion and/or review of earlier drafts of this thesis were provided by S. J. Hannon, P. Morrison, I. S.
Otvos and J. R. Spence. Financial support was provided through a NSERC Operating Grant to J. H. Myers, and an NSERC
Postgraduate Scholarship to the author. 1
CHAPTER ONE
GENERAL INTRODUCTION
The debate over the role which natural enemies play in regulation of insect populations continues, as evidenced by two recent papers. The first, by Dempster (1983), regards natural enemies as having little regulatory effect on host insect populations. The second by Hassell (1985) argues that the density-dependent (regulatory) pattern of attack by natural enemies can be a very powerful factor in insect population dynamics but is largely obscured by our inability to measure these patterns adequately.
The initial intent of the long-term studies of the winter moth, Operophtera brumata L., in Britain, was to determine the role of natural enemies in the regulation of host numbers (Varley and Gradwell 1958). Nineteen years of population data from this study have formed the basis of many conceptual and explicit models of insect population regulation (Varley and Gradwell 1968, May 1978, Hassell
1980). In the last fifteen years (1971-1985), 59 papers related to insect host-parasitoid interaction and host population regulation by insect parasitoids, have appeared in the Journal of Animal Ecology. Of these, 42 papers (71%) draw support from studies of the winter moth and the parasitic fly Cyzenis albicans Fall. (Diptera: Tachinidae) and models based on these studies. The study of this one 2 insect has, therefore, strongly influenced much of the current thought on insect host-parasite interaction and population regulation.
The role that natural enemies play in natural regulation will determine the degree to which success can be expected in attempts at biological control. Our ability to predict successfully the outcome of these attempts reflects, to a large extent, our understanding of population regulation. Predictions resulting from the long-term study of winter moth should ideally be applicable to host- parasitoid systems in general, but should be most relevant to winter moth and C. albicans introduced to new habitats.
This was done for winter moth and C. albicans in Nova Scotia with poor concordance between prediction (Varley 1963,
Varley and Gradwell 1 968, 1971 ) and outcome (Embree 1965,
1971 ) .
The question of what promotes or precludes success of biological control has been previously addressed by comparing among case histories of successes and failures
(e.g. Beirne 1975, Hall and Ehler 1979, Hall et al. 1980,
Ehler and Hall 1982). This method, however, suffers from the myriad of confounding factors when one compares among results from many different hosts and their natural enemies.
The approach I have taken, is to compare the same host and parasitoid in situations where: (1) the parasitoid appears to have controlled its host successfully, and (2) where it clearly has not.
Experience from the introduction to Nova Scotia 3 indicated that the impact of parasitoids was highly unpredictable. Despite the dramatic decline of winter moth in Nova Scotia oakwoods after parasitoid introduction, there has been little decline in host density in orchards (MacPhee
1967, and pers. comm.). Also, the control in oakwoods
(Embree 1965, 1966) was surprising because parasitoids had virtually no effect on winter moth populations in oakwoods in Britain (Varley and Gradwell 1958). These paradoxes indicate the present difficulty that we have in predicting the effect of natural enemies in attempts at biological control, and in insect population regulation.
Winter moth was introduced into southern Vancouver
Island, British Columbia in the early 1970's (Gillespie et al. 1978), and has increased to levels causing serious defoliation of a variety of deciduous trees. Cyzenis albicans and Agrypon flaveolatum Fall. (Hymenoptera:
Ichneumonidae) were introduced from 1979 through 1981
(Williamson 1981a, 1981b, 1984, Embree and Otvos 1984) in an effort to control moth populations. Only C. albicans has been recovered in significant numbers from winter moth,
A. flaveolatum is virtually absent from samples of winter moth (pe r s. obs. , and I.S. Otvos pers. comm.). Moth densities have been declining over the past three years
(Fig. 1), particularly in oakwoods. The pattern and timing of population increase and decline closely resembles that observed in Nova Scotia. A brief description of the life cycle of winter moth in British Columbia is provided in
Appendix A. 4
Figure 1. Mean density per m2 of ground area for first-, instar larvae (L), pupae (P) and adult (A) winter moth from 1983 through 1985 in the oakwood (•) and apple orchard (O). Methods for estimation of density described in Chapter 2. o apple
4.0 • oak 6
The comparisons I used for my study arose from the apparent paradoxes described above, and were conducted on populations of winter moth and Cyzenis albicans in British
Columbia from 1982 (three years after the initial release of the parasitoids) through 1985. Field observation and experiments, were used to examine factors which differ between winter moth populations where control was apparently successful, and where control was less evident. Two comparisons were made: first between Cyzenis attacking hosts on oak trees and attacking them on apple trees. For herbivorous insects, the principal differences between oak and apple trees are the earlier leafing-out of apple trees, and differing plant chemicals. Both of these characteristics could affect the Cyzenis-winter moth interaction since leaf phenology will affect host insect phenology and hence host- parasite synchrony, and plant chemicals could affect oviposition on foliage by Cyzenis.
The second comparison is between the case where there is strong density-dependent mortality of the parasitized stage and the case where this mortality is weak. The major difference between life tables for winter moth in Britain and Nova Scotia was the presence, in Britain, of a large density-dependent mortality of pupae in the soil (Varley
1968, Hassell- 1980). Because Cyzenis develops within the host pupa, and is in the soil for almost ten months as opposed to five months for the host, it is potentially subject to greater mortality. The result of this is alleged 7 to be that Cyzenis can not build up to the densities capable of regulating host numbers (Hassell 1980). In Nova Scotia, flies reached very high density and hence could have had a large impact on host numbers. The principal known difference, therefore, was the presence/absence of a density-dependent mortality acting on the stage in which the parasitoid develops.
Phenological difference in leaf flush, and its effect on host development and host-parasite synchrony are examined
in Chapter 2. Difference between oak and apple foliage, and
its effects on oviposition by Cyzenis is examined in Chapter
3. Difference in effectiveness of Cyzenis in Nova Scotia and
in Britain was attributed to differing mortality of host and parasite in soil; this pattern is examined experimentally in
Chapter 4. The impact of mortality from parasitism within generations is examined in Chapter 5, and the potential
impact of parasitism over time is investigated with simple mathematical models in Chapter 6. 8
CHAPTER 2
OAK VS APPLE: THE EFFECT OF PLANT PHENOLOGY
ON HOST-PARASITOID SYNCHRONY
INTRODUCTION
The winter moth in Nova Scotia oak forests declined dramatically in numbers following the introduction of the tachinid fly Cyzenis albicans, in the late 1950's (Embree
1965, 1966). Populations have remained at low density since then (Embree pers. comm.). Despite the apparently successful control of winter moth on oak trees, high densities have persisted in orchards (MacPhee 1967, and pers. comm.).
One characteristic difference between oak and apple trees is the timing of leaf-flush in spring; apple buds open approximately two weeks before oaks. Variation in leaf-flush among oak trees and among years was the major factor determining winter moth population size in Britain (Varley and Gradwell 1958). Winter moths are polyphagous (Wint
1983), and feed on a large number of tree and shrub species.
Variation in leaf-flush is much greater among tree species than within species. Therefore, populations developing on different host plants may differ greatly in their phenology of development, and hence synchrony between the parasitoid and the vulnerable stage of the host.
If winter moths hatch more than six to ten days before leaf-flush, they will disperse and suffer greater mortality 9
than if they hatch after leaf-flush (Wint 1983). However,
the magnitude of this mortality is generally compensated for
in later larval stages (Wint 1983) and the period of food deprivation does not affect growth of surviving larvae and may in fact promote growth (Wint 1983).
On Vancouver Island, Cyzenis albicans emerges in mid-
March to early April, and requires approximately four weeks
for egg maturation. Oviposition on foliage damaged by host
feeding occurs from late April through the end of May.
Winter moth hatching from eggs on apple trees can begin development earlier than those on oaks, where foliage is not yet available at hatching. Through earlier development on apple, a portion of the larval population may escape parasitism by pupating prior to parasitoid attack (Fig. 2).
Because Cyzenis eggs remain viable for up to six weeks after oviposition (Embree and Sisojevic 1966), they will be asynchronous with their host only if host larvae pupate early, not late. There is a good chance, therefore, for
Cyzenis to be poorly synchronized with host populations on apple, a characteristic which might account for its
inability to control these populations.
Modelled host-parasitoid populations suggest that asynchrony has two effects: (1) it reduces parasitoid effectiveness (lower rate of parasitism [Griffiths 1969,
Hassell 1969]), and (2) it provides a mechanism for population stability by providing a refuge in time for part of the host population (Varley 1947, Hassell 1969, Munster-
Swensen and Nachman 1978) thus preventing local extinction. 1 0
Figure 2. Schematic temporal pattern of (a) pupation of early (apple) and late (oak) populations of winter moth, and (b) abundance of Cyzenis albicans eggs in the foliage. Arrow and dashed line indicates peak abundance of parasitoid eggs. 11
time 1 2
This short-term study cannot answer the question of increased stability of populations, but can examine the effect of asynchrony on the magnitude of host mortality from parasitism. This chapter examines whether winter moth populations on different host plant species do differ in phenology; and whether resultant asynchrony of host and parasitoid determines the potential success of biocontrol of winter moth by Cyzenis albicans.
METHODS
Study sites
Two principal study sites were used in this study (site
5, Fig. 3): (1) a stand of native Garry oak (Quercus garryana [Dougl.]) at Mt. Tolmie Park, which was a release site for C. albicans and A. flaveolatum, and (2) an unsprayed apple orchard at the University of Victoria. The orchard was not a release site, but by the time this study began, levels of parasitism by Cyzenis were equal to those in the oakwood.
The native moth Operophtera bruceata Hulst. is also found on Vancouver Island (Gillespie et al. 1978). On the basis of pupal characteristics (Eidt and Embree 1968), however, this species comprised less than 5% of the
Operophtera populations at my study sites. The native tachinid fly, Cyzenis pullula Tsnd. (Gillespie and Finlayson
1981, Humble 1985a, 1985b), was absent from all my collections of Cyzenis (D.M. Wood, Biosystematics Research 1 3
Figure 3. Saanich Peninsula showing the five study sites used and Cyzenis release sites (from Williamson I98la,b and 1984, Embree and Otvos 1984). (1) West Saanich Road at Deep Cove Road, (2). Pat Bay Highway at Ware Road, (3) West Saanich Road at Senanus Road, (4) West Saanich Road at Stelly's Cross Road, (5) Mt. Tolmie Park (oak), and 2400 CedarvHill Road (apple). 14 15
Institute, Ottawa, pers. comm.) The population status of the endemic Aqrypon provancheri D.T. (Humble 1985a), is not known.
For all observations and experiments, only trees less than 2.5 m tall were used to facilitate complete sampling of each tree. In the orchard, trees were arranged in a grid with 5 m spacing between trees (30 X 50 m grid); in the oakwood, spacing was more irregular, but all trees were within an area 100 m X 50 m. The two sites are separated by approximately 1 km. Four additional sites on the Saanich
Peninsula (Fig. 3) were included for estimates of parasitism of winter moth. Each of these sites had an unsprayed apple orchard within 5-30 m of stands of Garry oak.
Time scale
Temperature, and hence development rate, varies among years. For this reason, the time scale used in analyses of phenology is based on degree-days above the developmental threshold for winter moth larvae. Kozhanchikov (1950), documented the number of days required for winter moth larvae to develop to pupation at different temperatures. I used these data (his.Table V) to calculate development rate
(1/days to pupation) for each temperature. Regression of development rate against temperature provides a predicted development threshold of 3.5°C (Fig. 4). Embree (1970) had estimated the threshold at 5°C but found it to result in poor predictions from models of larval development. He suspected a temperature between 3.5 and 4.0°C to be more Figure 4. Development rate of winter moth (l/days to pupation as a function of ambient temperature, calcualted from Kozhanchikov (1950, Table V). (rate=-0.012+.0034*temp, N=6, r2=0.992, P < 0.0001). Development threshold temperature (intercept of x-axis) is 3.45°C (± 1.6, 95% confidence limits). .10
.08 development 06 rate (days"1) .04
.02
0 5 10 15
temp (°C) 18 correct. Maximum and minimum daily temperatures used were those at Gonzales Heights Meteorological Station, Victoria
(Environment Canada, Atmospheric Sciences Service), which is located 1.5 km from the two principal study sites. . Daily accumulated degree days above the threshold were calculated from the known development threshold temperature and fitting a sine-wave function to the daily maxima and minima (Gilbert and Frazer 1976). In each year, "day zero" was set at March
1 since winter moth hatch begins in the second to third week of March.
Population phenology
Development of larval populations and timing of parasitoid attack were monitored in both apple trees (var.
Close) and Garry oaks in 1983, 1984 and 1985. In 1985, an additional apple variety (Spartan) with leaf-flush approximately one week later than var. Close was also included. By including this variety, I was able to control for characteristics other than phenology which might differ between oak and apple trees.
The sampling unit for host populations was the individual leaf cluster consisting of four to five leaves produced from a single bud. Winter moth larvae remain within clusters during the day, and feed and move among clusters at night. Densities of winter moth larvae were estimated for each of 32 apple trees in 1983 and 1984; 23 in 1985. Eight oak trees were sampled in 1983, sixteen in 1984, and nine in
1985. To ensure that estimates were representative of the 19 entire tree, I selected clusters randomly from within each of sixteen sections of each tree. Sections were delineated by the four cardinal directions (N, E, S, W), which were subsequently divided into top and bottom half of the tree crown. The resulting eight sections were further divided longitudinally in two, based on distance from the trunk.
Within each of the sixteen sections, a branch was selected arbitrarily. A random number between one and ten was selected, and the first cluster on the branch was assigned the number "1". Clusters along the branch were counted in sequence up to the randomly chosen number. If the branch terminated, or entered another section of the tree, a second branch was chosen and counting continued. The number of larvae in each instar was recorded as was total number of larvae. Sampling for each tree proceeded in multiples of sixteen until the standard error of the density estimate was less than 15% of the mean.
The pattern of Cyzenis emergence was measured by daily removal of flies from two cone emergence traps, 0.25 m2, placed under each tree. Phenology of winter moth pupation was measured by weekly removal of pupae from 0.416 m2 peat- filled pupation trays placed under each apple tree; 0.16 m2 trays under oaks. The cumulative proportion of the population which had pupated was calculated for each date.
Comparison of development between populations on apple and oak was based on the time (degree-days) required for 50 percent of the population to reach a particular stage
(second through fifth-instar, and pupae). 20
Paras i t i sm
Timing of oviposition by Cyzenis was monitored by periodic sampling of apple and oak leaves and by searching for eggs on foliage with the aid of a microscope. This was complemented by sampling of winter moth larvae and dissection for Cyzenis maggots. In this way, the time at which hosts began ingesting parasite eggs was determined each year. Percent parasitism of winter moth for the orchard and oakwood was estimated from pupae collected in pupation trays. Four additional sites (Fig. 3), where oaks and unsprayed apple trees occurred together were sampled for the estimation of parasitism in 1984 and 1985. This allowed comparison of parasitism rates on oak and apple at five widely separated sites. Fifth-instar larvae were collected from trees at each site and dissected for Cyzenis maggots.
The relation of phenology of host development on each tree species with timing of parasitoid "attack" measures the potential synchrony. A better measure of synchrony is the comparison of the timing of pupation for parasitized and unparasitized larvae (MacDonald and Cheng 1970). Because parasitism does not affect the timing of pupation (Hassell
1968), this comparison would identify whether individuals pupating early are able to avoid parasitism. I removed pupae
from pupation trays each week through the one-month period of larval drop, and scored each of these samples for parasitism. The cumulative proportion of parasitized larvae pupating through time was compared with that for unparasitized larvae. A refuge on apple was indicated if 21
larvae pupating early were less parasitized than the population as a whole. Data for this analysis were available
for the orchard only. The greater shade in the oakwood prevented pupation trays from drying after rainstorms, and
they could not be sifted at weekly intervals.
Control of phenology
The role that temperature plays in the control of
winter moth hatch and Cyzenis adult emergence was determined
by exposure of over-wintering insects to warm temperatures
for increasing lengths of time. Eight vials containing
approximately 100 winter moth eggs each, and eight vials
with 10 to 20 Cyzenis pupae each, were kept in an outdoor
shade house. On February 12, 1985, one vial of each set was
taken indoors, where temperature was approximately 22°C.
Each week, one more set of each was taken in. The number of
hatched larvae or emerged Cyzenis was checked daily after
they had been taken indoors. After all eight vials had been
taken indoors, Cyzenis had not begun to emerge in any great
numbers, so an additional three sets of pupae (eleven total)
were made from pupae which had been kept at outdoor
temperatures. Thus, the flexibility of each insect's
appearance in the spring, and the degree to which hatch and
emergence are temperature-dependent was determined.
Effect of foliage type on development
In 1984, four treatments were used to determine the
suitability of foliage from each tree species for winter 22 moth development. Third-instar winter moth larvae were collected from apple trees and oak trees. Each of these samples was divided in two and reared on either apple or oak foliage. The four treatments therefore were:
larvae from apple reared on apple (N=10)
larvae from apple reared on oak (N=10)
larvae from oak reared on apple (N=10)
larvae from oak reared on oak (N=10)
Larvae were reared individually in petri dishes, and fed ad
1ibiturn. Larvae were weighed each week, after a 24-hour period of food deprivation to clear the gut. Mean growth rate (weight gain) was estimated for each treatment.
Effect of foliage type on winter moth development in the field was assessed by comparing weights of pupae recovered from pupation trays under apple and oak trees.
Effect of foliage type on survival
Density of first-instar larvae was estimated from samples of foliage in early April. Density of fifth-instar larvae was estimated from the number dropping into pupation trays. Mortality during the larval stage was expressed as k- values (loglO of the initial density minus loglO of the final density [Varley and Gradwell 1960]). Use of k-values allows easier comparison with previous studies of winter moth, and permits comparison of proportionate losses regardless of absolute loss. The degree to which larval mortality is density-dependent was estimated by plotting k against log of the initial density of first-instar larvae 23
(Varley and Gradwell 1968). These data thus provide a description of the pattern of mortality on each tree species prior to attack by Cyzenis.
RESULTS
Population phenology
Populations of winter moth larvae began and completed development earlier on apple trees than on oaks in 1983 and
1984 (Fig. 5, Table I). These two years were warm, and there was a large difference in timing of leaf flush. In 1985, there was little difference in phenology on apples and oaks.
That year was unusually cold, resulting in leaf-flush on the two tree species being more similar than in other years. In
1985 samples were taken too infrequently on oak to determine when 50% of the population had reached the fifth instar. For this reason, comparison of development in 1985 was made using fourth-instar larvae.
Parasitism
Timing of attack
The time at which Cyzenis began parasitizing winter moth varied among years both with respect to date and degree-days. In each year, oviposition began prior to host pupation in either stand, but peak oviposition (14 days after initiation [Embr.ee and Sisojevic 1966]), occurred during the period of larval drop and pupation (Fig. 5). 24
Figure 5. Phenology of winter moth larval development on apple(O) and oak (#) trees in each of three years. Points are proportion of sample in each life stage on each date. In 1985, samples were not taken frequently enough for comparison of fifth-instar larvae, so fourth ihstars were used instead). Regressions are based on logit-transformed proportions reaching each stage plotted against degree-days above the the development threshold. Arrows indicate time of peak ovipostion by Cyzenis. (L2=second-instar, L4=fourth-instar, L5=fifth-instar, P=pupae). Data for pupae were not collected in 1983 and 1984 for the oakwood; curves for those years were based on the difference between stands observed for fifth-instars, and applying that difference to the pupation curve for the orchard. 25
degree - days Table I. Degree-day difference (S.E.) between winter moth populations on apple and oak reaching various stages of development. Differences are based on the number of degree-days required for 50 percent of the population to reach each stage. year L2 L4 L5 pupa*
1983 79.0(18.2) 58.7(18.8) 38.2(7.1) 38.2 1984 45.5(27.0) 46.0(19.3) 52.3(8.3) 52.3 1985 29.5(17.6) 12.6(14.6) - 12.6
*same interval as for last measured larval instar since pupae from oaks were only collected at the end of the season.
Table II. Percent parasitism of winter moth populations on apple and oaks in Victoria, and degree-day differential of winter moth to reach 50% pupation on each tree species Parasitism estimated from collections of pupae.
percent degree-day year tree N parasitism differential(S
1983 apple 6742 28 38.2 (7.1) oak 751 28
1 984 apple 790 65** 52.3 (8.3) oak 418 74
1985 apple 646 52*** 12.6 (14.6) oak 1 42 73
** P<0.01, *** P<0.001, X2 test for parasitism on apple vs oak. 27
Cyzenis eggs remain viable on foliage for up to six weeks
(Embree and Sisojevic 1966). Winter moth larvae on oak were exposed to parasitism far longer in 1983 and 1984, less so in 1985 (Fig. 5).
Levels of parasitism
If synchrony of the parasitoid with its host is the factor determining successful control on oak and lack of success on apple, the above results would predict little difference in parasitism rates in 1985, greater parasitism on oaks in 1983, greatest in 1984. Parasitism in fact differed most in 1985 (Table II).
When winter moth larval development was much earlier on apple than on oak, parasitism was equal to or higher than that on apple at all sites (1984, Table III). This is opposite to what would be predicted if early development on apple permitted escape from parasitism. Although host- parasitoid synchrony differs between the two populations, the effect on parasitism levels is minimal. In 1985, when winter moth development was slow and Cyzenis oviposited later than in other years (Fig. 5), those pupating early in the orchard suffered less parasitism than those pupating late (Fig. 6). Because phenology in the oakwood was virtually identical to that in the orchard (Fig. 5), a similar pattern was probable there. Therefore, a refuge in time likely occurred for both populations.
