Host Plant Variation and Population Limitation of Two
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HOST PLANT VARIATION AND POPULATION LIMITATION OF TWO INTRODUCED INSECTS by PETER D.S. MORRISON B.S. Stanford University 1978 A THESIS SUBMITTED IN PARTIAL FULFILLMENT THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept this thesis as conforming to th required standard THE UNIVERSITY OF BRITISH COLUMBIA December 1986 @ Peter D.S. Morrison, 1986 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 7nn-jnzv The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date February 22, 1987 r>F-fin/ft-n ABSTRACT The response to host plant variation shapes the long-term success of phytophagous insects. Two gall-forming tephritid flies, Urophora affinis and U. quadrifasciata, oviposit in flower buds of Centaurea diffusa and C. maculosa (Asteraceae). Females of both fly species chose among plants, among groups of buds on plants, and among buds. Among plant choices were correlated with buds per plant. Among bud choices corresponded to larval developmental requirements. Insect attack led to gall formation, bud abortion, and reduced seed production. Bud abortion, caused by probing females, limited gall densities. Increased densities of U. affinis females relative to oviposition sites led to more U. affinis galls, increased bud abortion, fewer U. quadrifasciata galls, and fewer seeds. A temporal refuge for seed production was observed. Plants compensated only slightly for aborted buds. Bud abortion may increase the search time between successful ovipositions. A simulation model based on this premise implied that bud abortion may dramatically reduce total gall formation. Plant quality was manipulated in an attempt to shift three population limiting factors. Plants responded to fertilization and watering with an increase in bud numbers. Except for two year-site-treatment combinations, galls per developed bud did not differ significantly between treatments. Treated plants did not differ in their propensity to abort buds. U. affinis larvae developed faster in fertilized plants. Among year comparisons showed that the density of buds available for oviposition was limited by precipitation, non- random insect attack, and, in the longer term, by the reduction in seed production due to fly attack. Bud densities, in turn, limited gall densities. iv TABLE OF CONTENTS ABSTRACT 11 LIST OF TABLES ix LIST OF FIGURES xiii ACKNOWLEDGEMENTS XV INTRODUCTION 1 ORGANISMS 4 Plants 4 Insects 5 GENERAL METHODS 8 Study sites 8 Bud descriptions 11 Statistical methods 14 I. THE EFFECT OF HOST SELECTION ON THE POPULATION DYNAMICS OF TWO INTRODUCED INSECTS 16 MATERIALS AND METHODS 19 Observation methods 19 Plant collections 21 Calorific content 21 RESULTS 23 Variation in resources 23 Insect choice 23 Among plants 23 Among buds on plants 27 Consequences of insect choice 32 Among plants 32 Among buds on plants 40 DISCUSSION 47 Variation in bud productivity 47 Bud abortion and population limitation 48 Basis for choice 49 Insect interactions 51 Summary 52 II. THE EFFECT OF TIMING OF ATTACK ON THE POPULATION DYNAMICS OF TWO INTRODUCED INSECTS 53 MATERIALS AND METHODS 57 Observation methods 57 Plant collections 58 Insect density manipulation 59 RESULTS 62 Insect attack and bud initiation 62 Insect attack 62 Bud initiation 65 Interaction 65 Changes in plant allocation 77 Bud growth and development 77 Compensatory reproduction 77 Insect density manipulation 81 DISCUSSION 85 Changes in insect density 85 Seed refuge 87 Compensatory reproduction 88 vi Evolutionary consequences 89 Summary 91 APPENDIX IIA. EFFECT OF COLLECTION DATE 93 APPENDIX IIB. EFFECT OF DENSITY ENCLOSURES 95 III. BUD ABORTION AND POPULATION LIMITATION OF UROPHORA AFFINIS (DIPTERA: TEPHRITIDAE) IN BRITISH COLUMBIA 97 MATERIALS AND METHODS 100 Bud collection and dissection 100 RESULTS 102 DISCUSSION 104 Oviposition behaviour 104 Search time between ovipositions 105 Model formulation 107 Model results 109 Summary 114 APPENDIX IIIA. LISTING OF THE NUMERICAL MODEL 116 IV. PLANT QUALITY AND THE POPULATION DYNAMICS OF TWO INTRODUCED INSECTS 120 MATERIALS AND METHODS 123 Weather 123 Experimental treatments in 1 979 123 Nutrient analysis 125 Experimental treatments in 1980 128 Observation of insects in 1979 129 Observation of insects in 1980 132 Plant collections and dissection 132 Additional statistical methods 134 vii RESULTS 135 Response of plants 135 Response to plants 139 Gall flies 139 Other herbivores 145 Change in interaction 146 Attack levels 1979 146 Attack levels 1980 149 Larval survival and development 152 DISCUSSION 154 Response of plants 154 Response to plants 154 Gall flies 154 Other herbivores 155 Change in interaction 