In 1984, there was a large difference in host population phenology on the two tree species (Fig. 5), but a Table III. Percent parasitism of winter moth populations on apple and oak in years when phenologies of host populations are different (1984) and similar (1985). Parasitism estimated from dissection of fifth-instar larvae.
1984 1985 (52.3 degree-days) (5.1 degree-days) di f ference di fference
% % site tree parasit N parasit N
1 apple 1 191 1 3 1 06 oak 0 161 7 28
apple 1 2 98 19 124 oak 4 1 16 20 55
apple 5g*** 1 13 36* 106 oak 20 65 65 1 7
apple 59*** 1 1 4 52** 97 oak 37 1 1 5 79 47
apple 66 1711 52*** 995 oak 67 1 066 84 410
* P<0.05, ** P<0.01, *** P<0.001 X2 test on parasitism on apple vs oak.
Table IV. Mean density of adult albicans emerging in the apple orchard and oakwood, 1983-85.
Cyzenis per m2 (S.E.)
apple oak
1983 1.75 (0.48) 5.76 (0.72) 1984 39.68 (5.16) 18.56 (6.48) 1985 15.78 (3.37) 45.00 (8.72) 29 small refuge occurred for early pupators in the orchard
(Fig. 6b). Because larvae on oaks would be susceptible later, there would also be a small refuge on oaks. This pattern would also be true in 1983 (Figs. 5 and 6a).
Results might have been confounded by differences in density of parasitoids. In 1984, parasitism levels were the
same on the two tree species (Table II) despite the prediction, on the basis of phenology, that it would be higher in the oakwood; only half as many flies emerged in
the oakwood as in the orchard (Table IV). In 1985, parasitism was much higher in the oakwood than predicted, and density of emerging flies was three times as high there.
It must be noted, however, that fly density at emergence may
bear little relation to density four to six weeks later when
they oviposit. At the other sites where parasitism was
estimated, trees of each species were only 5 to 30 m apart,
so the same density of flies was available for oviposition
on both tree species. At these sites, mortality from Cyzenis
was higher on oak in the year when parasitism rates were
predicted to be the same on the two tree species (Table
III) .
The two apple varieties showed differing phenology of
winter moth populations (Fig. 7) and large difference in the
patterns of pupation for unparasitized and parasitized
individuals (Fig. 6d). A refuge is apparent for var. Close,
but not var. Spartan. Parasitism did not differ in the two
populations, despite the differences in synchrony. 30
Figure 6. Cumulative proportion of unparasitized (•) and parasitized (O) winter moth pupating over time on Close and Spartan apple trees in the orchard. Significant difference tested with Kolmogorov-Smirnov two-sample test (see text for results). (b) 1984
Close
N = 280
/ yu N = 510 i— o- '— o- 7 15 23 31 8 16 IB 24 9 17 May June (d) 1985 Spartan }
• unparasitized
o parasitized
N-139
N = 131
16 24 I 9 17 May June 32
Figure 7. Phenology of winter moth larval development on apple var Close (O) and var Spartan (•). Regressions are based on logit-transformed proportions reaching each development stage plotted against accumulated degree-days above the development threshold. Curves for apple and oak are not significantly different from each other. 800 degree - days 34
Control of spring emergence
Hatch of winter moth eggs was accelerated, in every case, when they were taken from outside temperatures in the shade house (less than 10°C), to indoor temperatures (22°C)
(Fig. 8a). This suggests that diapause of eggs (Kozanchikhov
1950), is broken before February 12. After a short period of development, winter moth can hatch at any time. The bulk of
Cyzenis emergence on the other hand, did not proceed until after March 28, regardless of accumulated degree-days
(Fig. 8b). This suggests that diapause of Cyzenis pharate adults is not broken until the end of March. Emergence of
Cyzenis is therefore more fixed in time than is hatch of winter moth.
Effect of host-plant on development
Growth rate experiments suggest that larvae do well on both types of foliage, but can reach greater weight on oak foliage (Fig. 9). Wint (1983) found no difference in pupal weight of larvae fed apple and oak (Q. robur [L.]) foliage.
Delayed development on oak, therefore, is not due to oak being a poorer quality food at this time of year, nor is successful control of populations on oaks due to poor host development on oak foliage.
In the two years of relatively high density (1983 and
1984), pupal weights were lower from oaks than apple
(Fig. 10, Table V). At lower density (1985), pupal weights were not different. It should be noted that at densities of
1 to 2 larvae per leaf cluster observed in 1985, larvae 35
Figure 8. Phenology of (a) winter moth hatch, and (b) Cyzenis emergence as a function of time of increased ambient temperatures. Triangles indicate date on which each set of eggs or pupae was taken from cool to warm temperature.
37
Figure 9. Mean weights (± S.E.) of winter moth larvae reared on apple foliage (O) and oak foliage (•). Samples of third-instar larvae used were collected from: (a) apple trees, and (b) oak trees. 38 39
Figure 10. Mean pupal weight (mg) of winter moth as a functon of initial (first-instar) larval density per leaf cluster for apple (O) and oak trees (#) (all years combined). 40 • oak o* o apple 6 30 6° COO GO O O 8 o «• o Q0o O O . o «o ° 8> weight 2Q[ m (mg)
10
0 2 4 6 8 10 12 14
L, / leaf cluster 41
Table V. Mean weight of pupae (mg) and mean density of first instar larvae per leaf cluster from apple and oak trees in 1983 through 1985. Sample size is number of trees; the mean weight for each tree is based on samples of 15 to 50 pupae. Statistical significance of weigt-difference is indicated.
density weight * year apple oak P apple oak P
1 983 8.96 3.36 0.001 22.2 21.3 ns 1 984 5.25 3.42 0.01 27.4 22.4 0.0001 1 985 1 .23 0.94 ns 32.7 33.8 ns
* Student's t-test. 42 still caused up to 50% defoliation. At low to moderate density, both foliage types were suitable for winter moth growth and development.
Effect of host-plant on survival
Apple trees, on which leaf flush tends to be early, have higher densities of early-instar larvae than do oaks
(Table V). However, strongly density-dependent larval mortality (Fig. 11, Table VI) results in similar densities of larvae on the two tree species at the time Cyzenis are ovipositing. In 1983, there was a marked difference in the range of initial larval densities, and mortality on oak was on average higher than on apple, but less strongly density- dependent (Fig. 11a, Table VI). Pattern of larval mortality did not differ on apple and oak in 1984 or 1985 (Fig. 11b and c, Table VI). The principal source of larval mortality in 1983 was starvation through competition for food; in
1984, it was bird predation (Roland et al. 1986).
Larvae reared in the laboratory showed no difference in survival when fed oak or apple foliage (oak=60%, N=20; apple=60%, N=20). This was also the case for apple and oak in Britain (Wint 1983). It appears, therefore, that differences between oak and apple foliage have little effect on growth and survival of winter moth. Once established on oaks, larvae do just as well as on apple trees. The apparent success of Cyzenis in controlling winter moth populations on oaks is not the result of deleterious effects of oak foliage on winter moth larval development or survival. 43
Figure 11. Intrageneration mortality of winter moth from first-instar larvae to pupation on apple (O) and oak (•) trees in 1983 through 1985. Mortality of larvae expressed as k-values; initial density of larvae is per leaf cluster. 44
3r (0 1985
lorv
0
"0 5 00 0.5 1.0 1.5
log larvae per leaf cluster 45
Table VI. Mean density (L'1 per leaf cluster) and mortality of winter moth larvae from hatch to pupation (klarv). Slope of the regression of mortality against log initial density (Fig. 11) is also indicated.
density L1 klarv slope2
apple oak sig1 apple oak apple oak
1983 8.96 3.36 *** 0.46 0.53 1.98*** 0.43 1984 5.25 3.42 ** 0.95 0.66 1.06*** 1.36*** 1985 1.23 0.94 0.40 0.23 0.75 0.39
density L5
apple oak
1983 3.38 0.95 *** 1984 0.89 0.87 1985 0.62 0.84
1 Student's t-test 2 least-squares regression ** P<0.01, *** P<0.001 46
Close vs Spartan apple varieties
Phenology of winter moth development was later on
Spartan apple trees than on Close apple trees (Fig. 9).
Degree-day difference to reach each stage was: L2=10.9,
L4=24.2, and pupae=32.4. In 1985, therefore, winter moth
larvae developed later on Spartan than on oaks. Mean pupal
weight from Spartan apple trees was 33.5 mg (N=4 trees,
S.E.=1.37); no different from those from Close apple trees
(32.8 mg, N=23, S.E.=1.37) or oaks (33.9 mg, N=7,
S.E.=1.07).
Parasitism would be expected to be greater for larvae
on Spartan trees since the vulnerable stage is exposed to
Cyzenis eggs longer. On var. Close, early-pupating larvae
were less parasitized than late-pupating larvae. On var.
Spartan, a refuge in time did not exist. Over-all parasitism
was not different for larvae developing on the two varieties
(Spartan=48 percent, N=272; Close=52 percent, N=646; Chi-
square=1.14, df=1, P=0.20). Difference in host phenology and
hence synchrony with Cyzenis oviposition, does not affect
the rate of parasitism.
DISCUSSION
The successful control of insect pests by natural
enemies requires both the suppression of host numbers and
the maintenance of numbers at the new low density.
Asynchrony of host and parasitoid has been suggested as a
mechanism by which populations can be stabilized since there
is a refuge in time for part of the host population (Hassell 47
1969, Munster-Swensen and Nachman 1978). This would, however, occur at the expense of reduced mortality through parasitism in each generation. Perfectly synchronized parasitoids would maximize attack rate (Griffiths 1969).
This would be desirable in biological control programs provided there was a mechanism for stabilizing host numbers.
These mechanisms could include interference among parasitoids, aggregation by parasitoids, spatial heterogeneity, and density-dependent mortality of hosts
(Hassell 1978).
Phenology of host plant can have a large effect on insect herbivore population size. Examples include the winter moth on oaks (Varley and Gradwell 1958), winter moth on apple (Holliday 1977), sycamore aphid on maple (Dixon
1976) and large aspen tortrix on aspen (Witter and Waisanen
1978). Trees which leaf out early allow high survival and large population size. Earlier bud-burst may account for the higher density of first-instar larvae on apple trees than on oaks (Table V). Difference in winter moth density was also observed among oaks with different times of leaf-flush
(Varley and Gradwell 1958). Early development of winter moth on early-leafing oaks should result in poorer synchrony of
Cyzenis with fifth-instar host larvae. Within years however, oaks with early bud burst (high larval density) suffer higher parasitism (Hassell 1968,1980) despite relatively poor synchrony. Less synchronized populations (late leaf- flush, low density) suffered lower rates of parasitism. This probably resulted from the aggregative behaviour of Cyzenis 48
in patches of high host density (Hassell 1968). Among years at Wytham Wood, good synchrony of host and parasite (Table
VII) resulted in lower rates of parasitism and lowered parasitoid efficiency relative to years with poor synchrony.
This suggests that synchrony is not an important element in
success or failure of Cyzenis as a controlling factor.
Another tachinid fly, Lypha dubia Fall, does cause
higher parasitism of winter moth on late developing (low
density) oak trees (Cheng 1970). This may reflect the fact
that dubia larvae are viable for a relatively short
period, and must seek a host during this short interval
(Cheng 1969). As a result, good synchrony with its host is
probably very important. Cyzenis eggs, on the other hand,
can be ingested up to six weeks after oviposition and do not
require so precise synchronization with host development.
Phenology of . host development clearly varies in
response to host plant development and the effect of
microclimate. The vagility of parasitoids, and their
tendency to aggregate in areas of high host density
(e.g. Hassell 1968, Stamp 1981, Heads and Lawton 1983)
regardless of host phenology, may negate any effect which
difference in synchrony might have. Cyzenis respond not to
host larvae but to levels of feeding damage (Hassell 1968).
In years when populations on the two tree species have
differing phenology, flies may simply oviposit on apple
trees first, and gradually shift to more delayed populations
on oaks. The very high fly density observed in this study
(40 to 400 times that observed in Britain) may simply ensure Table VII. Density of Cyzenis albicans and winter moth in Wytham Wood, and mortality from parasitism in years of good and poor synchrony of host and parasitoid.
b b synchrony year Cyz/m2 larvae/m2 k2 ) poor 1958 1 .41 191 .07(15) .114 1 959 2.51 58 .10(21) .092 1 965 0.10 269 .03 (7) .690 good 1 966 0.63 51 01 (2) 037 a. from Hassell 1969 b. from Varley, Gradwell and Hassell 1973 (Table F) c. from Varley, Gradwell and Hassell 1973 (Fig. 7.8) d. a=2.3(k2)/Cyz (nr2) (Varley, Gradwell and Hassell 1973 p 107) . 50 that the portion of the host population at risk to parasitism, is parasitized. In years when phenology is similar on the two tree species, flies would remain in the area where they emerged, and parasitism levels would reflect parasitoid abundance at emergence.
Larvae are at first more abundant on apple than on oak.
When strongly density-dependent mortality precedes parasitism in population models (Wang and Guttierez 1980;
May e_t al. 1981), greater population suppression will result than if density-dependent mortality were absent. The two populations in this study suffer similar mortality of the larval stage (Fig. 11), and no advantage is conveyed to
Cyzenis on oaks.
In Nova Scotia, parasitism rates on larvae from both tree types were compared at one site only and in one year
(MacPhee 1967); parasitism was 6% on apple and 77% on oaks.
Winter moths have since remained at low density on oaks
(Embree pers. comm.), which suggests they are stable. The differences in host-parasitoid synchrony observed between winter moth populations in my study did not result in difference in host mortality from parasitism. The observed magnitude of this mortality was generally opposite to what was predicted from pattern of host development and timing of parasitoid attack. Other factors are likely to determine the efficiency of Cyzenis attack as it relates to success or failure of biocontrol.
Three models have attempted to address the impact of host-parasitoid synchrony on population dynamics (Griffiths 51
1969, Hassell 1969, Munster-Swensen and Nachman 1978). All predict reduced mortality when parasitoids are poorly synchronized with their host. The latter two are based on random search by parasitoids and predict increased stability with asynchrony. In these, asynchrony is the only stabilizing mechanism. Griffiths' (1969) model uses aggregated attack by parasitoids (appropriate for Cyzenis albicans [Hassell 1968, May 1978, Hassell 1980]). This model predicts reduced stability with reduced synchrony. That is the scenario which most closely resembles the pattern observed in Nova Scotia apple orchards. Parasitism was lower than or equal to that on oaks, and was associated with greater asynchrony of the parasitoid with its host. If
Cyzenis aggregates its "attacks" on apple trees with high host density (as seen among oaks in Britain), then asynchrony may not promote stability, but rather reduce it.
Reduced synchrony observed on apple populations is associated with higher host density and continued population outbreak. Unless pattern of aggregation by Cyzenis differs between apple and oak trees (see Chapter 3), asynchrony could allow outbreaks in the orchards in warm years when leaf-flush and winter moth development on apple are very rapid, and very much ahead of that on oaks. 52
CHAPTER 3
OAK VS APPLE: EFFECT OF HOST-PLANT SPECIES
ON RATE AND PATTERN OF CYZENIS OVIPOSITION
INTRODUCTION
Females of the tachinid fly Cyzenis albicans, do not oviposit directly on host larvae, but rather lay microtype eggs at the margins of leaves damaged by caterpillar feeding
(Hassell 1968). This behaviour results in the aggregation of parasitoid eggs in areas of high density and feeding activity of their host (Hassell 1968). Cyzenis eggs are ingested by winter moth larvae as they feed. Females oviposit in response to the sap flux at the damaged leaf edge (Hassell 1968), suggesting a chemical cue for oviposition by females. Because mechanical damage of oak leaves can induce the same response (Hassell 1968), the cue is strictly associated with chemicals of the plant, not directly with the feeding insect. Chemical differences among plant species affect oviposition by many herbivorous insects
(Rausher 1983); parasitoids such as Cyzenis, which use foliage cues for oviposition might respond to such differences as well.
Hassell (1980) concluded, from an analysis of models of winter moth populations, that the apparent success of
Cyzenis in Nova Scotia appeared due to (i) its ability to increase rapidly in numbers after introduction, and (ii) the 53 strong aggregation of parasitoids in areas of high local host density. Aggregation of attacks is thought not only to produce a strong stabilizing effect on host population size
(Hassell & May 1974; May 1978; Beddington, Free & Lawton
1978; Hassell 1980), but also to increase total host mortality through increased parasitoid efficiency (Hassell
1978 p. 99, Commins & Hassell 1979). Others have argued that although parasitoid aggregation does occur in the field, successful control can occur in the absence of spatially density-dependent attack by natural enemies (Morrison and
Strong 1980, Murdoch et al. 1984, Reeve and Murdoch 1985).
Stability may be difficult to define or measure (but see
Connell and Sousa 1983), but the prediction of increased efficiency, measured as host mortality per parasitoid, can be more easily tested.
Clumped spatial distribution of Cyzenis albicans eggs on oak leaves has been reported from Nova Scotia (Embree &
Sisojevic 1965) and England (May 1978, Hassell 1980) and was considered the cause of the statistical clumped distribution of Cyzenis maggots among samples of winter moth larvae.
Spatial aggregation of eggs on defoliated oak clusters was suggested to be greatest when overall levels of defoliation were low (Hassell 1980), that is, when damaged clusters were rare most parasitoid eggs were placed on the few clusters upon which winter moth larvae had fed. The statistical distribution of attacks among host larvae was correlated with these patterns, and was observed to be more clumped when average defoliation (and host population size) was low 54
(Hassell 1980).
Successful control of winter moth by Cyzenis may be mediated both by the total number of eggs laid and by their spatial distribution. If differing success is due to differing pattern of attack, we would predict that either:
(i) more Cyzenis eggs are laid on oak than on apple leaves for a given level of feeding damage or density of hosts, or
(ii) more Cyzenis eggs are ingested by winter moth larvae on oak than on apple. The former implies that the stimulus for oviposition by the parasitic fly differs between the two tree species. The latter implies either differential feeding rates by host larvae on the two plant species, or that
Cyzenis females are better able to aggregate their "attacks" on those oak leaves with high damage, and that those eggs oviposited on apple foliage are distributed randomly among damaged and undamaged leaves. Wint (1983) demonstrated no difference in duration of feeding by winter moth larvae on oak and apple, but the pattern of oviposition by the parasitoid on plants other than oaks has not been measured.
This chapter examines oviposition by C_^ albicans on oak and apple trees to identify patterns which might explain the differing success of Cyzenis in controlling its host on the two plant species.
METHODS
Cage experiments
The rate of oviposition by Cyzenis on oak and apple 55 foliage was compared directly by the use of cage experiments. Females were placed in 17 X 12 X 6 cm clear plastic cages screened on one side. Each cage contained one cluster of either oak or apple leaves. Prior to the experiments, leaves were exposed to one of two treatments:
(i) Trial I - leaves were artificially damaged by cutting an amount approximately equal to about four times the leaf margin, (ii) Trial II and III - approximately 40% of the leaf area was eaten by winter moth larvae. Each cage contained one cluster of 4-5 leaves from which any Cyzenis eggs already present had been removed. Female flies used in the trials had been reared from parasitized winter moth larvae collected the previous year, and had been kept in cages with males for one month prior to the experiment. Four
females were introduced in each experimental cage. Total number of parasite eggs was counted for each cage at the end of each trial.
Field studies
Host abundance
The response of Cyzenis to host density and level of defoliaton was examined in the field utilizing natural densities of winter moth in the oakwood in all years, and in the apple orchard in 1983 and 1985. In 1984, densities were manipulated on some apple trees by the application of an
insecticide spray during the host's first instar stage (one month prior to Cyzenis oviposition). Insecticide formulation 56 was: 0.5 ml 50.5% Safer's Insecticidal soap and 3.0 ml methoxychlor 25% emulsifiable concentrate per litre of water. Three treatments were used: (1) high density of winter moth larvae on 16 trees (no. spray), (2) medium density of larvae on 8 trees (one spray application, March
24), (3) low density on 8 trees (two spray applications,
March 24 and April 1). Any effect of insecticide treatment on number of Cyzenis eggs oviposited and on host parasitism was tested for by analysis of covariance using level of defoliation as the covariate. There was no difference in the mean number of Cyzenis eggs per leaf cluster for sprayed and unsprayed treatments with similar levels of defoliation
(F=1.74, df=2, P=0.18). This is reflected in the fact that there was also no difference in mortality from parasitism
for sprayed and unsprayed treatments with similar levels of defoliation (F=1.23, df=2, P=0.31).
Because feeding damage, rather than presence of host
larvae, is the stimulus for oviposition (Hassell 1968), the percent reduction in leaf area of each of these leaf clusters was visually estimated. Winter moth larvae do not
totally consume leaves, and so the proportion of leaf area
removed from each cluster could be easily determined. I define defoliation as the percent reduction in leaf area.
Number of trees used in each year is as indicated in Chapter
2.
Parasitoid abundance
The density of emerging Cyzenis was estimated each 57
spring as described in Chapter 2.
Ovipos it ion and parasit ism
Phenology of winter moth populations is usually seven
to fourteen days later in the oak wood compared with the
apple orchard (Chapter 2). This is probably due to warmer
temperatures in the open orchard than in the shaded oakwood,
and because apple buds leaf out earlier than do oaks
(Chapter 2). For this reason, sampling of Cyzenis eggs in
foliage was done one to two weeks later on oaks than on
apple trees.
In 1984 and 1985, leaf clusters collected for estimates
of late-instar larval density (Chapter 2) were examined for
Cyzenis eggs with the aid of a dissecting microscope. Sample
size for estimation of Cyzenis eggs was 8 apple trees and 8
oak trees in 1984; 10 apple trees and 8 oak trees in 1985.
Sampling was done approximately six weeks after Cyzenis
adult emergence and winter moth larval hatch, for three
reasons: (i) peak feeding and defoliation by fifth-instar
larvae occurs at this time, (ii) Cyzenis females require at
least four weeks of feeding to mature their oocytes (Hassell
-.1968), and (iii) maximum number of Cyzenis eggs occurs in
the foliage seven to fourteen days after the start of the
oviposition period (Embree & Sisojevic 1965). Field data on
fly oviposition on the two tree species were compared using
an analysis of covariance, with host density and level of
defoliation as covariates of number of eggs laid.
Because I did not know the spatial scale on which 58
Cyzenis flies detect differences in defoliation or host abundance, I analyzed data at three levels of spatial scale:
(1) among all leaf clusters, all trees combined, (2) among
leaf clusters, within trees, and (3) among trees. These
levels are analogous to those used by Murdoch et a_l. (1984) and Reeve and Murdoch (1985) for parasitoids attacking
citrus scale and olive scale in orchards. Dispersion of
Cyzenis eggs among leaf clusters and among trees was
estimated by k. from the negative binomial distribution. The
greater the value of k_, the less clumped the distribution,
the distribution being effectively random at k greater than
about eight. Negative values of k suggest a uniform
distribution. Just prior to larval drop, samples of fifth-
instar larvae were collected from each of thirty-two apple
and sixteen oak trees in 1984, and from 23 apple trees and
eight oaks in 1985. Cyzenis maggots, like those of other
tachinids which oviposit on foliage, hatch within a few
minutes of being ingested (Severin et al. 1915) and localize
in the host salivary glands within one hour. They do not
develop until the host pupates. Total number of attacks can
therefore be determined for each individual host. Dissection
of fifth-instar larvae (described in Hassell 1980) provided
a measure of the distribution of Cyzenis maggots among host
larvae for each tree, and allowed estimates of k_ for the
actual attacks, permitting comparison with data from Britain
(May 1978; Hassell 1980). Samples of larvae late in
development can still underestimate total parasitism of the
population since the time during which hosts are normally 59 exposed to parasitoids is shortened by collection.
Parasitism by Cyzenis was therefore also estimated from samples of winter moth pupae collected in pupation trays
(Chapter 2). Collected pupae were allowed eight weeks for development; cocoons were then opened and pupae scored for parasitism by Cyzenis. These data do not include estimates of multiple attacks since only one fly develops regardless of the number of parasite eggs ingested by the host.
In addition to the two principal study sites, samples of winter moth larvae were taken from four other sites on the Saanich Peninsula where unsprayed apple orchards were adjacent to oak woods (see map, Fig. 4 Chapter 2). At these sites, larvae were taken from apple and oaks on the same day, and were sampled in both 1984 and 1985.
All statistical comparisons between apple and oak are one-tailed with the prediction that more eggs or greater aggregation of eggs occurs on oak. Comparisons within a tree species (e.g. number of eggs vs defoliation) are two-tailed.
Oak-leaf extract on apple trees
Hassell (1968) clearly demonstrated that a water- soluble chemical in the sap flux of damaged oak leaves acted as an oviposition cue. I designed a preliminary experiment in 1985 to determine whether the chemical "identity" of apple trees could be altered to resemble that of oak trees by the application of an oak-leaf extract. Approximately 500 g of fresh oak leaves were shredded and soaked in two litres of water for 16 hours. I filtered this extract through a 60 cloth, and sprayed approximately 600 ml on each of two apple
trees with similar densities of winter moth larvae. Six
sprays were applied over the first two weeks of Cyzenis
oviposition. At the end of this period, density of fifth-
instar larvae was estimated on 15 control trees and from the
two experimental trees. Samples of larvae were collected
from all trees, and were dissected for Cyzenis maggots.
Parasitism rates for experimental trees were compared with
those for control trees, with the prediction that rates on
experimental trees would be higher.
RESULTS
Cage experiments
Within each oviposition trial, the number of eggs
oviposited was highly variable (Table VIII). Although twice
as many eggs laid on oak leaves, this difference was not
significant owing to the high variance among cages. Apple
foliage can clearly elicit a strong oviposition response
from Cyzenis females as indicated in trial II (Table VIII).
No significant difference between oak and apple was observed
whether compared within trials or when all trials are
combined (Mann-Whitney U test).
Oviposition in the field
Egg numbers
In 1984, fewer adult Cyzenis emerged in the oak stand
than in the apple orchard (Table III, Chapter 2). On 61
Table VIII. Mean number of eggs oviposited on apple and oak foliage per female Cyzenis albicans. Numbers are means for each cage.
trial I trial II trial III
apple oak apple oak apple oak
0.0 0.0 15.0 4.8 0.0 0.0 0.0 0.0 23. 3 69.0 0.5 5.5 3.0 0.5 36.0 94.0 0.5 53.5 13.5 6.3 1 33.5 161.3 3.5 131.5 62 average, however, more eggs were oviposited on oak foliage
than on apple foliage (1984 F=6.66, df=1, P<0.001; 1985
F=9.82, df=1, P<0.001). Greater oviposition on oak was especially marked at high levels of defoliation (Fig. 12a and b). Difference in egg density was not due to a difference in surface area of oak and apple leaf clusters
(oak=125.8 cm2, ± 8.6 [S.E.]; apple=118.1 cm2, ± 5.8).
Oviposition in response to defoliation
Regression of egg number against level of defoliation among all oak clusters is positive and highly significant
(Figs. 12a and b), but was not significant among apple clusters (Figs. 12c and d, Tables IX and X). Regressions for
oak and apple are significantly different from each other in
both years (1984 F=17.4, P<0.001; 1985F=24.8, P<0.001).
When number of eggs and defoliation are regressed among clusters within each tree, the regression coefficients tend
to be positive, but few are significant (Tables IX and X).
Within oak trees, Cyzenis females appear better able to aggregate their eggs on clusters with high levels of feeding
damage than in apple trees. This is evident from the consistently steeper (although not significant) slopes for
the regressions of number of eggs vs defoliation within oaks
than for those within apple trees.
Among oak trees, Cyzenis laid most eggs in highly
defoliated trees (Fig. 12e and f). Among apple trees, eggs
were distributed randomly with respect to level of
defoliation (Fig. 12e and f). Regressions among trees differ 63
Figure 12. Regression of number of Cyzenis albicans eggs on oak and apple- leaf clusters against percent defoliation of those leaf clusters: (a) and (b) among all oak leaf clusters, (c) and (d) among all apple leaf clusters, (e) and (f) mean number of Cyzenis eggs against mean defoliation of oak trees and apple trees. Not all points are shown in (a) through (d). Regression statistics: 1984 among clusters: oak r2=0.17, P<0.001 apple r2=0.0l, ns among trees: oak r2=0.59, ns apple r2=0.l8, ns
1985 among clusters: oak r2=0.32, P<0.001 apple r2=0.lO, ns among trees: oak r2=0.93, P<0.001 apple r2=0.02, ns 50 1984 (a) 1985 .(b) oak 40 oak
30
Cyzenis 20
eggs l0
0 (c) apple (d) 20 apple
10 o 00 b = 0.03
0 20 40 60 80 100 20 4 0 60 80 100 2or . (e) (f) mean 15 Cyzenis eggs 10
'°^--o°---bo-0-05
0 20 40 60 0 20 40 60 % defoliation % defoliation 65
Table IX. Regressions of number of Cyzenis eggs on oak and apple leaf clusters as a function of level of defoliation (percent) in 1984. Apple and oak trees arranged in order of increasing level of defoliation. Dispersion of eggs among leaf clusters or among trees estimated by k from the negative binomial distribution.
tree mean regr. Cyzenis regression type N defoliation coeff K eggs Ii2) among all apple 128 34.9 -.010 .58 2.20 (9.68) clusters oak 128 26.4 .139*** .52 5.50(66.06) among clusters, within trees apple 1 6 17.6 .016 .20 1.06 (6.44) 16 20.5 . 160* .89 4.75(25.60) 1 6 30. 1 .054 1 .21 2.50 (8.53) 1 6 35.4 .065 1 . 1 1 3.50(11.06) 1 6 36.6 .009 8 .40 1.75 (2.07) 1 6 41 .3 -.016 .55 2.19 (9.26) 1 6 43.2 .007 . 14 1.00 (5.20) 1 6 55.0 .002 2 .18 1.13 (1.58)
average k=1.83
oak 16 4.3 .110 .24 0.63 (1.46) 16 10.4 .071 .39 2.13 (9.88) 16 13.1 .103 .56 2.37(11.42) 16 13.8 .297** .72 5.56(39.98) 16 31.1 .132** 1.38 4.38(19.18) 16 40.9 .124 1.09 7.00(47.04) 16 45.9 .113 1.25 16.94(221.24) 16 51.6 -.088* 2.23 4.88(14.18)
average k= .98 among apple 8 34.9 -.048 -4.88 2.20 (1.50) trees oak 8 26.4 .183 1.67 5.50(26.73)
*= P<.05, **= P<.01, ***= P<.001 66
Table X. Regression of number of Cyzenis eggs on oak and apple leaf clusters as a function of level of defoliation (percent) in 1985.
regr. Cyzenis regression type N defol, coef f. eggs Cs2) among all apple 160 12.1 .028*** 0.43 0.67 (1.58) clusters oak 128 13.9 1i2*** 0.40 2.25(15.78) among clusters, within trees apple 1 6 8.2 .039 4.18 0. 88 (1 .04) 1 6 9.7 .047 0.24 0. 63 (1 .99) 1 6 9.8 . 041 * 0.52 0. 63 (1 .04) 1 6 10.1 .063** 0.40 0. 44 (0 .80) 16 11.4 .012 0.23 0. 19 (0 .29) 1 6 14.3 .020 0.82 0. 81 (1 .74) 1 6 14.8 .011 0.46 0. 63 (1 .19) 1 6 15.8 .124*** 0.66 1 . 50 (5 .71 ) 1 6 17.1 .006 0.30 0. 50 (1 .06) 1 6 17.8 . 1 52 0.34 0. 50 (0 .94)
avg. k = 0.82
1 6 3.6 .170** 0.12 0.25 (0.59) 1 6 6.6 .040 0.36 0.75 (1.66) 16 8.3 . 129* 0.82 1.56 (4.54) 1 6 9.2 .004 1.21 1.81 (4.04) 16 10.0 .319** 0.42 1.44 (4.33) 16 13.2 .364** 0.63 2.81(33.41) 1 6 15.1 .091** 0.77 2.75 (8.47) 1 6 55.5 .090 0.73 6.63(46.92)
avg k=0.63 among trees apple 10 12.1 .016 7.35 0.67* (0.12) oak 8 13.9 .115*** 1.66 2.25 (3.90)
* P<.05, ** P<.01, *** P<.001 67 between the two tree species (1984 P=0.06; 1985 P=0.01).
Amount of feeding damage to oak leaves appears to be a
strong cue which Cyzenis females use to aggregate their
"attacks" at all levels of scale measured. Leaf damage on apple trees, however, appears not to be a cue for aggregation of eggs. The distribution of eggs in apple
foliage is random with respect to level of defoliation at all spatial scales measured (Tables IX and X)
Oviposition in response to host density
Comparison among trees of the number of Cyzenis eggs vs
density used estimates of density per leaf cluster at the
time parasitoids began ovipositing. This density was
obtained for each tree by interpolating between density
estimates for sampling dates before and after the date of
initial fly oviposition.
Host density, rather than defoliation, is a better
predictor of number of Cyzenis eggs oviposited on apple
trees, but only within individual trees (Tables XI and XII).
Host density per se does not predict the number of eggs
oviposited on oak foliage regardless of the spatial scale
used for analysis (Table XI and XII).
Among oak trees, number of eggs laid tends to be
greater at high host density, but not so on apple (Fig. 13,
Tables XI and XII). This may simply reflect the .fact that
host density itself is correlated with level of defoliation.
Although the regressions of egg number vs host density,
among trees, are not all significant, they are significantly 68
Table XI. Regressions of number of Cyzenis eggs on oak and apple leaf clusters as a function of density of fifth-instar larvae per leaf cluster (density) in 1984. Apple and oak trees arranged in order of increasing host density. Dispersion of eggs among clusters or among trees estimated by k from the negative binomial distribution.
regr. Cyzeni s regression tree N density coef f. k eggs (s2) among all apple 1 28 1.9 .62*** .58 2.20 (9.68) clusters oak 1 28 1 .4 .68 .52 5.50(66.06) among clusters, within trees 16 1 . 1 1 .44 .20 1.06 (6.44) 1 6 1 .4 1m27*** .55 2.19 (9.26) 16 1 .4 .33 1.11 3.50(11.06) 16 1 .5 -.21 2.18 1.13 (1.58) 16 1 .6 2.11* .89 4.75(25.60) 16 2.2 .91* . 1 4 1.00 (5.20) 16 2.4 .72* 1 .21 2.50 (8.53) 16 3.4 .35* 8.40 1.75 (2.07)
oak 1 6 0.4 . 1 7 1 .09 7.00(47.04) 1 6 0.6 .05 .39 2.13 (9.88) 16 0.7 4.54* .72 5.56(39.98) 1 6 0.9 .03 .24 0.63 (1.46) 1 6 1 .3 2.15 1 .25 16.94(221.24) 16 1 .9 1 .03 1 .38 4.38(19.18) 16 2.3 .90 .56 2.37(11.42) 16 2.8 .48 2.23 4.88(14.18) among trees apple 8 3.31 -.40 -4.88 2.20 (1.50) oak 8 2.7 1 .22 1 .67 5.50(26.73)
*= P<.05, **= P< .01, *** = P< .001
1 regressions among trees use density of winter moth larvae at the time Cyzenis began ovipositing, rather than density at the time parasite eggs were sampled. 69
Table XII. Regression of number of Cyzenis eggs on oak and apple leaf clusters as a function of density of fifth-instar larvae per leaf cluster (density) in 1985.
regr. Cyzenis regression tree N density coeff. k eggs (s2)
among all apple 1 60 0.64 0.44 0.43 0.67 (1 .58) clusters oak 1 28 0. 18 1 .99 0.40 2.25( 15.78)
among clusters, within trees apple 16 0.31 0.53 0.52 0.63 (1.04) 16 0.38 -0.73 0.46 0.63 (1.19) 1 6 0.44 -0.30 0.46 0.63 (1.99) 1 6 0.56 -0.25 0.30 0.50 (1.06) 1 6 0.56 0.85* 0.40 0.44 (0.80) 1 6 0.63 0.19 4.18 0.88 (1.04) 16 0.63 -0.33 0.23 0.19 (0.29) 16 0.88 0.46* 0.34 0.50 (0.94) 16 0.88 2. 18*** 0.66 1 .50 (5.71 ) 16 1.13 0.27 0.82 0.81 (1.74)
oak 16 0.00 -(a) 0.42 1 .44 (4.93) 1 6 0.00 - 0.77 2.75 (2.87) 16 0.00 - 0.12 0.25 (0.59) 16 0.00 0.36 0.75 (1.66) 16 0.06 2.3- 3 1.21 1.81 (4.04) 16 0.25 1 .53* 0.82 1 .56 (4.54) 16 0.25 5.92 0.63 2.81(33.41) 1 6 1 .38 -1 .65 0.73 6.63(46.92)
among trees apple 1 0 0.621 0.34 7.35 0.67 (0.12) oak 8 0.50 4.29* 1 .66 2.25 (3.90)
* P<.05, ** P<.01, *** P<.001
(a) samples on oaks were taken after most larvae had dropped to to pupate, so regressions within these trees could not be calculated.
1 regressions among trees use density of winter moth larvae at at the time Cyzenis began ovipositing, rather than density at the time parasite eggs were sampled. 70
Figure 13. Regression of mean number of Cyzenis albicans eggs per leaf cluster against mean number of fifth instar winter moth larvae per cluster for apple (O) and oak (•) trees in 1984 and 1985. Regression statisitics: 1984 oak r2=0.l9, ns apple r2=0.28, ns
1985 oak r2=0.63, P<0.001 apple r2=0.07, ns
72 different between tree species (1984, P=0.045; 1985,
P<0.01). Pattern of Cyzenis oviposition in response to host density, therefore, closely parallels the relation between oviposition and level of defoliation.
The slope of the regression of number of parasitoid eggs vs host density on oaks was far steeper at low mean host density (1985) than at high host density (1984)
(Fig. 14). No tendency for increase in slope was apparent among years in the apple orchard (Fig. 14). Slopes are different between habitats at low mean density (1985,
F=7.41, P=0.008, one-tailed), but are not different in years of high mean host density (Fig. 14, 1984, F=1.83, P=0.10, one-tailed). If aggregative behaviour improves the degree of control, its greatest effect at low mean host density.
Statistical aggregation of Cyzenis eggs
Aggregation on foliage
Data on oviposition in the field were gathered in 1984 and 1985 only. At all levels of scale, eggs were more clumped (smaller k) on oak foliage than apple foliage
(Tables IX and X). Clumping of eggs among leaf clusters was especially strong within oak trees that had low overall feeding damage, while distributions were more random (larger k) in oaks with greater total feeding damage (Table X,
Spearman's rank correlation: 1984, r =0.93, P<0.01; 1985, r =0.41, ns). No such trend is apparent among apple trees
(1984, r =0.17, ns; 985, r =-0.20, ns), which suggests again 73
Figure 14. Slope of the regression between number of Cyzenis eggs and host density on oak (•) and apple (O) trees as a function of mean host density at the time Cyzenis began ovipositing. Standard errors of slopes are indicated. Cyzenis eggs were not sampled in 1983; fly numbers were low and slope of the number attacked vs number per cluster was used with the assumption that they reflected the slope of the number of eggs vs number of host larvae. 74
slop
0 1 2 3
l_5 per leaf cluster 75 that Cyzenis does not respond to differing levels of feeding damage on apple foliage. Within apple trees, host number affected the number of eggs oviposited in 1984, but the degree of clumping of "attacks" within trees did not increase with reduction in mean host density (1984, r =0.41, ns; 1985, r =0.18, ns). Little relation existed between number of larvae on each cluster and number of Cyzenis in
1985.
Aggregation of "attacks" among hosts
At all sites where both oak and apple occur (and where parasitism rates were greater than ten percent), the distribution of fly maggots was more clumped (smaller k_) among caterpillars from apple than among those from oaks.
This relationship held for the same site in different years
(Table XIII) and among sites in the same year (Table XIV).
No measure of distribution of Cyzenis eggs in foliage was made at sites 1-4, but they are assumed to be aggregated on oaks with feeding damage and random among apple trees. The suggestion that the statistical aggregation of attacks among hosts reflects the aggregation of parasitoids among patches of high host density (May 1978, Hassell 1980), is not supported. The apparent paradox could result if flies move randomly among apple trees, but lay eggs in clumps on leaf clusters (Kobayashi 1966, Iwao and Kuno 1971), or because the negative binomial distribution can arise more naturally if parasitoids aggregate independently of host density
(Reeve and Murdoch 1985). 76
Table XIII. Percent parasitism (%) and distribution of Cyzenis attacks among fifth instar host larvae from all apple trees at University of Victoria Experimental Farm, and oak trees at Mt. Tolmie Park among years.
# Cyzenis maggots tree year spec ies N 0 1 2 3 4 5 6 >6 %par k
1 982 apple - - - oak 99 0 -
1 983 apple 3274 2750 466 50 6 2 0 0 0 2.20 oak 1 587 1 1 92 298 72 21 4 0 0 0 25 1.10
1 984 apple 1 690 570 578 309 1 34 49 34 8 8 66 3.50 oak 1 067 357 364 215 85 29 1 0 6 1 67 6.77
1985 apple 1 1 40 523 331 1 72 67 25 1 3 6 3 54*** 1 .91 oak 410 65 89 88 84 43 21 10 1 1 84 6.31
*** P<0.001, X2 test for difference in parasitism between apple and oak. 77
Table XIV. Parasitism rates (%) and clumping (k) of attacks among fifth-instar winter moth larvae on apple, and oak at five sites on southern Vancouver Island. Site numbers refer to Figure 1, Chapter two. Site 5a are trees at site 5 on which Cyzenis eggs were counted. mean attack site tree N % k per host(s2 1 984 1 apple 191 1 - 0.01(0.01 oak 161 0 - 0.00
2 apple 98 1 0 5.77 0.13 (0.14 oak 1 1 6 4 .14 0.05 (0.07
3 apple 1 1 3 5g*** 3.00 0.97 ( 1 .44 oak 65 20 5.30 0.23 (0.24
4 apple 1 1 4 gg*** 4.82 1 .35 (1.71 oak 1 1 5 37 a 0.48 (0.48
5 apple 1690 66 3.32 1.21 (1.71 oak 1067 67 6.54 1.18 ( 1 .40
5a apple 422 65 2.89 1.21 ( 1 .78 oak 500 63 4.54 1.11 ( 1 .38
1 apple 1 06 1 3 a 0.14 (0.14 oak 28 7 a 0.07 (0.07
2 apple 1 24 19 -3.37 0.21 (0.20 oak 55 20 -3.77 0.22 (0.21
3 apple 1 06 36* 6.32 0.46 (0.50 oak 1 7 65 7.19 1.12 (1 .36
4 apple 97 52** 1 .84 0.98 ( 1 .37 oak 47 79 2.86 2.21 (3.65
5 apple 1 1 40 54*** 1 .91 0.96 (1 .45 oak 410 84 6.31 2.28 (3.06
5a apple 685 g2*** 1 .63 0.93 (1 .43 oak 343 85 6.78 2.28 (3.02
a - no suitable estimate of k, s2=x, ie. random
* P<0.05, ** P<0.01, *** P<0.001, X2 test for difference in parasitism between apple and oak. 78
Among trees, the degree of clumping among hosts
generally does not increase when clumping in foliage
increases. (Spearman's rank correlation: oak 1984 r =-0.05
ns, 1985 r =0.28 ns; apple 1984 r =1.00 P<0.01; 1985 r =0.30
ns).