156 Effect of bud abortion 156 Larval survival and development 158 Effect of plant quality on population dynamics 159 Fertilization as a management tool 160 Summary 161 V. POPULATION LIMITATION OF TWO INTRODUCED INSECTS: PROCESSES WITHIN AND BETWEEN YEARS 163 MATERIALS AND METHODS 166 Weather 166 Plant collections in 1979 166 Plant collections in 1980 167 Plant collections in 1981 168 viii Plant dissections 168 RESULTS 169 Among year differences 169 Effect of rainfall 173 DISCUSSION 176 Bud density 176 Effect of bud density on gall density 182 Gall distributions 183 Interaction between gall fly species 187 Summary 193 APPENDIX VA. ESTIMATION OF BUD AVAILABILITY 194 Methods 194 Results 195 CONCLUDING DISCUSSION 202 LITERATURE CITED 207 i x LIST OF TABLES Table 1.1 Calorific content of developed diffuse knapweed buds by branching category 24 Table 1.2 Observed and predicted distributions of U. affinis (UA) and U. quadri fasc iata (UQ) adults among branching categories 30 Table 1.3 Distribution of U. affinis (UA) and U. quadr i fasc iata (UQ) adults by size of diffuse knapweed buds 31 Table 2.1 Dates on which buds were first observed and corresponding bud initiation categories 58 Table 2.2 Mean day of observation for different categories of Urophora flies on diffuse knapweed plants, Robertson's 1980 62 Table 2.3 Counts of Urophora flies observed in density enclosures, Robertson's 1980 81 Table 2.4 Effect of gall fly density manipulations on diffuse knapweed characteristics, gall production, seed production, and bud abortion 82 Table 2.5 Effect of plant collection date on diffuse knapweed characteristics and gall fly attack, Robertson's 1980 94 Table 2.6 Effect of Urophora enclosures on diffuse knapweed characteristics and gall fly attack 95 Table 3.1 Urophora eggs and larvae and proportion of buds X aborted for terminal buds of diffuse knapweed, Robertson's 1980 102 Table 3.2 Duration of probing into spotted knapweed buds by female U. affinis 106 Table 3.3 Listing of the numerical model 117 Table 4.1 Soil characteristics at the study sites 128 Table 4.2 Effect of fertilization and watering on the total number of diffuse knapweed buds and the number of developed buds, 1979 135 Table 4.3 Effect of fertilization and watering on diffuse knapweed characteristics and insect attack, Ned's Creek 1979 136 Table 4.4 Effect of fertilization and watering on diffuse knapweed characteristics and insect attack, Robertson's 1979 137 Table 4.5 Effect of fertilization and watering on diffuse knapweed characteristics and insect attack, Robertson's 1980 139 Table 4.6 Effect of fertilization and watering on the total number of spotted knapweed buds and the number of developed buds, 1979 140 Table 4.7 Effect of fertilization and watering on spotted knapweed characteristics and insect attack, Chase 1979 .141 Table 4.8 Effect of fertilization and watering on spotted knapweed characteristics and insect attack, Chase 1980 .144 Table 4.9 Effect of fertilization and watering on the proportion of knapweed buds chewed, 1979 146 xi Table 4.10 Effect of fertilization and watering on the proportion of knapweed buds aborted, 1979 147 Table 4.11 Effect of fertilization and watering on the number of U. affinis galls per developed bud, 1979 148 Table 4.12 Effect of fertilization and watering on the proportion of knapweed buds developed, 1979 149 Table 4.13 Effect of fertilization and watering on the number of U. quadrifasciata galls per developed bud, 1979 150 Table 4.14 Contents of Urophora galls from control and treated diffuse knapweed plants, Robertson's 1980 152 Table 5.1 Diffuse knapweed characteristics and Urophora attack levels, Ned's Creek 1979-1980 169 Table 5.2 Diffuse knapweed characteristics and Urophora attack levels, Robertson's 1979-1981 171 Table 5.3 Spotted knapweed characteristics and Urophora attack levels, Chase 1979-1981 172 Table 5.4 Urophora galls per developed bud at the three study sites, 1973-1981 184 Table 5.5 Proportion of diffuse knapweed buds unattacked and estimated proportion of buds unavailable to ovipositing gall flies, Ned's Creek 1973-1980 198 Table 5.6 Proportion of diffuse knapweed buds unattacked and estimated proportion of buds unavailable to ovipositing gall flies, Robertson's 1977-1981 199 Table 5.7 Proportion of spotted knapweed buds unattacked and estimated proportion of buds unavailable to xi i ovipositing gall flies, Chase 1973-1981 200 xiii LIST OF FIGURES Figure 0.1 Location of the study sites 9 Figure 0.2 Numbering scheme for knapweed buds 12 Figure 1.1 Distribution of both species of gall flies among diffuse knapweed plants 25 Figure 1.2 Total gall flies and total buds on diffuse knapweed plants 28 Figure 1.3 Total U. affinis adults and U.