Host mortality from parasitism
Percent parasitism vs defoliation
Rates of parasitism in 1983 reflected the pattern of
Cyzenis oviposition, with increased levels on oaks at high
level of defoliation, and reduced levels on apple at high
defoliaton (Fig. 15a, Table XV). In other years, there was
no relationship between level of defoliation and mortality
from parasitism (Figs. 15b and c, Table XV).
Percent parasitism vs host density
In 1983, those trees with the highest defoliation
(Fig. 15a), had the lowest density of fifth-instar larvae
(Fig. 16a). This resulted from severe defoliation of some
trees to the extent that losses of larvae (most likely due
to starvation) over-compensated for differences among trees
in the initial larval density (see Chapter 2, Fig. 10a).
Trees with high densities of first-instar larvae were almost
totally defoliated, resulting in low densities of fifth-
instars. Parasitism was closely correlated with density of
fifth-instar larvae in 1983, both on oaks and on apple trees 79
Figure 15. Percent parasitism of winter moth pupae as a function of percent defoliation of apple (O) trees and oak trees (#).'in 1983 through 1985. (a) • oak 100 1983 o apple 80
60
40 o»->o--o . • "V<£DQ O 20 o * 0<3*-o
(b) 100 1984 80
60 • o o o o o % o parasitism 40 o
20 -
100 (c) 1985 80
60
40 <8>
20
0 20 40 60 80 100 % defoliation 81
Table XV. Regression statistics for percent parasitism of collected pupae as a function of defoliation (percent) and host density (fifth instar larvae per leaf cluster) among trees.
parasitism vs defoliation parasitism vs density year coef f. r2 coef f. r2
1983 apple -0.44 4 1 *** 10.5 .69*** oak 0.41 .46* 13.7 .56**
1 984 apple 0.23 .07 1 .6 .03 oak 0.34 .19 -4.7 .09
1985 apple 1.01 .09 10.5 .04 oak 0.05 .002 9.0 .04
*=P<.10, **=P<.05, ***=P<.01 82
Figure 16. Percent parasitism measured from collections of winter moth pupae, as a function of mean number of fifth instar winter moth larvae per leaf cluster for apple (O) and oak (•) trees in 1983 through 1985. (b) 100 1984 80 • • • ao o
o % 60 o o *o parasitism4 0
20
_l L (c) 100 1985 80
QO 60 °1
40
20
0 12 3 4 5 6 7
L5 per leaf cluster 84
(Fig. 16a, Table XV). The lack of any relation between percent parasitism and host density in the latter two years is probably due to the very high density of Cyzenis flies
(Table IX). High density of flies (and fly eggs) would ensure that all winter moths at risk to parasitism are parasitized. The fact that percent parasitism on apple in
1983 is negatively correlated with defoliation (Fig. 15), but positively correlated with host density (Fig. 16) supports the contention that on apple, Cyzenis cue on host presence rather than level of leaf damage. When Cyzenis is at low density (1983) these behaviours are reflected in patterns of parasitism.
Despite large differences in number and distribution of
Cyzenis eggs in apple and oak foliage, there was little or no effect on total host mortality resulting from parasitism
in 1984. Estimates of parasitism from samples of winter moth larvae were slightly higher on oak in 1983. The more accurate estimates from pupae in that year were identical for apple and oak (Table XVI). In 1984, slightly higher parasitism rates occurred on oaks than on apple, but were virtually identical on those oak and apple trees where
Cyzenis eggs had been counted. In 1985, winter moth densities were much lower than in 1984, but Cyzenis density was still high. For each tree species, parasitism levels did not differ among trees with different amounts of defoliation
(Fig. 15a) or density of host larvae (Fig. 16a). Parasitism rate did differ, however, between populations on oak and apple. This suggests that Cyzenis could not locate hosts Table XVI. Total percent parasitism (%) estimated from dissection of fifth instar winter moth larvae and from collections of pupae in pupation trays for apple and oak trees in 1983 through 1985.
estimated estimated from larvae f rom pupae 1 983 % N % N
all apple 1 6** 3277 28 6742 trees oak 25 1 583 28 751
1 984
all apple 66 1711 65** 790 trees oak 67 1066 74 418
trees from which egg distributions apple 65 422 72 189 were estimated oak 60 500 74 88
1985
all trees apple 54*** 1 1 40 52*** 646 oak 84 410. 73 1 42
trees from which egg distributions apple 52*** 685 58** 260 were estimated oak 85 343 75 120
X2 test, * P<0.05, ** P<0.01. *** P<0.001 86 efficiently at low host density on apple trees.
The efficiency of.many parasitoids, estimated as the
"area of discovery" (Varley et al. 1973), tends to decrease with increase in parasitoid abundance (Hassell 1978). At low mean parasitoid density, aggregation in response to host density would result in greater number of hosts killed per parasitoid (higher efficiency) than if they attack randomly.
At high parasitoid abundance, there would be greater
interference among parasitoids which aggregate, and so they would be less efficient than if they searched at random
(Fig. 17a). I have calculated a for Cyzenis on apple and oak
trees in 1984 and 1985. Parasitoid abundance for each tree was calculated from the mean density emerging in each habitat, and weighting this value by the proportion of
Cyzenis eggs observed in each tree (this assumes equal mortality and net movement in or out of both habitats
between the time flies emerge and the time they oviposit).
Area of discovery is simply calculated as (2.3*k2)/Pt, where
k2 is mortality due to parasitism expressed as k-values, Pt
is Cyzenis density per m2, and 2.3 is the constant
converting base 10 logarithms to natural logarithms.
Parasitoid efficiency was higher on oaks than on apple at
low parasitoid' density, but the same on the two tree species
at high parasitoid density (Fig. 17b).
Oak-leaf extract on apple trees
Among control (unsprayed) trees, there was a slight
positive relation between larval density and percent 87
Figure 17. (a) Theoretical efficiency of randomly attacking parasitoids (——) and parasitoids aggregating in areas of high host density (-»") as a function of parasitoid density (from Hassell 1978, p. 99), (b) observed efficiency "a" of Cyzenis as a function of parasitoid density for apple trees ( O ) and oak trees (#) in 1984 and 1985. Means for each year are also indicated (apple , oak ). (apple a = 0.31*Pt" 6 " , r2 = .67; oak a= 1 . 9 1 *Pt1 1 , r2=.89, slopes are significantly different: F=8.64, df=1, P=0.006) (a)
N N \ \ \
(b) 89 parasitism of winter moth larvae (Fig. 18). Winter moth on the two experimental trees that had been sprayed with oak- leaf extract had significantly higher levels of parasitism than those from control trees with similar host density.
Parasitism was increased by as much as 50 percent by the application of this crude plant extract. Resultant levels of parasitism were similar to those of winter moth populations
from oaks (Table XVI).
DISCUSSION
Parasitoid behaviour
Host-plant characteristics have been shown to alter host-parasitoid interactions through a variety of mechanisms
(for a review see Price et §_1. 1980). These can be indirect
through the plant's effect on herbivore growth rate or
survival (Lawton and McNeill 1979), or direct by promoting
or hindering parasitoid attack (for example the effect of plant phenology on host insect phenology, Chapter 2).
Although Cyzenis females use leaf damage as a cue for
selection of oviposition sites on oaks, they do not cue on
apple foliage. The chemical involved in site selection on
oaks, as suggested by the work of Hassell (1968), appears to
be absent from apple foliage. Holliday (1975), studied
winter moth populations in apple orchards in Britain, but
recovered only one tachinid fly from his collection of 1396
host larvae, which suggests there is little oviposition by
Cyzenis on apple foliage. 90
Figure 18. Percent parasitism (logit scale) of fifth instar larvae on control trees (O) and on trees sprayed with oak leaf-extracts (#). Ninety-five percent prediction limits one-tailed) for the regression among controls are indicated Sample size for each tree is 50-103 larvae. 91 92
The statistical distribution of attacks among winter moth larvae has previously been used as a mathematical embodiment of Cyzenis search behaviour (May 1978, Hassell
1980). Statistical clumping of parasites among hosts was believed to reflect spatial aggregation of parasitoids that caused some hosts to be more susceptible to parasitism than others. Although mathematically tractable, it does not appear to provide a consistent reflection of the pattern of oviposition observed in this study. This supports the observation of Reeve and Murdoch (1985) that the negative binomial distribution of parasites among hosts arises most naturally when parasites aggregate independently of host density (larger zero-class of attacks when there is stronger aggregation). Models which explicitly distribute parasitoids among patches of host larvae may be more realistic
(e.g. Hassell and May 1974). Such models have not been used
for Cyzenis (Hassell 1980) because no data on the pattern of
fly search have previously been recorded. If the pattern of
Cyzenis oviposition is assumed to reflect search behaviour, then my data could be used to describe relationships in these more explicit models.
Host mortality
In this chapter, I have tested whether the failure of
Cyzenis to reduce winter moth populations on apple trees in
Nova Scotia could have resulted from the reduction of the number or change in distribution of parasitoid eggs on the 93 apple trees as compared with oak trees. I assume that the behaviour of ovipositing Cyzenis on oak and apple in Nova
Scotia is the same as that observed in British Columbia. Two
results from my study suggest why Cyzenis albicans might be better able to control winter moth on oak than on apple:
firstly, in the field, more eggs were observed on oak
foliage than on apple foliage (Fig. 12) and secondly,
Cyzenis eggs were more clumped on oak foliage with high
feeding damage at all spatial scales measured (Table IX).
Both of these may reflect the searching behaviour of
females. On theoretical grounds both would be considered characteristics which promote successful biological control.
Aggregated attack would improve control by promoting greater
stability at the new, low equilibrium (Hassell & May
1973,1974; Beddington, Free & Lawton 1978; May 1978; Hassell
1980), and by increasing efficiency of searching parasitoids
(Hassell 1978, Commins and Hassell 1979), or by both of
these mechanisms.
Successful biological control can occur without spatial density-dependence (e.g. Murdoch et a_l. 1984, Reeve and
Murdoch 1985). Many field examples of host-parasitoid
interaction show no or even frequently density-dependence
(Morrison & Strong 1980). Spatially density-dependent parasitism has been observed for parasitoids involved in
successful biological control attempts (Hassell 1982; Heads
& Lawton 1983), but whether density-dependence actually contributes to success is unclear. Greater mortality of hosts can occur when searching parasitoids aggregate in 94 patches of high host density compared with when they do not
(e.g. Burnett 1958). In this study, I have shown that different spatial distributions of "attack" by Cyzenis on host populations has little effect on host population mortality, at least at high host and parasitoid density.
Fewer Cyzenis eggs were observed on apple leaf clusters than on oak, yet the resultant parasitism levels were the same in two of the three year (1983 and 1984). This paradox could occur for one of at least three reasons:
(i) caterpillars eat more apple foliage than oak foliage. In my study, amounts of foliage eaten were not measured directly, but pupae from apple trees were 5 to 10 percent heavier than those from oak at high density, but not different at low density (Chapter 2). Wint (1983) also found that pupae of larvae which fed on late-spring apple foliage were significantly heavier (15%) than those which had fed on oaks. He also found that the duration of feeding through the season was the same on the two tree species. It is unlikely that the small difference in amount of foliage eaten could make up for the 2.5-fold difference in number of Cyzenis eggs oviposited.
(ii) Clumping of eggs on oak could result in greater incidence of superparasitism. Although several C. albicans eggs can be ingested by a single host, only one will successfully develop to maturity. As a result, many Cyzenis
"attacks" do not contribute to host mortality. The proportion of all attacks which result in superparasitism would be expected to be greatest when parasitoids show 95 aggregative behaviour (on oaks) since flies will continue to
lay eggs on the leaves with greatest defoliation. From the dissection of winter moth larvae, the percent of the Cyzenis maggots which contributed to superparasitism, rather than to host mortality, could be calculated. The proportion of all
Cyzenis maggots which occur as redundant attacks increases
linearly with percent parasitism for both oak and apple
(Fig. 19) and shows no difference in either slope (F=1.13,
P=0.29) or adjusted mean (F=0.14, P=0.71). On oaks,
therefore, there is no evidence of greater interference among Cyzenis through increased competition within host
larvae and cannot explain why parasitism level is the same despite differing number of parasite eggs in foliage,
(iii) Cyzenis eggs were sampled earlier on apple than on oak
for reasons discussed above. Additional oviposition could have occurred in the orchard after sampling and increased
the rate of parasitism among larvae which had not yet pupated. At the other study sites, however, there was no difference in sampling date, and the magnitude of parasitism on apple and oak still reflected that of the primary study
sites.
In Britain, density of emerging Cyzenis was typically
less than 1/m2 (Varley, Gradwell & Hassell 1973). At my primary study sites, Cyzenis was estimated at 5-45/m2 in
1984 and 1985. At these densities, the distribution of parasitoid eggs in relation to host abundance may make
little or no difference to total host mortality. Certain proportions of the host population may be at risk to 96
Figure 19. Regression of percent of all attacks by Cyzenis albicans which did not contribute to host mortality (superparasitism), against percent parasitism measured from dissections of fifth instar larvae from apple (O) and oak (•). Sample size for each point was approximately 200 in 1983, and 50-103 in 1984 and 1985. 1.0
0.8
arcsin superparasitism 0.4
0.5 1.0 1.5 arcsin parasitism 98 parasitism, and the massive numbers of eggs on all foliage ensures that they are parasitized. Cyzenis densities were not estimated in Nova Scotia, but mean number of parasite eggs in foliage ranged from 0.2 to 6.4 per leaf cluster among sites (Embree & Sisojevic 1965), and resulted in
levels of parasitism of 72 to 78 percent at one site (Embree
1965). These results closely resemble those observed in
British Columbia.
The outcome of the introduction of Cyzenis in British
Columbia is not yet apparent, although moth densities and
levels of defoliation have declined more on oaks than on apple trees (Fig. 1, Chapter 1). Four possible scenarios ex i st:
(1) Reduction of winter moth on oak but not on apple (as in
Nova Scotia). The difference in reduction is correlated with
the difference in ability to aggregate attacks. If reduced densities are stable on oak, it might be attributed to
strong spatial aggregation;
(2) No reduction on either oak or apple; clearly, the
observed aggregation by parasitoids does not promote
successful control;
(3) Reduction in both oak and apple; aggregation has little effect on reduction since it is absent from apple;
(4) Reduction on apple but not oak (opposite to what
occurred in Nova Scotia). This would be the least probable
outcome if aggregation were an important element for
successful control.
The lack of success by Cyzenis albicans, in at least 99
the initial reduction of winter moth populations on apple
trees, is not due to an inability to inflict high rates of parasitism. Two features suggest why winter moth populations might remain at low density in oak woods following initial
suppression: (1) stronger aggregation by Cyzenis among oak
trees than among apple trees, and (2) greater parasitoid
efficiency on oaks within trees by aggregating at host
feeding damage. When winter moth density was greater than 1
per cluster, parasitism was similar in oak and apple
populations. Many Cyzenis eggs were laid on each apple leaf
cluster, and so most larvae became parasitized. When density
dropped below one caterpillar per leaf cluster, parasitism
was reduced on apple, presumably because the absence of a
chemical cue resulted in many wasted eggs. Holliday (1977)
found no parasitism by Cyzenis in apple orchards where
density was about 0.25 larvae per leaf cluster. At this
relatively low density, in the absence of chemical cues,
hosts might easily escape parasitism.
Levels of parasitism by Cyzenis still vary greatly
among years and sites. Despite this variation, patterns of
parasitism of winter moth on oak and apple are consistent,
with rates on apple equal to or higher than on oaks at high
mean host density. At low mean host density, aggregation in
response to host density (observed on oaks) is associated
with higher levels of parasitism. Whether the observed
pattern of oviposition and magnitude of parasitism can
account for the decline of winter moth populations is
addressed in Chapter 6. 100
CHAPTER 4
MANIPULATION OF PREDATORS:
EFFECT ON HOST AND PARASITOID PUPAL SURVIVAL
INTRODUCTION
From previous long-term studies of winter moth populations (Varley, Gradwell & Hassell 1973, Embree 1965),
there emerged a paradox about whether parasitoids regulate winter moth density. The phenomenal reduction of winter moth
populations which occurred in Nova Scotia over a two-year
period (Embree 1965, 1966) was attributed to parasitism by
Cyzenis albicans, which had been introduced five to six
years prior to the decline. This large impact of Cyzenis on
winter moth populations was surprising since, in Britain,
level of parasitism by Cyzenis rarely exceeded 10% and
showed no relationship with host density among years
(Varley, Gradwell and Hassell 1973). In Nova Scotia,
however, parasitism reached 61% at the principal study site
(Oak Hill) the year prior to the crash, but more typically
averaged 30-60% among sites (Embree 1965).
Using life-table data from these two studies, Hassell
(1980) examined features which differed between the two
populations and might have affected the host-parasitoid
interaction. The principal difference between the two
populations was the presence in Britain of significant
density-dependent mortality of winter moth pupae in the 101 soil. Although mortality of pupae in the soil was the only observed regulating factor in Britain, it was much lower in
Nova Scotia, and was not density-dependent among years.
Because both parasitized and unparasitized larvae pupate in the soil, this mortality affected not only winter moth, but also Cyzenis. Mortality was greater on Cyzenis because it spends ten months in the soil compared with only five for its host. The magnitude of this mortality, and the fact that it was density-dependent, was thought to prevent build-up of Cyzenis to the level at which it would be an important regulating factor (Hassell 1980). In the absence of density-dependent soil mortality in Nova Scotia, Cyzenis could build up over a few years to very high densities, and so drive winter moth populations down.
Various agents have been considered as the cause of soil mortality. Principal among these is predation by carabid and staphylinid beetles (Varley and Gradwell 1963a,
1963b; Frank 1967a; East 1974; Kowalski 1976, 1977).
Predation by small mammals also occurs, but its impact is small (Frank 1967b). Despite the number of studies which implicate beetle predation in winter moth population regulation, no one has previously tried an experimental removal of beetles.
If Hassell's (1980) explanation of the paradox in the role of C. albicans in winter moth population regulation is correct, we should be able to predict the degree of success which Cyzenis would have in British Columbia by measuring the magnitude and degree of density-dependence of soil 102 mortality.
In this chapter, I examine the pattern of mortality in the soil of both winter moth and Cyzenis in British Columbia with two objectives: (1) to determine the contribution which predaceous beetles make to winter moth and Cyzenis mortality in the soil, and (2) to test the predictions regarding the likely success of Cyzenis as a controlling factor in the decline of winter moth populations in British Columbia. If winter moth pupal mortality in the soil in British Columbia is large and density-dependent, then two predictions can be made: (1) Cyzenis will not build up to densities capable of causing 70 to 80 percent mortality, and (2) reduction of predatory beetle abundance will result in a large increase in the parasitoid relative to its host.
METHODS
Both the University of Victoria apple orchard and the native oak stand at Mt. Tolmie Park were used in this study.
Only observational studies of soil mortality were conducted at Mt. Tolmie; observational and experimental studies were conducted in the orchard because of its more homogeneous nature.
Manipulation and measurement of beetle abundance
Experimental design, 1983-84
Beetle abundance was manipulated from April 1983 103 through April 1985. The initial design (1983-84) consisted of two grids of 16 apple trees enclosed by "beetle-proof" barriers (Fig. 20). Barriers consisted of 15 cm-high plastic
"turf-edge" fence sunk approximately 7.5 cm into the ground.
This provided a barrier to above-ground and below-ground movement of beetles, but would not prevent beetle flight. On each side of this fence, a 5-cm strip was kept clear of vegetation by clipping and by the application of a soil sterilant (Later's Hybor-D granular sterilant; 91.0% borax,
2.0% boracil [10.2% boron equivalent], and 5.0% 2,4-D acid equivalent).
Within each of the two plots, beetles were caught in
10-cm diameter pitfall traps set under trees (four per tree) and along the inner fence margin (16 traps per plot) for a total of 80 traps per plot. Traps were protected from rain and sun by a small board placed above each. Traps were checked weekly, and beetles removed from one plot (removal plot), and added to the other (addition plot). On several occasions, the efficacy of the beetle fences was tested by marking beetles released into the addition plot. Marking and recapture also permitted me to estimate absolute beetle abundance with a Petersen estimate (Caughley 1980, p. 141)
Experimental design 1984-85
Experimental design was altered in 1984. The area under each of 33 trees was split in two (Fig. 21), with a beetle removal plot being established in one half, and a control 1 04
Figure 20. Experimental design for the manipulation of beetle abundance in the apple orchard, 1983-84.
\ 105
trunk crown beetle fence 5 m
beetle addition beetle removal 1 06 plot on the other. The side of the tree (east or west) which had the removal plot was randomly assigned among trees within each of the four rows of the experimental grid
(Fig. 21) to eliminate possible bias of directional effects of prevailing wind on the numbers of larvae dropping to pupate under each tree. Each of the 33 removal plots (1m X
0.5m) was enclosed by a beetle-fence. A Diazinon-drench, applied four weeks prior to winter moth larval drop, ensured removal of beetle adults and larvae from the removal plot of each tree. Formulation was 30 ml Greenleaf 12.5% diazinon per 5 litres water, sprayed at a rate of approximately 500 ml/m2. Pitfall traps were set in both removal and control plots (two per plot) for a total of 66 pitfalls per treatment.
Beetle trapping in the oakwood
Because the orchard is a somewhat artificial environment, beetle abundance and host and parasitoid mortality were also studied in a natural oakwood (Mt. Tolmie
Park). Beetles were trapped in the oak stand only in 1984 and 1985. Trapping was more difficult in this habitat because of the rocky substrate. Pitfall traps could not always be set under specific trees, and so were set where soil conditions would permit. A total of 26 traps were set in a 20 X 20 m area adjacent to five of the trees on which winter moth populations were monitored. 1 07
Figure 21. Experimental design for the manipulation of beetle abundance in the apple orchard, 1984-85. 108 1 09
Host and parasitoid abundance and survival
Both parasitized and unparasitized winter moth larvae drop to the ground to pupate. The number of larvae dropping per m2 of tree crown area was measured by providing one
0.415 m2 peat-filled tray under each apple tree, and one
0.168 m2 tray under each oak. Larvae readily spin cocoons and pupate in these. Trays were checked weekly during the period of larval drop. Cocoons were later opened and pupae scored for parasitism. Therefore, the number of healthy and parasitized pupae per m2 was determined for each tree in each year.
The natural range of pupal density was used in 1983 and
1985. In 1984, however, densities of pupae were manipulated to provide a wide range of densities among plots. Prepupae were removed from foliage in 16 trees and forced to pupate,
in equal numbers in the two plots (beetle removal and control) under each tree; 200 per plot were added under 8 trees, and 50 per plot were added under 8 trees. These were
in addition to the natural density of prepupae dropping per m2.
To estimate mortality of pupae in the soil I needed to
know density of adult moths and adult flies emerging in the
fall and spring respectively. Two methods were used to estimate density of adult moths in fall. (1) In 1983 and
1985 the total number of flightless females climbing a tree was measured using funnel stocking-traps similar to those
used by Varley and Gradwell (1963a). Two or three traps were
used to encircle the trunk completely. Some females were 110 able to bypass funnel traps. Number of females in stocking traps was adjusted by the known trap bias (Appendix B) to give total number of females ascending each tree. The number of females was multiplied by two (assuming an equal sex ratio) and divided by the area of the tree crown to give the density of winter moth adults per m2.
(2) In 1984, the number of female and male moths emerging per m2 of soil was measured directly for 33 trees by the use of cone-shaped screen emergence traps covering 0.25 m2 each.
One cone trap was provided per plot. Stocking traps would not permit an estimation of the effect of beetles on moth density, since female moths from both treatments would crawl up each tree. Seventeen trees (each alternate tree in the grid) also had traps on the trunks to measure number of
females ascending. Density of females was calculated as the number of females per m2 of tree crown area. To estimate total mortality for the entire tree, I used the initial density based on natural density plus added prepupae, averaged over the tree crown area. In the oakwood, traps on
tree trunks were used in all three years. In 1984, cone emergence traps were also used to estimate number of adult moths per m2 of soil below the trees.
Density of flies emerging in spring was measured with
two 0.25 m2 cone-shaped emergence traps under each tree.
Traps were checked daily during the one-month period of fly emergence.
Mortality of each life stage was expressed as k-values.
The four intervals over which mortality was calculated are: 111
(1) k2, mortality due to parasitism by Cyzenis estimated from pupae in the pupation trays.
(2) k5, soil mortality from pupation of unparasitized larvae
(in pupation trays) to the time females reach a tree to oviposit (measured in traps on tree trunks). This is the same as k5 of Varley and Gradwell (1963), and kL+kP+kA of
East (1974).
(3) k5a, soil mortality from pupation of unparasitized larvae (in pupation trays) to emergence from the soil of both female and male winter moth (measured in cone emergence traps). This is the same as kL+kP of East (1974).
(4) kcyz, soil mortality of Cyzenis albicans from pupation of parasitized winter moth (estimated from pupation trays), to emergence of female and male adult flies (measured in cone emergence traps).
Effect of diazinon
Diazinon used to remove beetles in 1984 may have had a residual effect on winter moth and Cyzenis survival despite being applied four weeks prior to larval drop. If this were the case, the number of moths or parasites surviving in the absence of beetles might be lowered owing to diazinon alone.
To test for this, an additional four trees with two plots under each were established in the orchard with diazinon sprayed on one half, but no fence excluding beetles
(Fig. 22). A two-metre strip of fencing between the two plots under each tree controlled for any effect which fences had, in the main experiment, particularly in preventing 1 1 2
Figure 22. Experimental design for the effect of diazinon alone on beetle abundance and on winter moth and Cyzenis mortali ty. •tree crown sprayed beetle fence unsprayed V. ' tree trunk 1 1 4 leaching of diazinon into the adjacent control plots. Since beetles were not prevented from re-invading these "removal" plots, the effect of diazinon alone on host and parasitoid survival could be tested by comparing "sprayed + beetle" plots with the "beetle" plots. Beetle abundance was estimated in these plots in the same manner as described above.
Fate of pupae
These experiments were designed to determine whether predation by beetles was independent of other sources of soil mortality. Experiments on host and parasitoid survival in the presence or absence of beetles were complemented by an examination of the specific fate of individual pupae in the soil. These studies examined: (1) the effect of pupal density on specific types of pupal mortality, and (2) the effect of beetle presence/absence on specific types of pupal mortality to determine if they were independent of predation.
Forced pupation
Five replicate sets of fifth-instar larvae were forced to pupate at three densities. Larvae were " placed in clear plastic cages (12 X 17 cm), open to the ground, at densities of 10, 30 and 60 per cage. Under each of three trees, five replicate cages for each density (one density per tree) were set immediately adjacent to each other to enhance the density effect. Once all larvae had entered the soil to 1 1 5 pupate (3-4 days), metal pins were set at the corners of each cage to allow relocation of each set when they were dug up on a later date.
One set of pupae for each density was dug up at each of
five time intervals. Intervals were: 4, 8, 13, 26 and 45 weeks after pupation. Winter moth adults emerge between week
26 and 30, Cyzenis adults emerge after week 45. Soil samples
(8-10 cm deep, see Frank 1967) were carefully searched, and
the depth of each pupa was recorded as was its fate (or where possible combined fates).
Fate categories were: healthy or emerged, parasitized, virus, fungus, desiccated, failed pupation, nematodes,
shredded, torn, holed, and missing from the cocoon. In addition, the difference between number of cocoons recovered and number of larvae forced to pupate was assigned as missing. Recovery of all pupal remains is unlikely, so the number of pupae lost through predation is an over-estimate.
The last four categories were later combined into a
"depredated" category.
Presence/absence of beetles
To test for independence of predation from other
sources of mortality, sod samples were dug up from beetle exclusion and control plots in 1984-85 from each of two
trees at week 45. Pupae were assigned to fate categories as above. Frequency of each fate category in the presence and absence of beetles was compared with a 2 X N contingency
test. 1 1 6
Predation cage experiments
Cage experiments were designed to test whether mortality from beetle predation is conditional on other fates which pupae suffer (e.g. parasitism and pathogens).
Two tests were made: (1) differential predation on parasitized and unparasitized pupae, and (2) differential predation on virus-infected and uninfected pupae.
Preliminary trials showed that two carabid species:
Pterostichus melanarius and Carabus nemoralis survived best in cages, and were facile at opening pupae. These two species were used in all cage experiments. Five parasitized and five unparasitized winter moth cocoons were placed in a cage 35 X 25 cm with ten P. melanarius and/or ten
C. nemoralis. Cocoons had been opened at one end to check for parasitism and then closed and buried approximately 0.5 cm below the soil surface. Losses of each type of pupa were scored daily for one week. Missing pupae were replaced by new pupae of the appropriate type (parasitized or unparasitized).
The second experiment was of the same design as above, except that pupae were either virus-infected or uninfected.
Infected pupae were recognized as being darker in colour, showed no movement, and were soft and pliable. Although not conclusively virus-infected, these pupae were likely infected by some pathogen; similar pupae filled with milky fluid contained virus inclusion bodies (I.S. Otvos, Pacific
Forest Research Centre, Victoria). 1 1 7
RESULTS
Beetle abundance
Total numbers
Predatory beetles were dominated by five species. These
included three carabids (Pterostichus melanarius Illig.,
Carabus nemoralis Mull., Calathus fuscipes Goeze), and two
staphylinids: (Staphylinus aeneocephalus DeGeer and Ocypus melanarius Heer). All are introduced from Europe (G.E. Ball,
A. Smetana pers. comm.)
Beetle numbers were consistently higher in beetle- addition plots than in removal plots (Fig. 23). The
experimental design for the period April 1984 to June 1985 provided a greater disparity in density of all beetles (10
to 100-fold) than did the design from April 1983 to April
1984 (2 to 3-fold). The biggest difference in abundance was
observed for adults of the three carabid species (10 to 100-
fold), less so for staphylinids (10 to 30-fold) and least
for beetle larvae (3-fold). In 1983, there was little
difference in abundance of staphylinids or larvae between
experimental and control plots; in 1984 and 1985 their
abundances differed markedly between treatments. Beetle
larvae were most abundant during the period when neither
host nor parasite were in the soil. Marked beetles released
in the beetle-addition plot remained within that plot; only
31 of 385 recaptures (8%) were in the removal plot. Absolute
densities from mark-recapture estimates were very high at
some times of the year (Table XVII), but because of the
small proportion of recaptures, estimates had large errors. 1 18
Figure 23. Total number of beetles trapped in beetle removal (O) and control (addition) (•) plots from 1983 through 1985 (log number of beetles +1). N=160 traps total in 1983-84, and N=132 traps total in 1984-85. log number beetles trapped 120
Table XVII. Population estimates from mark-recapture studies of each major beetle species. Estimates (standard errors) are for the 400m2 beetle addition plot in 1983. date P^ melanar ius C. nemoralis C. fuscipes Staphylinid
May 12 1 07 (55) 68 (29) 37 (18) 30 (17) 18 336 (181 ) 105 (38) __ -- -- 24 238 (58) 561 (238) — 204 (132) 30 385 (79) 691 (302) — — 590 (396)
July 1 3 669 (367) — — — — 9 (7) 19 502 (171 ) 663 (431) 4332 (4272) 26 744 (204) 1364 (492) 18281(18190) 40 (45)
Aug 2 4753(3291) 3864(2170) 87250(87075) — — 9 1 639 (455) 4894(2744) 29613 (5428) — —
Nov 3 91 (84) — — 186 (76) — — 6 55 (21 ) 2886 (2849) — 9 46 (15) — 1419 (675) 1 596 (888) 1 5 41 — — 11245(11158) 21 55 (12) 1605 (663) 4230 (2363)
'all captures were recaptures, hence no S.E. can be calculated 121
Plots sprayed with Diazinon but not fenced, had the same mean density of beetles as unsprayed, unfenced plots
(Fig. 24). Diazinon itself, therefore did not have a residual effect on beetle numbers.
Through most of the study, total beetle density was lower in the oakwood than in the apple orchard (Fig. 25).
Differences were, however, species-dependent: P. melanarius and C. nemoralis were similar in abundance in both habitats, whereas C. fuseipes was far more abundant in the orchard. The orchard also supported higher densities of staphylinids and beetle larvae (Fig. 25).
Functional response
Regression analysis was used to determine whether any of the beetle species were aggregating in areas of high host density at any time during the period May 1983 through June
1985. For each beetle species, the total monthly capture in pitfalls (in beetle-addition plots [1983-4] and control plots [1984-5]) was regressed against density of pupae for each tree. No evidence of aggregation by any beetle species in response to prey density occurred at any time of year. Of the resulting 83 regressions, only two were significant
(P<0.05), and only one was positive (P. melanarius in
November, 1983). Significance in one of 83 regressions is probably due purely to chance. 1 22
Figure 24. Total number of beetles trapped in diazinon sprayed (O) and unsprayed (#) plots, 1984-95 (log number of beetles + 1); (a) number of beetles trapped in plots where fences and diazinon are both used to remove beetles (N=76 traps total), (b) number of beetles trapped where fence is set around sprayed plots (N=16 traps total).
1 24
Figure 25. Mean number of beetles per pitfall trap (log number of beetles + 1 ) in the apple orchard (O) control plots (N=66 traps) and oakwood (•) (N=26 traps). Standard errors are indicated.
1 26
Host and parasitoid mortality
Effect of diazinon
In 1984, beetle removal included the application of a diazinon spray to the soil. Before analysis of winter moth and Cyzenis survival in response to beetle abundance could be done, the residual effect of diazinon spraying (without beetle fences) on both host and parasite survival had to be determined. The number of winter moth and Cyzenis adults emerging per m2 from plots in the diazinon-only experiment was used to determine the magnitude of this effect. Since location (east-west) of plots was thought to have an effect on numbers of larvae entering the soil to pupate, it was
included with presence and absence of diazinon as a factor
in an analysis of variance of numbers emerging.
There was a residual effect of diazinon on both the number of winter moth emerging (F=5.17, df=1, P=.037, one- tailed) and the number of Cyzenis emerging (F=4.76, df=1,
P=.047, one-tailed). Since beetle numbers were the same in sprayed and unsprayed plots of this experiment (Fig. 24), I conclude that the difference in numbers emerging was due solely to residual diazinon. The mean number of winter moth emerging per m2 was: 8.0 for sprayed plots, and 19.0 for unsprayed plots. For Cyzenis the means were 10.0 and 19.0
for sprayed and unsprayed plots, respectively. In the analysis of survival for the main experiment, the effect of diazinon was used to adjust observed numbers of host and parasite, emerging from the removal plots in the main experiment: 127
winter moth = obs winter moth*(l9/8)
Cyzenis = obs Cyzenis *(19/10)
Effect of beetles
The effect of beetle presence/absence on mortality was determined by analysis of covariance with combined density of parasitized and unparasitized pupae used as a covariate in all analyses. Density was used as a covariate because pupal mortality in Britain was known to be density-dependent within generations (East 1974). Mortality of winter moth pupae was not affected by the presence or absence of beetle predators (Table XVIII). Mortality of Cyzenis albicans was, however, reduced in the absence of beetles in both years of the experiment (Table XIX). This difference between host and. parasite could arise from one of four reasons: (1) Cyzenis is in the soil twice as long as winter moth, (2) beetles prey preferentially on parasitized pupae, and so beetle manipulation would have the greatest effect on Cyzenis, (3) parasitized pupae are more easily detected by beetles by virtue of altered behaviour of their host (e.g. shallower depth of pupation), and (4) winter moth suffer some other large mortality which masks the effect of beetle predation, a mortality which does not affect the parasitoid to the same extent.
Even in the absence of beetles, soil mortality was strongly density-dependent for both the winter moth and
Cyzenis (Figs. 26 and 27). This again suggests that a large mortality occurs in the soil which masks the impact of 128
Table XVIII. Analysis of covariance table for effect of beetle abundance on winter moth mortality in the apple orchard. All tests are two-tailed except the effect of beetles, which is one-tailed with the prediction that removal of beetles reduces mortality.
1983-84 source mean k5 (N) SS df MS F-ratio Prob. beetle 0.031 1 0.031 1 .08 0.15 removal 66 (16) addition 75 (16) prey density 1 .026 1 .026 36.34 <0.0001 beetleXdensity 0.012 0.012 0.40 0.53 residual 4.978 28 0.807 total 1 .908 31
1984-85 source mean k5a(N) SS df MS F-ratio Prob. beetle 0.139 1 0.139 0.77 0.19 removal 0.49 (33) addition 0.57 (33) location(E,W) 0.716 1 0.716 3.98 0.06 tree 11.948 32 0.385 2.14 0.02 beetleXdensity 0.201 1 0.201 1.12 0.30 residual 5.401 30 0.180 total 20.349 65 1 29
Table XIX. Analysis of covariance table for the effect of beetle abundance on soil mortality of Cyzenis albicans (kcyz) in the apple orchard. All tests are two-tailed except for the effect of beetles, which is one-tailed with the prediction that removal of beetles reduces mortality.
1983-84
source kcyz (N) SS df MS F-ratio P beetle 0.402 1 0.402 2.89 0.05 removal 0.41 (16) addition 0.67 (16) prey density 2.279 1 2.279 1 6.38 0.0004 beetleXdensity 0.007 1 0.007 0.05 0.83
residual 4.029 28 0. 144 total 6.878 31
1984-85
beetle 1.639 1 1 .639 6.84 0.007 removal 0.92 (33) addit ion 1.23 (33) location(E,W) 2.525 1 2.525 1 0.54 0.003 tree 16.676 32 0.538 2.25 0.01 beetleXdensity 0.142 1 0. 142 0.59 0.45
residual 7.189 30 0.240 total 35.685 65 1 30
Figure 26. Soil mortality of winter moth and Cyzenis for each tree in the presence (9) and absence (O) beetle predators in 1983-84. 131
winter moth 3h Cyzenis albicans
o removal L • addition
0 0
log pupae (m2) 1 32
Figure 27. Soil mortality of winter moth and Cyzenis for each tree in the presence (•) and absence ( O ) beetle predators in 1.984-85. winter moth 3 Cyzenis albicans
2 f 0 I 0
log pupae (m2) 1 34 beetles.
Effect of prey type
Two measures of winter moth mortality were made: (1) k5a was mortality only while in the soil, and did not include losses while females moved to trees to oviposit. R5a was measured in 1984, and provided the best comparison with mortality of Cyzenis (kcyz) while in the soil. In both the oakwood (P=.053) and orchard (P<.0001) k5a was lower than kcyz in plots with beetles (Table XX). This difference probably reflects the difference in time each spends in the soi 1.
Total mortality, to oviposition by winter moth (k5), provided a less consistent pattern, with no difference in mortality for the host and parasite in 1983 or 1984 (Table
XXI). In the oakwood, however, winter moth mortality (k5) was higher than for Cyzenis (Table XXII) despite the shorter period in the soil. Mortality of both winter moth and
Cyzenis were density-dependent (Fig. 28), suggesting that they are subject to the same causes of death. Presumably, pupal mortality of winter moth in the oakwood is independent of beetle abundance as was observed in the orchard.
Effect of habitat type
Winter moth mortality (k5) was significantly higher in the oakwood than in the orchard in 1983 and 1985, but not different in 1984 (Table XXIII). The habitat with higher beetle density (orchard) (Fig. 25), therefore, never 1 35
Table XX. Analysis of covariance table for effect of prey type on soil mortality (k5a vs kcyz) 1984-1985. All tests are two-tailed.
oakwood source mean k (N) SS df MS F-rat io Prob. prey type 0.241 1 0.241 4.33 0.054 moth 0.50 (10) Cyzenis 0.72 (9) prey density 0.950 1 0.950 24.12 0.0002 typeXdensity 0.006 1 0.006 0.14 0.71 residual 0.624 1 5 0.042 total 1 .822 18
apple orchard prey type 7. 108 1 7. 108 24.76 <0.0001 moth 0.57 (33) Cyzenis 1 .23 (33) prey density 6.040 1 6.040 21 .04 <0.0001 typeXdensity 0.397 1 0.397 1 .39 0.24 residual 17.688 62 17.688 total 31.234 65 1 36
Table XXI. Analysis of covariance table for the effect of prey type (winter moth vs Cyzenis) in the apple orchard control plots. In 1984-85, only trees with stocking traps are used in the analysis. All tests are two-tailed.
1983-84 source mean k (N) SS df MS F-ratio Prob. prey type 0.046 1 0.046 0.43 0.51 moth 0.75 (16) Cyzenis 0.67 (16) prey density 1 .865 1 1 .865 17.44 0.0002 typeXdensity 0. 128 1 0. 128 1 .20 0.28 residual 2.973 28 0. 106 total 5.012 31
1984-85 prey type 0.384 1 0.384 1.53 0.22 moth 0.85 (17) Cyzenis 1.06 (17) prey density 4.246 1 4.246 16.87 0.0003 typeXdensity 0.706 1 0.706 2.99 0.09 residual 7.801 31 0.252 total 12.431 33 1 37
Table XXII. Analysis of covariance table for the effect of prey type (winter moth vs Cyzenis) in the oakwood. Cyzenis mortality was measured in the oakwood in 1984-85 only. All tests are two-tailed.
1983-84
source mean k(N) SS df MS F-ratio Prob.
prey type moth 0.92 (8) prey density - 0.019 1 0.019 0.16 0.70
residual 0.682 6 0.114 total 0.701 7
1984-85
prey type 0.305 1 0. 305 5. 63 0. 03 moth 0.97 (11) Cyzenis 0.72 (9) prey density 1 .076 1 1 . 076 18. 43 0. 0005 typeXdensity 0.013 1 0. 013 0. 21 0. 66
residual 0.980 1 6 0. 061 total 2.373 19 1 38
Figure 28. Soil mortality of winter moth (#) and Cyzenis (O) in the oakwood in 1983-84 and 1984-85. Regression statiistics: winter moth: 1983-84 r2=0.03, P=0.70 1984-85 r2=0.49, P=0.02 Cyzenis 1984-85 r2=0.62, P=0.01 139
winter moth 1983-84
21 k
I i- •
1984- 85
winter moth 8tJ
^° Cyzenis albicans Oh
log pupae (m ) 1 40
Table XXIII. Analysis of covariance table for the effect of habitat type on soil mortality of winter moth (k5). All tests are two-tailed.
1983-84 source mean k (N) SS df MS F-ratio P habitat 0.497 1 0.497 6.87 0.016 apple 0.75 (16) oak 0.92 (8) prey density 0.380 1 0.380 5.25 0.03 habXdensity 0.147 1 0.147 2.13 0.16 residual 1.374 20 0.069 total 2.062 23
1984-85 habitat 0.010 1 0.010 0.20 0.66 apple 0.85 (17) oak 0.97 (11) prey density 1.483 1 1.483 30.16 <0.0001 habXdensity 0.002 1 0.002 0.03 0.86 residual 1.228 24 0.051 total 2.807 27
1985-86 habitat 2.712 1 2.712 18.48 0.0002 apple 0.60 (23) oak 1.35 (9) prey density 1 ..57 4 1 1 ..57 4 1 0.73 0.003 habXdensity 0. .001 1 0. .001 0.01 0.97 residual 4. .255 28 0, . 1 52 total 9. .456 31 141 exhibited a greater mortality of winter moth. In 1984, the only year in which Cyzenis mortality was estimated in both habitats, it was lower in the oakwood (Table XXIV), opposite to winter moth mortality. Therefore, the potential for
Cyzenis increase, relative to winter moth, is greater in the oakwood. Differences in mortality between the two habitats
for Cyzenis reflects the difference in beetle abundance; winter moth mortality does not.
Production of Cyzenis relative to its host
In the orchard, the number of adult Cyzenis surviving
in the soil, per surviving winter moth adult, was no different whether beetles were present or absent in 1983 or
1984 (Fig. 29). Mean number of surviving adult Cyzenis per
surviving adult host was greater in the oakwood than the
orchard (Fig. 30), despite lower beetle abundance in the
oakwood (Fig. 25). Because beetles prey longer on Cyzenis
than on winter moth, greater disparity in host and parasitoid mortality would be expected where predators are more abundant. Since this did not occur, it appears that
beetle abundance has little effect on whether parasitoids can build up to greater numbers relative to its host.
Fates of pupae
Fate in the presence/absence of beetles
Not surprisingly, there was a lowered incidence of predation in the beetle-removal plots than in control plots
(Table XXV). The only other sources of mortality which 1 42
Table XXIV. Analysis of covariance table for the effect of habitat type on soil mortality of Cyzenis albicans in 1984-85. All tests are two-tailed.
source mean k (N) SS df MS F-ratio P habitat 2.110 1 2.110 6.054 0.02 apple 1.23 (33) oak 0.72 (9) prey density 4.483 1 4.483 12.86 <0.001 habXdensity 0.633 1 0.*633 1*85 o! 18 residual 12.960 38 0.341 total 19.894 41 1 43
Figure 29. Number of adult Cyzenis albicans emerging per adult winter moth in the presence (#) and absence ( of predatory beetles in 1983-84 and 1984-85. (Ancova on log-transformed data; 1983-84, F=0.61, df=1, P=0.44; 1984-85, F=0.001, df=1, P=0.98). 40 1983-84
32
24
16
8
0
Cyzenis 15 moth 1984-85 12
6r
3L
art ^ .g 0 240 480 720 960 1200 density of pupae (m2) 1 45
Figure 30. Number of adult Cyzen i s albicans emerging per adult winter moth in the presence of beetle pedators in the apple orchard (O) and oakwood (•), 1984-85. (Ancova on log-transformed data; F=4.86, df=1, P=0.04). Oak=4.28 parasitoids surviving per winter moth Apple 1.08 parasitoids surviving per winter moth 40 • oak o apple 32
24 Cyzenis moth 16
CD 8
0 r» ml n In JCQ- 240 480 720 960 1200
density of pupae (m ) 147
Table XXV. Frequency of specific fates of pupae from beetle removal plots and control plots in the orchard, 1984-85.
beetle fate control removal total X2 df sig 1 emerged 1 34 23 1 57 0.44 ! ns depredated 90 6 96 7.39 1 <.05 virus 16 1 3 29 16.80 1 <.01 failed pupation 9 9 18 14.40 1 <.01 2 other 7 0 7 1 . 44 1 ns total 256 51 307 40.47 4 <. 001
1 multiple comparisons use critical values of Chi-square based on Sidak's multiplicative inequality (Rohlf and Sokal 1981). 2 includes fungus, nematodes and desiccation. 1 48 differed between the two treatments were virus infection and failed pupation. For both of these, a higher incidence was found in the absence of beetle predators (Table XXV), suggesting that predators remove virus-infected pupae and failed pupations preferentially over healthy pupae. Hence, pupae with these fates are only detectable in quantity where beetles are removed. Failed pupation may have resulted from pathogens; the result of combining these two fates is also highly significantly different between the two treatments
(Chi-square=31.0, df=1, P<.01). Although several species of pupal parasites have been recorded from winter moth on
Vancouver Island (Gillespie and Finlayson 1981, Humble
1985a, 1985b), none were recovered in this study.
Effect of density
The proportion of pupae preyed upon did not differ significantly among densities in the forced pupation experiment, nor did the proportion of moths emerging successfully (Table XXVI). This suggests that the density- dependence seen in the main experiment results from mechanisms occurring outside the soil environment (possibly among late-instar larvae). The proportion of pupae recovered which were infected by virus similarly did not differ among density treatments, again suggesting that if virus is the source of density-dependence, the mechanism of density- dependence lies outside the soil environment (i.e. late-
instar larvae are infected). 1 49
Table XXVI. Specific fates of pupae as a function of density at which larvae were forced to pupate. Categories with an expected frequency of less than five (fungus, nematodes, desiccation) are not included.
fate 10/cage 30/cage 60/cage total X2 df sig depredated 35 1 1 3 232 380 0.36 2 ns emerged 1 3 31 54 98 1 .55 2 ns virus+failed 2 2 12 16 2.20 2 ns total 50 1 46 298 494 4.11 4 ns
1 multiple comparisons use critical values of X2 based on Sidak's multiplicative inequality (Rohlf and Sokal 1981).
Table XXVII. Contingency table for incidence of predation among parasitized and unparasitized winter moth pupae from forced pupation experiments. Only samples up to the time of winter moth emergence (week 26) are used in the analysis.
parasitized not parasitized total depredated 25 34 59 not depredated 92 177 269
total 117 211. 328
Chi-square = 1.41, df=1, P=0.24 1 50
Interaction of predation and parasitism
If parasitized pupae are preferred to unparasitized pupae or are more easily encountered, Cyzenis would suffer relatively more than the host. Parasitism and predation were independent of each other (Table XXVII) even though parasitized pupae tend to be closer to the surface
(Fig. 31a). The intensity of predation at different depths in the soil did not differ significantly from the distribution of pupae generally (Fig. 31b). Therefore, pupae at all depths (whether parasitized or not) are equally susceptible to predation. Predatory beetles did not apparently distinguish between parasitized and unparasitized winter moth pupae (see cage experiments below).
Predation over time
Because time itself will reduce the likelihood of recovery of pupae, analysis of incidence of predation over time did not include the number of "missing" pupae from the initial number of larvae forced to pupate in each cage.
Incidence of predation was estimated only from recovered remains. Predation was 15% (k=0.07) by the end of June, and had reached 46% (k=0.27) by 30 November, when winter moths emerge as adults. By the time Cyzenis emerged (30 March), predation had reached a level of 65% (k=0.46). These values are appreciably lower than total mortality observed in the soil presence of beetles in the main experiment (winter moth mortality=73%, k=0.57; Cyzenis mortality=94%, k=1.25).
Predation therefore accounts for about two-thirds of the 151
Figure 31. Comparison of the depth of pupation of (a) parasitized (#) and unparasitized (O) winter moth larvae (Kolmogorov-Smirnov two-sample test, Dmax=0.28l, P<.025) and (b) depredated pupae (•) and non-depredated pupae (O) (Dmax=0.17, ns). cumulative proportion 1 53 total soil mortality, bearing in mind that many depredated pupae may have been infected by pathogens and would have died anyway.
Predation cage experiments
Pterost ichus melanarius and Carabus nemorali s are voracious predators (Thiele 1977) and were very adept at finding, opening and consuming winter moth pupae in cages.
When provided a choice between parasitized and unparasitized pupae, they were either unable to detect differences between the two types, or had no preference for either (Table
XXVIII). There was, however, a tendency for beetles to prey more heavily on virus-infected pupae than on uninfected pupae (Table XXIX). It is not known whether this is the result, of a preference for, or greater ease of finding, infected pupae. Beetles do, however, forage largely by olfaction and infected pupae do have a very strong, foul odour, suggesting they would be easily located.
DISCUSSION
Clearly, mortality in the soil of both, winter moth and its parasitoid Cyzenis albicans was large and strongly density-dependent at my study sites. Average soil mortality during the years studied ranged from k=0.60 (75%) to k=0.85
(86%) in the orchard, and k=0.92 (88%) to k=1.35 (95%) in the oakwood. Spatial density-dependence (slope of k vs log pupal density) was generally large (b=0.60 to b=1.00 for winter moth, b=0.40 to b=1.23 for Cyzenis). In Britain, Table XXVIII. Predation by caged carabid beetles on parasitized and unparasitized winter moth pupae. All tests are two-tailed since field data suggested no preference.
Trial 1: Pterostichus melanarius
depredated not depredated total parasitized 7 8 15 unparasitized 4 11 15 total 11 19 30 Fisher's Exact Test: P=.22
Trial 2: P_^ melanar ius and Carabus nemoralis
depredated not depredated total parasitized 17 3 20 unparasitized 16 4 20 total 33 7 40 Fisher's Exact Test: P=.50
Pooled data: N=70, Fisher's Exact Test: P=0.46 155
Table XXIX. Predation by caged Pterostichus melanarius and Carabus nemoralis on virus-infected and uninfected winter moth pupae. Test is one-tailed since field data suggested possible preference for virus-infected pupae.
depredated not depredated total
infected 16 3 19 uninfected 12 7 19
total 28 10 38
Fisher's Exact Test: P=.06 (one-tailed) 1 56 density-dependent mortality of these stages was attributed, to predatory beetles (Varley and Gradwell 1963a, Kowalski
1976, Frank 1967a 1967b, East 1974, Hassell 1980). Despite the implied regulatory effect of predaceous beetles (Varley and Gradwell 1963b), very different winter moth densities were observed in areas with similar beetle communities
(Wytham Wood and Wistman's Wood) (Frank 1967a).
Reduction of beetle densities in this study, by as much as 100-fold, had no effect on the magnitude of soil mortality of winter moth pupae, but did affect the mortality of its dipteran parasitoid Cyzenis albicans. The number of emerging adult parasites per emerging adult moth was not
significantly affected by manipulating beetle abundance despite the parasite's being exposed to predation twice as
long. Beetles do feed on pupae, as demonstrated by cage experiments and from the abundance of pupal remains bearing evidence of predation, but beetles appear to be removing pupae which were destined to die regardless. Small mammals caused significant mortality in Britain (Frank 1967, 20% mortality; Buckner 1969, 40-60% mortality). In November,
1985, I set 70 snap-traps for four nights in the orchard,
but captured no mice or voles, suggesting that small-mammal
predation is minor.
The magnitude of soil mortality in this study is
greater than that observed in Britain. In Britain, winter moth tended to be at endemic densities and seldom cause
serious defoliaton (Buckner 1969). Total soil mortality was moderate. Average k=.61 (75%), and was only slightly 157 density-dependent among years (b=0.35, [Varley et. al. 1973]), and within years (b=0.28 [East 1974]). The bulk of pupal mortality was attributed to beetle predators. The mechanism was attributed to behaviour of individual beetles
(observed in cage experiments, Frank 1967b), and to aggregation of beetles in areas of high prey density (East
1974).
Using cage experiments and measurement of beetle abundance, Frank (1967b) found that beetles accounted for
37% (k=0.20) mortality of unparasitized pupae in 1965. In that year, total soil mortality was 87% (k=0.89) (calculated
from Table F in Varley e_t al. 1973), suggesting that beetles accounted for less than half the total soil mortality of winter moth. Experiments conducted by East (1974) showed
that pupae which he himself buried, suffered about 74% mortality (k=0.59) due to beetle predators. In British
Columbia, beetles removed about two-thirds of pupae, many of which would have died from other causes. In my study, the habitat with higher beetle numbers (apple) exhibited lower
soil mortality than where beetles were less abundant (oak).
Soil mortality of winter moth in Nova Scotia varied
from k=0.2 (37%) to k=1.1 (92%) among years, increasing
sharply during the years of decline (Embree 1965). Only predation by small mammals was measured, and accounted for
k=0.04 to 0.16 (9% to 31%). Clearly some other factor accounted for the bulk of soil mortality. Embree (1966) measured mortality of Cyzenis only in 1961 at his Oak Hill
site, and found it to be 37% (k=0.20). In that year at Oak 158
Hill, mortality of winter moth in the soil was 94% (k=1.2)
despite their being in the soil only half as long as the
parasitoid. This suggests that in Nova Scotia, death of
pupae was caused by factors other than predation. The sharp
increase in soil mortality in Nova Scotia was coincident
with increased parasitism by the introduced parasitoids
C. albicans and Agrypon flaveolatum and by the nematode
Complexomermis sp. (Fig. 32a). The inclusion (by Embree) of
mortality by endemic nematodes with mortality by introduced
parasitoids gives the impression that introduced parasitoids
played a larger role than they, in fact, did. When I
reanalyzed his data, placing nematode-caused mortality with
other sources of soil mortality, a very different picture
results (Fig. 32b). Soil mortality rather than parasitism is
more strongly implicated in the "crash", soil mortality
being caused largely by factors other than predation.
Parasitism by introduced parasitoids plays little role in
winter moth decline, and the contribution of predation to
total mortality in the soil is small.
Soil mortality of Cyzenis in British Columbia is
greater and more strongly density-dependent than was
observed in Britain even when the abundance of beetle
predators is greatly reduced. The magnitude of soil
mortality in my study is similar to that observed in the
year prior to decline in Nova Scotia. Given Hassell's (1980)
analysis, the strong density-dependence and large average
magnitude of soil mortality I have observed would predict
that Cyzenis could not have built up to densities causing 1 59
Figure 32. Contribution to generation mortality by soil mortality (k5) and parasitism by Cyzenis (k2) in Nova Scotia: (a) data from Embree (1965, his Figure 27), (b) my reanalysis of his data placing nematode-caused mortality with soil mortality, and parasitism only from introduced parasitoids. Calculations are provided in Appendix C. 160
K
1954 1956 1958 I960 year 161 the 70-80% parasitism observed in Nova Scotia. C. albicans has, however, built up to densities 20 to 40 times that observed in Britain, and parasitism of winter moth has reached 50-90% (k=0.30 to 1.00) among trees, and averaged
65-75% (k=0.46 to 0.60) among sites in some years. The prediction that large density-dependent mortality of the parasitoid will preclude successful control fails and an alternative explanation should be sought. An alternative would ideally be consistent among the three winter moth populations studied. The consistent factor in these three studies is the role which soil mortality plays in population regulation, and the possible link between parasitoid abundance and some element of soil mortality.
I suggest that this link is the potential of parasitoids to act as vectors of viruses which infect fifth- intar larvae and kill pupae. The decline of winter moth populations in British Columbia and in Nova Scotia is undeniably well correlated with the introduction and increase of insect parasitoids. In both populations, the decline began four to five years after parasitoid introduction. If soil mortality by pathogens is responsible for control of winter moth, and if this increases only after parasitoids are introduced, then the parasitoids would be implicated in the introduction and possibly the transmission of this virus. A trial spray of winter moth nuclear polyhedrosis virus (NPV) was made in my study orchard in
1979 (Cunningham et al. 1981), before Cyzenis was present.
Although mortality of winter moth increased in the year of 162 virus application, there was no transmission into the next year (1980). If Cyzenis does act as a vector of pathogens, application of virus, in the presence of the parasitoid, might be more successful.
Incidence of virus infection in "removal" plots was of the same magnitude as beetle predation in control plots
(Chi-square=1.65, df = 1, P>0.10). The proportion of the population emerging successfully was the same whether beetle predation was high or low. This pattern is supported by both
life table data from the beetle manipulation experiment
(Figs. 26 and 27), and by proportions successfully emerging
in the presence or absence of predation (Table XXVII).
Virus infections are typically density-dependent
(Anderson and May 1980), which would explain the observed pattern of mortality in this study, even in the absence of beetles (Figs. 26 and 27). Virus would be transmitted to the
late larval stage (i.e. outside the soil environment).
Results from forced-pupation experiments at different densities (Table XXV), imply that the mechanism of density- dependence occurs prior to larvae entering the soil. Other
factors acting during the late-larval stages might have pre• disposed winter moth at high density to suffer higher mortality in the soil, possibly competition for food or activation of pathogens by stress at high population density. Many insect species exhibit high virus infection after a few years of high density (Smith 1976). Density-
induced stress can act as a factor activating latent
infection causing population decline (Steinhaus 1958, 163
Steinhaus and Dineen 1960). Host stress and infection could also affect the suitability of winter moth as hosts for
Cyzenis and result in similar density-dependent pattern of mortality for the parasitoid.
If virus mortality is density-dependent and is due to transmission by parasitoids, then stronger aggregation of
flies observed in the oakwood (Chapter 3, Fig. 12, p. 63) would result in stronger density-dependence and greater average soil mortality than in the orchard. Mortality in the
soil was not more strongly density-dependent in the oakwood, but was significantly higher in magnitude (Chapter 4, Table
XXIII). Embree (1966) noted the appearance of a polyhedrosis virus in larvae of his winter moth populations in 1961 (the year prior to the crash), and "by 1964 the virus was
generally present throughout the area of winter moth distribution" (Embree 1966, p. 1167). He suspected Cyzenis albicans was the vector of this virus.
Dipteran parasitoids have been implicated as vectors of pathogens in other studies. The sarcophagid fly, Sarcophaga
aldrichi Parker, has been shown to transmit virus which
kills its host, the forest tent caterpillar Malacosoma
disstria Hbn. (Stairs 1966). Areas with high fly populations
had high incidence of virus infection, and virus which
killed forest tent caterpillar was cultured from field-
caught adult flies. The tachinid fly Drino bohemica
Mesn. transmits virus which kills the European spruce sawfly
Diprion hercyniae Htgr. (Balch and Bird 1944, Bird and Elgee
1953, Bird 1961). Similar transmission of virus among 1 64 alfalfa caterpillars occurs via braconid wasps (Thompson and
Steinhaus 1950). The combined effect of pathogens and
parasitoids drives the. eight to ten-year oscillation in
numbers of the spruce budworm Chor i stoneura fumi ferana
Clem. (Royama 1984). There appears, therefore, to be a
pattern for populations of important forest defoliators to
be determined, to a large extent by the combined, and
possibly synergistic, effect of parasitoids and pathogens.
The fact that parasitoids act as vectors of pathogens
suggests that parasitoids have played a role in control of
winter moth which goes far beyond the parasitism they
inflict. Virus may have gone unnoticed in other studies
because: (1) mortality is expressed in the pupae rather than
larvae, and (2) predatory beetles remove infected pupae
preferentially over healthy pupae. Incidence of observed
pathogen infection can be many times greater for insect
populations protected from predation than when exposed to
predators (Weiser 1958). Infection of the pupal stage is
typical if the late-instar larvae are the infected stage
(Jaques and Stutz 1966; Vaughn 1974). It is during this
larval stage that Cyzenis is actively seeking feeding-
damaged foliage on which to oviposit and during which most
larval feeding occurs.
In both Nova Scotia and British Columbia, parasitoids
were introduced approximately five to six years after winter
moth had reached high density. In both areas, population
decline began four to five years after parasitoid release,
and approximately ten years after moth populations were 165
first recorded at high density. The hypothesis of stress-
induced susceptibility to pathogens must, therefore, be considered as an alternative explanation for the timing of host decline relative to the time that parasitoids were
introduced. The timing of parasite introduction (in the absence of population data from areas without parasites), may simply give the appearance of their being an important
factor.
Soil mortality has been a consistent factor controlling winter moth populations among the three populations studied.
Soil mortality was the principal regulating factor in
Britain (Varley and Gradwell 1963b); reanalysis of Embree's data show that mortality in the soil increased much more during the decline than did parasitism. Soil mortality in
British Columbia was also of far greater magnitude, was more
strongly density-dependent within generations (Fig. 33) and
showed greater increase among generations (Fig. 34) than did parasitism.
To invoke mortality from parasitism as the cause of the decline in Nova Scotia and British Columbia requires a different scenario for each case. Although parasitism may not have controlled winter moth populations, the parasitoid may have contributed more to the decline of host populations as a vector of pathogens than through parasitism per se. The experimental removal of Cyzenis and its effect on soil mortality and generation mortality are examined in the next chapter. 166
Figure 33. Within-generation soil mortality (k5, [#]) and mortality from parasitism by Cyzenis (k2, [O]) in the apple orchard and oakwood in this study.
Regression statistics:
habitat year mortality slope r 2 sig.
apple 1 983 soil 0.81 0. 42 *** parasitism 0.16 0. 25 ** 1 984 soi 1 0.72 0. 73 *** parasitism -0.01 0. 01 1985 soil 0.76 0. 53 * * * parasitism 0.08 0. 08
oak 1 983 soi 1 0.17 0. 03 parasitism 0.31 0. 52 * 1 984 soil 0.65 0. 49 * parasitism -0.10 0. 02 1 985 soil 0.75 0. 1 7 parasitism -0.15 0. 04
* P<0.05, ** P<0.01, *** P<0.001 1985
1985 1 68
Figure 34. Average soil mortality (k5) and mortality from parasitism by Cyzenis (k2) among years. Soil mortality in 1982 (O) was not estimated from direct measure of prepupae entering the soil to pupate, but from levels of defoliation and the known relationship between defoliation and density of pupae calcualted in Appendix D. apple orchard oakwood
1982 1983 1984 1985 1982 1983 1984 1985 170
CHAPTER 5
EFFECT OF FLY EXCLUSION ON MORTALITY IN THE SOIL
AND ON GENERATION MORTALITY
INTRODUCTION
The greatest increase in winter moth mortality after parasitoid introduction in British Columbia and Nova Scotia, has been mortality of unparasitized pupae in the soil
(Chapter 4). A large proportion of mortality in the soil seems due to pathogens (Chapter 4). These facts implicate
introduced parasitoids as disease vectors, or that winter moth increased and declined as a result of factors unrelated to the introduction of parasitoids.
Other mechanisms of pathogen transmission would likely also be acting, including transmission by birds (Enwistle et al. 1977a, 1977b), transovarial transmission from adult
females (Cannings and Barker 1982) either in the egg or on the egg surface (Wigley 1979) and fouling of foliage on which host larvae feed (Entwistle and Adams 1977, Wigley
1979). These mechanisms would, however, have been operating even if parasitoids had not been introduced which suggests
that the introduction of Cyzenis may have also resulted in
the introduction of host pathogens, but may not have been
the only mode of transmission once the pathogen had been
introduced.
Clearly, Cyzenis does contribute to winter moth 171 mortality directly through parasitism. The extent to which parasitism is able to control winter moth will depend on the presence of any subsequent compensatory mortality of the host (Wang and Guttierez 1980, Royama 1981, May et al. 1981). If such a pattern of mortality exists, it could nullify any contribution which parasitism makes to total host mortality. Although the parasite might inflict high rates of loss, host numbers could still decline in its absence.
By excluding parasitic flies from some trees, I tested both the above questions regarding winter moth mortality with the following predictions: (1) excluding Cyzenis will reduce soil mortality of unparasitized pupae in the soil
(i.e. reduce prevalence of pathogens), and (2) removing parasitism as a source of mortality will increase total survival from first-instar larvae to adult emergence.
METHODS
Fly exclusion
In 1984, a preliminary experiment was conducted to determine whether the exclusion of Cyzenis from trees would affect total generation mortality. One apple tree was totally covered by fly screening (2 mm mesh) during the one- month oviposition period of Cyzenis. This treatment also excluded other predatory or parasitic insects as well as insectivorous birds, and is therefore called the "fly+bird" exclosure. As a control, one tree was covered with fly 1 72 screening which had many 15-cm holes cut in it to permit
flies to enter the tree canopy ("bird" exclosure). Birds were never observed in this tree, and winter moth density remained as high as on the screened tree (Roland e_t al. 1986).
In 1985, three trees were covered by fly screen. One tree was covered by fly screen with holes, and three trees covered by one-inch-mesh gill net to exclude birds but not
Cyzenis. Experimental and control trees were chosen to span the range of densities of first-instar larvae observed in the study grid. Fifteen trees without screening or netting were used as controls. In the oak wood, two trees (one low density and one high density) were covered by fly screening.
Screen and netting were put over trees just prior to the
time Cyzenis began ovipositing.
Density and survival estimates
Density of winter moth larvae and pupae for all trees was estimated as described in Chapter 2. Parasitism was
estimated from numbers of parasitized and unparasitized pupae in pupation trays set under each tree. Mortality (k- values) of various winter moth stages was determined for controls, bird exclosures, and fly+bird exclosures. Stages
for which mortality was calculated were: (1) larval mortality (klarv) from first-instar larvae to pupae, (2) mortality from parasitism by Cyzenis (kpar), (3) soil mortality of unparasitized pupae (ksoil) to adult emergence,
(4) total pupal mortality from parasitism and soil mortality 173
(kpup), and (5) total generation mortality from first-instar
larvae to adult emergence (ktot).
Because several of these sources of mortality were
suspected to be density-dependent, the magnitude of each was plotted against the log of the initial density on which each mortality factor acted. Mortality on experimental trees was compared with the 95 percent prediction limits for each
regression. Because reduced mortality was predicted in the absence of parasitic flies, 95 percent prediction limits for
regression lines use t-values for P<0.10 (i.e. one-tailed
tests)
RESULTS
Apple orchard 1984 (Fig. 35a~e)
Soil mortality of unparasitized winter moth (ksoil) was
not reduced by excluding Cyzenis albicans and other insects,
nor by excluding insectivorous birds, and was the same for
the two experimental treatments (Fig. 35c). Generation
mortality (ktot, Fig. 35e) was lower for both the fly
exclusion and bird exclusion, probably due to the effect
which cages had on excluding insectivorous birds (Fig. 35a)
(Roland et a_l. 1986). The difference in total mortality
between the two experimental trees reflects the difference
in parasitism rates (Fig. 35b). The effect of parasitism,
therefore, appears not to be compensated for through the
action of soil mortality. 174
Figure 35. Mortality (k-values) as a function of host density (log density per m2), for control trees (•), trees from which birds are excluded (A), and trees from which flies and birds are excluded (O). Solid lines indicate significant regressions with 95% prediction belts, dotted lines indicate non-significant regressions and their 95% prediction belts. Top row: orchard 1984; middle row: apple orchard 1985; bottom row: oakwoods 1985. Each mortality is defined in the text. 2 3 1 2 3 12 3-1 2 3 1 2 3 log L. m2 log pupae m2 log pupae m log pupae m log L, m 176
Apple orchard 1985 (Fig. 35f-j)
Again, unparasitized pupae died just as fast in both
experimental treatments as in the controls (Fig. 35h, Table
XXX). At high host density, mortality in the soil was less
on fly-exclusion trees than on controls (Fig. 35h) but not
different from those where birds were excluded (Fig. 35i).
In two of the trees where flies were excluded, soil
mortality was in fact higher than on control trees
(Fig. 35h), suggesting that some compensation for the
effects of parasitism can occur through mortality in the
soil.
Total generation mortality was reduced on one tree at
high density by the exclusion of flies, but was also reduced
when only birds were excluded (Fig. 35j). Reduction in total
mortality from fly exclusion was principally through reduced
parasitism (Fig. 35g). The difference in mortality from
parasitism rate between fly and bird exclosures (difference
in k=0.29 [49%]) was somewhat compensated for on two of the
fly-exclusion trees, since the difference in overall pupal
mortality (kpup) was only k=0.04 (9%) (Table XXX).
Oakwood 1985 (Fig. 35k~o)
In the oakwood, only fly+bird exclosures were used;
there were no controls for the effect of excluding birds but
not flies. As it turned out, 1985 was a year with low bird
predation on winter moth and larval survival in the orchard
was similar whether birds were present or absent (Table XXX,
Fig. 35f). Soil mortality of unparasitized pupae (Fig. 35m) Table XXX. Average mortality (k-values) for control trees, bird-exclusion trees and fly-exclusion trees. Mortality for control trees are estimated from the relationships between mortality and density (Figure 35) and the mean density observed for experimental trees. Each mortality defined in the text.
klarv kpar ksoi 1 kpup ktot 1984 apple control 0.53 0.50 1 .47 1 .87 2.40 bird 0.05 0.46 1 .54 1 .98 2.03 fly + bird 0.11 0.00 1 .52 1 .53 1 .64
1985 apple control 0.36 0.31 0.64 1.01 1 .28 bird 0.26 0.30 0.60 0.90 1.16 fly+bird 0.16 0.01 0.92 0.94 1 .09
1985 oak control 0.23 0.69 1 .35 2.03 2.26 fly+bird 0.17 0.00 1.12 1.12 1 .29 178 was not reduced by the exclusion of parasitic flies. Total
generation mortality (Fig. 35o), was significantly reduced,
but largely reflected the difference in parasitism level
(Fig. 351). Parasitism was therefore not compensated for by
subsequent soil mortality. The difference in total mortality
between the two fly exclosure trees (Fig. 35o) reflected the
difference in larval mortality (Fig. 35k), suggesting that
very little compensation occurred through the entire
generation. Mortalities are therefore additive. This could
be the result of winter moth density in 1985 in the oakwood
being generally low, with little density effect being
expressed.
DISCUSSION
Removal of parasitic flies was initially designed to
determine whether differences in mortality from parasitism
were compensated for in later stages. Difference in
parasitism was compensated for in one year, but losses due
to parasitoids are additive to other sources of mortality.
Any compensation which does occur is not due to the effect
of predatory beetles since they do not distinguish between
parasitized and unparasitized pupae (Chapter 4). If strong
compensation had been observed, it would imply that low
rates of parasitism would be followed by high mortality in
the soil, and high parasitism followed by low soil
mortality. If parasitoids are acting as vectors of
pathogens, however, low levels of parasitism would be
followed by low mortality in the soil. 179
The second aim of this experiment was to determine whether flies act as vectors for pathogens. In general, soil mortality was not reduced by the exclusion of either parasitic flies or insectivorous birds. If either flies or birds are acting as vectors of pathogens which cause winter moth pupal mortality, their effect was not evident from this experiment. Fly screening and bird netting were put up approximately four weeks after fly emergence, but prior to
fly oviposition. Flies and birds therefore were likely
feeding in these trees (on sap and insects respectively) during the period trees were not enclosed. As a result, virus could still have been introduced into the winter moth
larval populations. To properly test for transmission by
flies, screening will in future have to be erected prior to
fly emergence (mid-March).
Characteristic symptoms of virus infection (flaccid bodies filled with milky fluid) were never seen in larval populations in either the apple orchard or in the oakwood.
In Britain, pathogens were also infrequent in larval populations; only 3 percent for virus (Wigley 1979), and 3
to 30 percent for microsporidian infection (Cannings and
Barker 1982). Microsporidian disease is particularly
interesting since infection was low in late-instar larvae
(3% to 30%), but was high (70% to 80%) in emerging adults
(Cannings and Barker 1982). There is, therefore, some
indication that pathogens might not be detectable in the
larval stage, but be very high in the pupal stage, a stage 180 which is typically not examined for disease.
Although parasitism of winter moth by Cyzenis is additive to other sources of mortality, and is only somewhat compensated for in later stages of the host life-cycle, the magnitude of this mortality is small relative to soil mortality. The potential for increased death of winter moth pupae through pathogens introduced by Cyzenis (as was also
suggested by Embree [1965]) is still a possibility, but
testing this properly would require the exclusion of Cyzenis during both the feeding and oviposition period. 181
CHAPTER 6
THE ROLE OF PARASITISM IN HOST SUPPRESSION,
EXPLORED WITH TWO SIMPLE MODELS
INTRODUCTION
Chapters three and four suggest that: (1) the maintenance of low host populations on oaks is associated with aggregation of parasitoids in areas of high host density, and (2) parasitism by Cyzenis has played a minor role in the suppression of winter moth both in Nova Scotia and British Columbia. To what extent then, if any, are the introduced parasitoids involved in successful biological control through first reducing the number of winter moth, and then keeping them down?
To answer these questions empirically would have required observation over the past four years in areas where
Cyzenis was not present, and a demonstration that density of winter moth •remained much lower in the oakwood than in the orchard over a period of several years after the decline.
Because these were not tested empirically, I will explore each with two simple mathematical models.
The first model has two objectives: (1) to determine whether reduction of winter moth numbers observed over the past four years would have occurred in the absence of parasitism, and (2) to determine the impact on population suppression if some soil mortality was caused by pathogens transmitted by the introduced parasitoids. 182
METHODS
Model I
This model is developed for each of the two habitats
studied: apple orchard and oakwood. The model is basically a
"book-keeping" exercise, given the density of first-instar
host larvae observed in 1983, and applying the appropriate
survival of each life-stage in each of three years, 1983
through 1985. Survival of some of these stages is density-
dependent, in others it is not (Fig. 36). The three major
survival periods used are: (1) survival of larvae from hatch
to pupation (Fig. 36a and 37a), (2) survival from parasitism
by Cyzenis (Fig. 36b and 37b), and (3) survival of
unparasitized pupae in the soil (Fig. 36c and 37c).
Survivorship curves are determined by the reformulation of
the linear regression of k-values against log of the initial
density:
k= a + b(log density) (1)
to give:
-a -b
s= 10 * density (2)
where "s" is the proportion surviving. In cases where
survival is predicted to be greater than 1.0 it is set equal
to 1.0 (Fig. 36). Survival of Cyzenis in the soil is
similarly determined from empirical relationships between k-
values and density (Chapter 4).
Fecundity (F), is calculated from the relationship 183
Figure 36. Survival in the apple orchard: (a) of larvae, from parasitism by Cyzenis and (c) of pupae in each of 1983 through 1985.
185
Figure 37. Survival in the oakwood (a) of larvae, (b) from parasitism by Cyzenis and (c) of pupae in each of 1983 through 1985.
187 between density of first-instar larvae (m~2) and the
fecundity of resulting females (Fig. 38). The number of eggs
oviposited per female is then divided by two for the estimate of the number of eggs oviposited per adult
(assuming an equal sex ratio). Therefore:
F = .5 * (630*density-15 for apple (3)
and, F = .5 * (1542*density-031 for oak (4)
The model assigns a normal distribution of first-instar
densities among twenty trees (with a mean of 1000 first-
instar larvae per m2, appropriate for what was observed in
spring 1983), and calculates mean density for each stage
(first-instar, pupa, adult) in each year. Number of eggs
oviposited is calculated at the end of each year for each
tree.
To determine the effect of parasitism from 1982 through
1985, the model is run with mortality due to parasitism, and
without, the prediction being that without parasitism,
winter moth populations would not decline.
The hypothesis that some soil mortality is due to
pathogens introduced and/or transmitted by Cyzenis is
examined by removing that portion of soil mortality which is
assumed to be due to these pathogens. Survival (s) is
simply:
s = 1 - m (5)
where m is the proportion dying. When a portion of the
mortality is removed, the adjusted survival s' is then:
s' = 1 - [ m - (m*p) ] (6)
where 2 is tne proportion of the soil mortality due to 188
Figure 38. Fecundity (number of eggs) of female winter moth as a function of density of first-instar larvae (L1) in the apple orchard and in the oakwood. Regressions are: apple , eggs=630* (density "•'5 , r2=.64; oak, eggs= 1 542* (density3 1 ) , r2 = 0.47. 189
500 -o apple N = 74 o* -• oak N = 31
400 rp
soop^0 CO 00 eggs O o 000 o o - _ tbo o«o o o ~ o-o- _o _ 200 > oo oo - — o o oo o o o
100
0' 1 1 1— 1 I L_ 0 500 1000 1500 2000 2500 3000 L, / m2 190
Cyzenis-borne pathogens. Field data suggest that as much as
50% of mortality in the soil is due to pathogens (Chapter 4,
Table XXVII). How much of this is the result of Cyzenis transmission is not known, so a range of values is used. The model is run with p_ set at values of 0.5, 0.2 and 0.1. The reduction of winter moth population in the absence of parasitoid-carried pathogens is then compared with population reduction with unadjusted soil mortality.
Model II
This is the model which Hassell (1980) developed for winter moth and Cyzenis in Nova Scotia. I use this model because the survival functions are based on patterns of intergenerational mortality observed over an eight-year period (Embree 1965), which is preferable to using the three years data from my study. This model is also used because it accurately describes the pattern of winter moth decline observed in Nova Scotia, and the because decline in the model is due largely to the impact of parasitoids. The model applies survival of larvae s1 , survival from parasitism s_2 and survival of pupae in the soil s5 to each appropriate stage. Survival from parasitism is calculated from the zero- term of the negative binomial distribution:
-k
s2 = [ 1 + (a*Pt)/k] (6) where a is the area of discovery (parasitoid efficiency), Pt is the density of parasitoids, and k is the coefficient of dispersion of attacks among hosts. 191
The impact of differing patterns of Cyzenis search on oaks and apple trees is compared by using the relation between search efficiency a and parasitoid density for each tree species (Chapter 3, Fig. 17b). Efficiency is equal at high parasitoid density, but is greater on oaks at low parasitoid density than on apple. The model is run iteratively for 50 years for each population (oak and apple).
RESULTS
Model I
Reduction of winter moth populations is almost as large in the absence of parasitism as when parasitoids are included in the model (Fig. 39a and b). Density of adult moths is slightly higher in the absence of parasitoids, but density still goes down. Some other factor has an over• riding effect on any change in level of parasitism, enough to counter the effect of the 70 to 80 percent parasitism of pupae observed in 1984 and 1985.
Some soil mortality is due to pathogens (Chapter 4). If it is due to parasitoid-transmitted pathogens, then its removal should greatly reduce the loss of hosts. When as little as 10 to 20 percent of soil mortality is removed, winter moth numbers remain high (Fig. 39c and d). Clearly, soil mortality has a very large impact on host population size from 1982 through 1985. Therefore, only a small increase in soil mortality after parasitoid introduction 1 92
Figure 39. Density of winter moth adults from model I for the oakwood and the apple orchard, (a) and (b) with and without parasitism in the model, (c) and (d) with soil mortality reduced by the proportion indicated. The assumption is that these proportions are the amount of mortality due to parasitoid-transmitted pathogens. Observed density of adults in 1982 are indicated (•). Oak Apple IOO r (a) (b)
adults 5o[ -2 • - no parasitism m ~ ~ -ex parasitism1 0l > 82 83 84 85 83 84 85 o » i o 'i ° 0.5 (d) / '\ (c) \ \\ / 1 / \ i 1 / \ i 100 1 / i \ / \ .1 \ / \ ' i.' \ / \\> i i \ » • ' < adults •o o 0.2 \i 50 -o. \ >' 0 \ -2 . -o \ vO-' °0.l •-°0 m \ » - -
82 83 84 85 82 83 84 85 year year 194 would cause a decline. For populations in the orchard, however, a greater proportion of soil mortality (50 percent) must be removed before numbers do not go down. This, I feel,
is the result of the very large larval mortality inflicted by insectivorous birds in 1984 (Roland et a_l. 1986) in the orchard but not in the oakwood, which limits the impact which other sources of mortality might have had on total
generation mortality.
Model II
Parasitoid efficiency differs between oak and apple at
low parasitoid density (Chapter 3), and in the model results
in an order of magnitude difference in host population
density (Fig. 40a). Populations are reduced far more in the
oakwood than in the orchard. Lower host density on oaks is
particularly striking given that it is maintained by a
parasitoid density approximately one-tenth that in the
orchard (Fig. 40b). The implication, therefore, is that
increased parasitoid efficiency, manifest through
aggregative attack, can have a dramatic impact on the level
to which host populations can be suppressed and at which
they can be maintained, even though initial decline might
not have been caused by parasitism.
DISCUSSION
From the simple models presented, it is clear that
decline of winter moth has not resulted from parasitism by
introduced parasitoids, and that soil mortality has been the 195
Figure 40. Result of simulations of model II for (a) density of winter moth, and (b) density of Cyzenis albicans with parameters for search efficiency "a" as a function of parasitoid density from Chapter 3. parasitoid
log density
(m2)
j i .—i 1 —<— 10 20 30 40 50 years 1 97 principal reason why numbers have gone down. Results from
these models support, in general, the conclusions based on
empirical evidence in Chapter 4.
Suppression of winter moth in Nova Scotia was attributed to introduced parasitoids (Embree 1965, Hassell
1980). Hassell's (1980) model of the Nova Scotia population
requires parasitism for decline to occur; removal of
parasitism from his model results in populations remaining
at outbreak levels. Hassell's model for Nova Scotia results
in only 6% of the population remaining unparasitized during
the period of greatest decline. Data from Nova Scotia,
however show that 40-50% of the population remained
unparasitized (Embree 1965, Table 22). The portion of the
population surviving parasitism was, in reality, seven to
eight times that which occurred in the modelled population,
suggesting that the contribution by parasitism to total
mortality in the model was generous.
In general, I feel that parasitoids have been given far
too much credit for the decline of winter moth populations.
The decline of this defoliator is well correlated with the
rise in parasitism up to as much as 70 to 80 percent. The
critical question of whether or not parasitism is necessary,
has not been previously addressed either empirically or
through simulation. Results from the simple models presented
here indicate that it is not necessary for suppression but
may play a role in maintaining populations at low density. 1 98
CHAPTER SEVEN
GENERAL CONCLUSIONS
The approach taken in this study to determine the role of parasitoids in controlling winter moth numbers (comparing apparent success with obvious failure), produced two primary results:
(1) I could not reduce mortality of winter moth pupae in the soil by removing predatory beetles, which suggests that predation has little impact on winter moth (or on Cyzenis)
(Chapter 4). Mortality of the host in the soil was, however, very large, so large in fact that the effects of parasitism in the suppression of host numbers over the three years of this study are relatively small (Chapter 5 and 6). Soil mortality of Cyzenis was much higher in this study than that in Britain (Varley et a_l. 1973), but Cyzenis still built up to very high numbers (Chapter 2). The fact that parasitism could not explain the resulting decline of winter moth either in Nova Scotia or British Columbia suggests that
Cyzenis numbers merely respond to an increase in resources
(host numbers) but do not cause the subsequent decline. This supports Dempster's (1983) contention that parasitoids have little effect on Lepidoptera populations, at least in population decline. The apparent cause of the observed decline is due to pathogens which kill the pupal stage
(Chapter 4). The correlation beween the time of parasitoid
introduction and time that host populations decline, 199 suggests that pathogens have been introduced and/or transmitted by the introduced parasitoid, or that the parasitoid introduction was coincident with a decline which would have occurred regardless.
(2) The fact that populations of winter moth in Nova Scotia have remained low in oakwoods for 25 to 30 years suggests a strong stabilizing mechanism is acting on these populations.
This contrasts with the pattern of continued high density observed in orchards. Stability at low density is correlated with the parasitoid's ability to aggregate "attack" in areas of high host density (Chapter 3). The strength of this aggregation increases as mean winter moth population declines in oakwoods; no aggregation is apparent in apple orchards at any density (Chapter 3).
In population models, stability at low population density as observed for the winter moth (and other examples of biological control) could be achieved only by aggregation of parasitoids in response to host density (Beddington e_t al. 1978). Other mechanisms which would contribute stability
(e.g. density-dependent mortality of the host) cannot provide as strong a stabilizing effect. My results, therefore, support Hassell's (1985) contention that natural enemies exert a strong stabilizing effect on host populations, at least at low density.
My re-analysis of Embree's (1965) data for winter moth in Nova Scotia indicates that parasitism by Cyzenis could not have caused host population decline. Unfortunately, data were not collected in Nova Scotia after winter moth 200 populations had declined. I suspect that at low density,
Cyzenis has a greater impact than at high density, an effect manifest either through aggregative attack or through aggregative transmission of pathogens. Because winter moth were little affected by Cyzenis at low density in Britain
(Varley e_t al. 1973), but suffered high mortality in the
soil, aggregative transmission of pathogens seems the more plausible of the two.
In population studies generally, it is rare that a
single factor is found which regulates numbers. It is perhaps too much to expect the introduction of a parasitoid
not only to bring down the host population, but also keep it
low. The numerous examples of insect population decline associated with an increase in both pathogens and parasitoids (Chapter 4) suggests that the synergistic
effects of more than one factor may be more generally
responsible for population decline and regulation. 201
LITERATURE CITED
Anderson, R.M. and R.M. May. 1980. Infectious diseases and population cycles of forest insects. Science, 210:658-661.
Balch, R.E. and F.T. Bird. 1944. A disease of the European spruce sawfly, Gilpinia hercyniae (Htgr.), and its place in natural control. Sci. Agric. 25:65-80.
Beddington, J.R., C.A. Free and J.H. Lawton. 1978. Characteristics of successful natural enemies in models of biological control of insect pests. Nature, 273:513-519.
Beirne B.P. 1975. Biological control attempts by introductions against pest insects in the field in Canada. Can. Ent. 107:225-236.
Bird, F.T. 1961. Transmission of some insect viruses with particular reference to ovarial transmission and its importance in the development of epizootics. J. Insect Pathol. 3:352-380.
Bird, F.T. and D.E. Elgee. 1953. Virus disease and introduced parasites as factors controlling the European spruce sawfly, Diprion hercyniae (Htg.), in central New Brunswick. Can. Ent. 89:371-378.
Buckner, C.H. 1969. The common shrew (Sorex araneus) as a predator of the winter moth (Operophtera brumataT near Oxford, England. Can. Ent. 101:370-375.
Burnett, T. 1958. Effect of host distribution on the reproduction of Encarsia formosa Gahan (Hymenoptera: Chalcidoidea). Can. Ent. 90:179-191.
Cannings, E.U. and R.J. Barker. 1982. Transmission of microsporidia between generations of winter moth Operophtera brumata. Proc. Brit. Soc. Parasit. 1981. Parasitology, 84:xiv (abstracts).
Caughley, G. 1980. Analysis of Vertebrate Populations John Wiley and Sons.
Cheng, L. 1969. The life history and development of Lypha dubia Fall. (Dipt., Tachinidae). Entomolgist, 101:25-32.
Cheng, L. 1970. Timing of attack by Lypha dubia Fall. (DipteratTachinidae) on the winter moth Operophtera brumata (L.) (Lepidoptera:Geometridae) as a factor affecting parasite success. J. Anim. Ecol. 39:313- 320. 202
Clark, E.C. 1958. Ecology of the polyhedroses of tent caterpillars. Ecology, 39:132-139.
Commins, H.N. and M.P. Hassell. 1979. The dynamics of optimally foraging predators and parasitoids. J. Anim. Ecol. 48:335-351.
Connell, J.H. and W.P. Sousa. 1983. On the evidence needed to judge ecological stability or persistance. Am. Nat. 121:789-824.
Cunningham, J.R., N.V. Tonks and W.J. Kaup. 1981. Viruses to control winter moth, Operophtera brumata (Lepidoptera: Geometridae). J. Ent. Soc. British Columbia, 78:17-24.
Dempster, J.P. 1983. The natural control of populations of butterflies and moths. Biol. Rev. 58:461-481.
Dixon, A.F.G. 1976. Timing of egg hatch and viability of the sycamore aphid, Drepanosiphum platanoidis (Schr.), at bud burst of Sycamore, Acer pseudoplatanus L. J. Anim. Ecol. 45:593-603.
East, R. 1974. Predation on the soil dwelling stages of the winter moth at Wytham Wood, Berkshire. J. Anim. Ecol. 43:611-626.
Ehler, L.E. and R.W. Hall. 1982. Evidence for competitive exclusion of introduced natural enemies in biological control. Env. Ent. 11:1-4.
Eidt, D.C. and D.G. Embree. 1968. Distinguishing larvae and pupae of the winter moth, Operophtera brumata, and the Bruce spanworm, 0. bruceata (Lepidoptera:Geometridae). Can. Ent. 100:536-539.
Embree, D.G. 1965. The population dynamics of the winter moth in Nova Scotia, 1954-1962. Mem. Ent. Soc. Can. 46:1-57.
Embree, D.G. 1966. The role of introduced parasites in the control of the winter moth in Nova Scotia. Can. Ent. 98:1159-1168.
Embree, D.G. 1970. The diurnal and seasonal pattern of hatching of winter moth eggs, Operophtera brumata (Geometridae:Lepidoptera). Can. Ent. 102:759-768.
Embree, D.G. 1971. The biological control of winter moth in eastern Canada by introduced parasites. I_n Huffaker, C.B. (ed) Biological Control. Plenum Press, New York, pp. 217-226. 203
Embree, D.G. and P. Sisojevic. 1965. The bionomics and population density of Cyzenis albicans (Fall.) (Tachinidae:Diptera) in Nova Scotia. Can. Ent. 97:631-639.
Embree, D.G. and I.S. Otvos. 1984. Operophtera brumata (L.), winter moth (Lepidoptera:Geometridae). (In) JTS. Keleher and M.A. Hulme. (eds) Biological Control Programmes Against Insects and Weeds in Canada 1969 ^ 1980. Commonwealth Agr. Bureaux, Slough, pp. 353-357.
Entwistle, P.F. and P.H. Adams. 1977. Prolonged retention of infectivity in the nuclear polyhedrosis virus of Gilpinia hercyniae (Hymenoptera, Diprionidae) on foliage of spruce species. J. Invert. Pathol. 29:392-394.
Entwistle, P.F., P.H. Adams and H.F. Evans. 1977a. Epizootiology of a nuclear-polyhedrosis virus in European spruce sawfly Gilpinia hercyniae) : the status of birds as dispersal agents of the virus during the larval season. J. Invert. Pathol. 29:354-360.
Entwistle, P.F., P.H. Adams and H.F. Evans. 1977b. Epizootiology of a nuclear-polyhedrosis virus in European spruce sawfly Gilpinia hercyniae : birds as dispersal agents of the virus during winter. J. Invert. Pathol. 30: 15-19.
Frank, J.H. 1967a. The insect predators of the pupal stage of the winter moth, Operophtera brumata (L.) (Lepidoptera:Hydriomenidae). J. Anim. Ecol. 36:375-389.
Frank, J.H. 1967b. The effect of pupal predators on a population of winter moth, Operophtera brumata (L.) (Hydriomenidae). J. Anim. Ecol. 36:611-621.
Frazer, B.D. and N. Gilbert. 1976. Coccinellids and aphids: A quatitative study of the impact of adult ladybirds (Coleoptera:Coccinellidae) preying on field populations of pea aphids (Homoptera:Aphididae). J. Ent. Soc. B.C. 73: 33-56.
Gillespie, D.R., T. Finlayson, N.V. Tohks and D.A. Ross. 1978. Occurrance of the winter moth, Operophtera brumata (Lepidoptera:Geometridae), on southern Vancouver Island, British Columbia. Can. Ent. 110:223-224.
Gillespie, D.R., and T. Finlayson. 1981. Final-instar larvae of native hymenopterous and dipterous parasites of Operophtera spp. (Lepidoptera:Geometridae) in British Columbia. Can. Ent. 113:45-55. 204
Griffiths, K.J. 1969. The importance of coincidence in the functional and numerical responses of two parasites of the European pine sawfly, Neodiprion sertifer. Can. Ent. 101 .-673-713.
Hall, R.W. and L.E. Ehler. 1979. Rate of establishment of natural enemies in biological control. Bull. Ent. Soc. Am. 25:280-282.
Hall, R.W., L.E. Ehler and B. Bisabri-Ershadi. 1980. Rate of success in classical biological control of arthropods. Bull. Ent. Soc. Am. 26:111-114.
Hassell, M.P. 1968. The behavioural response of a tachinid fly ( Cyzenis albicans (Fall.)) to its host, the winter moth C Operophtera brumata (L.)). J. Anim. Ecol. 37:627-639.
Hassell, M.P. 1969. A study of the mortality factors acting upon Cyzenis albicans (Fall.), a tachinid parasite of the winter moth (Operophtera brumata) (L.). J. Anim. Ecol. 38:329-339.
Hassell, M.P. 1978. The Dynamics of Arthropod Predator-Prey Systems. Princeton Univ Press, Princeton. 237 pp.
Hassell, M.P. 1980. Foraging strategies, population models and biological control: a case study. J. Anim. Ecol. 49:603-628.
Hassell, M.P. 1982. Patterns of parasitism by insect parasitoids in patchy environments. Ecol. Ent. 7:365-377.
Hassell, M.P. 1985. Insect natural enemies as regulating factors. J. Anim. Ecol. 54:323-334.
Hassell, M.P. and R.M. May. 1973. Stability in insect host- parasite models. J. Anim. Ecol. 42:693-726.
Hassell, M.P. and R.M. May. 1974. Aggregation in predators and insect parasites and its effect on stability. J. Anim. Ecol. 43:567-594.
Heads, P.A. and J.H. Lawton. 1983. Studies on the natural enemy complex of the holly leaf-miner: the effects of scale on the detection of aggregation responses and the implications for biological control. Oikos, 40:267-276.
Holliday, N.J. 1977. Population ecology of winter moth (Operophtera brumata) on apple in relation to larval dispersal and time of bud burst. J. Appl. Ecol. 14:803-813. 205
Humble, L.M. 1985a. Final-instar larvae of native pupal parasites and hyperparasites of Operophtera spp. (Lepidoptera:Geometridae) on southern Vancouver Island. Can. Ent. 117:525-534.
Humble, L.M. 1985b. Seasonal activity of Ichneumonid pupal parasites of Operophtera spp. (Lepidoptera:Geometridae). J. Ent. Soc. British Columbia. 82:44-46.
Iwao, S.I. and E. Kuno. 1971. An approach to the analysis of aggregation pattern in biological populations. (In) Statistical Ecology Vol. I. Spatial Patterns and Statistical Distributions. G.E. Patil, E.C. Pielou and W.E. Waters. Teds! Penn. State Univ. Press, University Park. pp. 461-513.
Jaques, R.P. and H.T. Stultz. 1966. The influence of a virus disease and parasites on Spilonota ocellana in apple orchards. Can. Ent. 98:1035-1045.
Kobayashi, S. 1966. Process generating the distribution pattern of eggs of the common cabbage butterfly, Pieris rapae crucivora. Res. Popul. Ecol. 8:51-61.
Kowalski, R. 1976. biology of Philonthus decorus (Coleoptera:Staphylinidae) in relation to its role as a predator of winter moth pupae [ Operophtera brumata (Lepidotera:Geometridae)]. Pedobiologia, 16:233-242.
Kowalski, R. 1977 Further elaboration of the winter moth' population models. J. Anim. Ecol. 46:471-482.
Kozhanchikov, I.V. 1950. Life cycle development and geographical distribution of frost spanworm, Operophtera brumata (L.). (In Russian) Entomol. Obozr. (Ent. Review) 31:178-197.
Lawton,. J.H. and S. McNeill. 1979. Between the devil and the deep blue sea: on the problem of being an herbivore. (In) Population Dynamics. R.M. Anderson, B.D. Turner and L.R. Taylor (eds), Blackwell, Oxford, pp. 223-244.
MacDonald, P.D.M. and L. Cheng. 1970. A method of testing for synchronization between host and parsite populations. J. Anim. Ecol. 39:321-331.
MacPhee, A.W. 1967. The winter moth Operophtera brumata (Lepidoptera:Geometridae), a new pest attacking apple orchards in Nova Scotia, and its coldhardiness. Can. Ent. 99:829-834.
May, R.M. 1978. Host-parasitoid systems in patchy environments: a phenomenological model. J. Anim. Ecol. 47:833-843. 206
May, R.M., M.P. Hassell, R.M. Anderson and D.W. Tonkyn. 1981. Density-dependence in host-parasitoid models. J. Anim. Ecol. 50:855-865.
Morrison, G. and D.R. Strong. 1980. Spatial variations in host density and the intensity of parasitism: some empirical examples. Env. Ent. 9:149-152.
Munster-Swendsen, M. and G. Nachman. 1978. Asynchrony in insect host-parasite interaction and its effect on stability, studied by a simulation model. J. Anim. Ecol. 47:159-171.
Murdoch, W.W., J.R. Reeve, C.B. Huffaker and C.E. Kennett. 1984. Biological control of olive scale and its relevance to ecological theory. Am. Nat. 123:371-392.
Price, P.W., C.E. Bouton, P. Gross, B.A. McPheron, J.N. Thompson and A.E. Weis 1980. Interactions among three trophic levels: influence of plants on interactions between insect herbivores and natural enemies. Ann. Rev. Ecol. Syst. 11:41-65.
Rausher, M.D. 1983. Ecology of host selection behavior in phytophagous insects. Variable Plants and Herbivores in Natural and Managed Systems Denno, R.F and M.S. McClure (eds). Academic Press, New York. pp. 223-257.
Reeve, J.D. and W.W. Murdoch. 1985. Aggregation by parasitoids in the successful control of the California red scale: a test of theory. J. Anim. Ecol. 54:797-816.
Rohlf, F.J. and R.R. Sokal. 1981. Statistical Tables. W.H. Freeman and Co., New York. 219 pp.
Roland, J., S.J. Hannon and M.A. Smith. 1986. Foraging pattern of pine siskins and its influence on winter moth survival in an apple orchard. Oecologia, (in press).
Royama, T. 1981. Evaluation of mortality factors in insect life table analysis. Ecol. Monog. 51:495-505.
Royama, T. 1984. Population dynamics of the spruce budworm Choristoneura fumiferana. Ecol. Monog. 54:429-462.
Severin, H.H.P., H.C. Severin and W. Hartung 1915. The stimuli which cause the eggs of leaf-ovipositing Tachinidae to hatch. Psyche, 22:132-137.
Smith, K.M. 1976. Virus-insect relationships. Longman Group Ltd. New York. 291 pp. 207
Stairs, G.R. 1966. Transmission of virus in tent caterpillar populations. Can. Ent. 98:1100-1104.
Stamp, N. 1982. Searching behaviour of parasitoids for web-making caterpillars: a test of optimal searching theory. J. Anim. Ecol. 52:387-395.
Steinhaus, E.A. 1958. Crowding as a possible stress factor in insect disease. Ecology, 39:503-514.
Steinhaus, E.A. and J.P. Dineen. 1960. Observation on the role of stress in granulosis of the variegated cutworm. J. Insect Pathol. 2:55-65.
Thiele, H.U. 1977. Carabid Beetles and Their Environments. Springer-Verlag, Berlin 369 pp.
Thompson, C.G. and E.A. Steinhaus. 1950. Further tests using a polyhedrosis virus to control the alfalfa caterpillar. Hilgardia, 19:411-445.
Varley, G.C. 1947. The natural control of population balance in the knapweed gall-fly (Urophora jaceana) . J. Anim. Ecol. 16:139-187.
Varley, G.C. 1963. The interpretation of change and stability in insect populations. Proc. R. Ent. Soc. Lond. (C) 27:52-57.
Varley, G.C. and G.R. Gradwell. 1958. Oak defoliators in England. Proc. 10th Int. Cong. Ent. 4:133-136.
Varley, G.C. and G.R. Gradwell. 1960. Key factors in population studies. J. Anim. Ecol. 29:399-401.
Varley, G.C. and G.R. Gradwell. 1963a. The interpretation of insect population changes. Proc. Ce-ylon Assn. Adv. Sci., 1962. 18:142-156.
Varley, G.C. and G.R. Gradwell. 1963b. Predatory insects as density dependent mortality factors. Proc. 16th Int. Cong. Zool., 1:240.
Varley, G.C. and G.R. Gradwell. 1968. Population models for the winter moth. In: Insect Abundance (Ed by T.R.E. Southwood), Symp. Roy. Ent. Soc. London, No 4, Oxford.
Varley, G.C. and G.R. Gradwell. 1971. The use of models and life tables in assessing the role of natural enemies. (In) Biological Control. C.B. Huffaker (ed.) Plenum Press, New York. 511 pp. 208
Varley, G.C., G.R. Gradwell and M.P. Hassell. 1973. Insect Population Ecology: an Analytical Approach. Blackwell Scientific Publications, Oxford.
Vaughn, J.L. 1974. Virus and rikketsial disease. (In) G.E. Cantwell (ed.) Insect Diseases. Marcel Dekker Inc, New York.
Wang, Y.H. and A.P. Gutierrez. 1980. An assessment of the use of stability analyses in population ecology. J. Anim. Ecol. 49:435-452.
Weiser, J. 1958. Protozoan diseases in insect control. Proc. 10th Int. Cong. Ent. 4:681-685.
Wigley, P.J. 1976. The epizootiology of a nuclear polyhedrosis virus of the winter moth, Operophtera brumata L., at Wistman's Wood, Dartmoor. D. Phil. Thesis Oxford University, England. 185 pp.
Williamson, G.D. 1981a. Insect liberations in Canada. Parasites and predators. 1979. Agriculture Canada, Research Branch. 19 pp.
Williamson, G.D. 1981b. Insect liberations in Canada. Parasites and predatores. 1980. Agriculture Canada Research Branch. 19 pp.
Williamson, G.D. 1984. Insect liberations in Canada. Parasites and predators. 1981. Agriculture Canada, Research Branch. 19pp.
Wint, W. 1983. The role of alternative host-plant species in the life of a polyphagous moth, Operophtera brumata (Lepidoptera:Geometridae). J. Anim. Ecol. 52:439-450.
Witter, J.A. and L.A. Waisanen. 1978. The effect of differential flushing times among trembling aspen clones on totricid caterpillar populations. Env. Ent. 7:139-143. 209
APPENDIX A
On Vancouver Island, adult winter moth emerge in November and December, and lay eggs on a variety of deciduous trees. Eggs hatch from mid-March through early April, and Cyzenis adults emerge from the soil. Cyzenis do start ovipositing until late April, at the time winter moth have reached the fourth and fifth larval instar. Parasite eggs are ingested by host larvae, and the fully developed first-instar maggot within these eggs emerges within minutes of being ingested. Within less than one hour, maggots localize in the salivary glands of the host. Development of the parasited maggot does not continue until the host pupates.
Both parasitized and unparasitized larvae drop to the ground to pupate in late May and early June. The parasite begins rapid development, and becomes a pharate adult by mid-July. Both host and parasite remain in the soil until emergence as adults. Winter moth emergence occurs within five months, but the parasitic fly remains in the soil for almost 10 months. 210
APPENDIX B
Some female winter moth are able to bypass stocking traps, and result in lowered estimates of adult density and inflated estimates of mortality of pupae in the soil. The magnitude of this bias was assessed in the fall of 1983 by placng sticky traps six inches above stocking traps on the trunks of 15 trees. Sticky traps consisted of masking tape wrapped around the trunk, and "tanglefoot" applied to the taped area. Sticky traps would thereby catch any females which were able to bypass the stocking traps. Total number of females ascending each tree was obtained by adding the number in the stocking trap and on the sticky trap. The relationship between total number trapped vs number trapped in stocking trap was determined by linear regression:
total = 27.5 + 1.19 * (number in stocking trap)
This regression was highly significant (F=268.3, df=1, P<0.0001, r2=0.95). All counts of females trapped in stocking traps were adjusted using the above relationship. 21 1
APPENDIX C
The following tables illustrate the calculations in the re-analysis of Embree's (1965) data.
Table 1. Parasitism from introduced parasitoids at site 1-2 at Oak Hill, Nova Scotia converted to k-values. (data from Embree [1965] Table XXII, p 53).
percent parasitism k-value
year Cyzenis Agrypon Total (total)
1 956 1 1 0.004 1957 2 2 0.009 1958 9 9 0.04 1959 24 1 25 0.12 1960 39 22 61 0.41 1961 50 50 0.30
Table 2. Partitioning of mortality (k-values) from parasitism into that from introduced parasitoids and from endemic nemato
total parasitism parasitism from parasitism introduced parasites nematode year (Embree's Figure 27) (from Table 1 above) (differen
1 956 0.004 1 957 0.05 0.009 0.04 1958 0. 10 0.04 0.06 1959 0.08 0.12 -0.04 1960 0.80 0.41 0.39 1 961 1 .20 0.30 0.90 212
Table 3. Inclusion of nematode-caused mortality with soil mortality.
mortality mortality in soil from nematodes total mortality year (Embree Fig 27) (Table 2 above) (soi1+nematodes)
1956 0.25 0, 25 1957 0.20 0.04 0, 24 1958 0.18 0.06 0, 24 1959 0.20 -0.04 0, 1 6 1960 0.35 0.39 0, 74 1961 1.20 0.90 2, 1 0
Column 2 of Table 2, and column 4 of Table 3 are plotted in Figure 32b, Chapter four. 213
APPENDIX D
In 1982, density of pupae in the soil was not measured in the orchard or oakwood, but level of defoliation was visually estimated at approximately 25 percent. Data from other years were used to establish the relationship between level of defoliation (estimated from each leaf cluster during routine sampling, [Chapter 2]), and the resulting density of pupae (estimated from number of pupae in pupation trays under each tree [Chapter 2]). The resulting regression was:
pupae/m2 = 9.4 * (defoliation) - 169
where defoliation is in units from 0 to 100. This regression was highly significant (F=67.1, df=1, P<0.0001, r2=0.8l).
With defoliation in the orchard being 25%, the estimated density of pupae in 1982 is 66 per m2. Mean density of adults emerging in autumn of 1982 was 46.2 per m2 (S.E.=1.74, N=32 trees). Mortality (k-value) in the soil in 1982 was therefore: log(66/46) or 0.16. This value is used for mortality in the soil for both the orchard and the oakwood in Figure 34, Chapter four.