FACTORS INFLUENCING EUROPEAN
CORN BORER POPULATIONS
IN OHIO
DISSERTATION
Presented in Partial Fulfillment of the Requirement for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
MOHAMED TAHIR KIRA, B. Sc., M. Sc.
******
The Ohio State University 1958
Approved by:
jffj Adviser Department of Zoology and Entomology ACKNOWLEDGMENTS
I wish to express my thanks to Dr. D. M. DeLong, Dr. C. R.
Neiswander, and to Dr. L. H. Rolston for their helpful super vision, suggestions, and criticisms in the preparation of this dissertation. Special thanks are also due to W. D. Guthrie for furnishing the corn borer egg masses required for this experi ment and for helping in obtaining the data presented herein; to
C. C. King for his continuous and valuable service in all phases of this problem.
The contributions of C. A, Triplehorn and the Statistical
Laboratory at Iowa Agricultural Experiment Station were greatly appreciated. I am grateful to Mrs, Hilda Lea for typing the dissertation and to Glenn Berkey and Clarke Robey for their photographic contributions. TABLE OF CONTENTS
Page
INTRODUCTION...... 1
REVIEW OF LITERATURE...... 3
Factors Influencing European Corn Borer Populations . 3
Natural Enemies and the Borer Populations ...... 9
Arthropods ...... 9
Enemies other than arthropods...... 11
Effect of Corn Resistance or Susceptibility on Borer
Populations . 11
Effect of Planting Date on Borer Populations. .... 19
The Damage Done by Borer...... 26
PROCEDURES AND TECHNIQUE...... 35
Preparing the Field of Experiment...... 35
Plot Layout ......
Planting and Maintaining the Corn Plants...... 40
Artificial Infestation...... *f0
Hand application...... ^0
Laboratory production of corn borer egg masses . k2.
Sampling and Data Recorded...... kk
RESULTS . . k9
Effect of Resistance and Susceptibility on European
Corn Borer Populations...... 51
First Generation Population...... 51
Natural oviposition ...... 51
Larval establishment and rate of development 55 Page
Summer population...... 62
Fall Population . . . . » . . •• ...... 66
Effect of Date of Planting on European Corn Borer
Populations...... 71
First Generation Population ...... , * 71
Natural oviposition...... 71
Larval establishment and rate of development 72
Summer Population . . . 78
Fall Population ...... 79
Combined Effect of Corn Hybrids and Planting Dates on
European Corn Borer Populations ...... 8l
First Generation Population ...... 8l
Fall Population and Second Generation . 90
Damage Done by the European Corn Borer ...... 100
Burrows and Leaf Lesions...... 101
Yield • ...... • • . • * . •• 110
SUMMARY...... Il8
APPENDIX ...... 120
LITERATURE CITED ...... 132
AUTOBIOGRAPHY...... 136
iv LIST OF TABLES
Table Page
1. Showing borer population per hundred stalks for all varieties and each planting date, 1926. , and 192*7 $ * • • • * • • • • • • • • • • • • . • • « 2Q
2. Yield of susceptible and resistant varieties. early planted treatment 4 compared with treatment 1. 31
3. Yield of.susceptible and resistant varieties late planted treatment 4 compared with treatment 1. 32
4. Reduction in growth of susceptible plants as a result of borer infestation. Early planting. . . 33
5 . Reduction in yield of susceptible plants by borer attack...... • . . * 34
6. First generation egg masses by hybrid^ x planting date « . . • . •...... « • 32
7» Firsts generation egg masses in hybrids within dates of planting ...... • 53
8. Larval establishment and rate of development in hybrids within dates ofplanting. Treatment 2. 58
9 . Larval establishment and rate of development in hybrids within dates ofplanting. Treatment 4. 59
10. Midsummer dissection. Analysis of variance. . 63
11. Number of first generation borers in hybrids within dates of planting...... 64
12. First generation pupae in hybrids within dates of planting...... 65
13. Fall population. Analysis of variance. . . . 67
14. Fall dissection. Borer population in hybrids within planting dates...... 68
15. First generation egg masses in date of planting within hybrids ...... 72
v Table Page
16. Rate of larval development on early and late plants within hybrids. Treatment 2 ...... 76
17. Rate of larval development on early and late plants within hybrids. Treatment ...... 77
18.‘ Number of first generation borers in dates of planting xd.th.in hybrids...... 78
19. First generation pupae in dates of planting within hybrids * ...... 79
20. Fall population in dates of planting within hybrids...... 80
21. Rate of larval development in hybrids and piantang date... . * • ... . «« ...... 82
22. Comparison of larval establishment in treat ments 2 , 3 ? and ...... ^5
23. First generation population by hybrids and planting dates ...... 88
2.k. First generation poioulation by treatment in hybrids and planting dates , ...... 88
25. First generation pupae in hybrids and date of planting ...... 90
26. Fall population. No. of larvae by hybrids and planting dates ...... 92
27* Fall population. Ho. of larvae within hybrids and dates of planting...... 93:
28. Fall population. Ho. of larvae by hybrids and planting dates ••••••••••■■•••»• 93
29* Fall population. Ho, of larvae in hybrids of each planting date in all treatments ...... 93
30. Second generation larval populations. .... 96
31. First generation borer population in treatments 3 and 8 ...... 98
32. Estimate of 1956 second generation larvae on treatment 3» • • ...... 99 vi Table Page
33. First generation borers, burrows, and leaf lesions, in hybrids of each planting date...... 102
34. No. of burrows and leaf lesions in hybrids within date of planting.:(1st generation damage) . . 105
35. Ho. of burrows and leaf lesions in dates of planting within hybrids.(1st generation damage). . . 106
36. No. of burrows in hybrids within dates of planting. (Second generation damage)...... 107
37* Ho. of burrows in dates of planting within hybrids. (Second generation damage) ...... 108
38. Comparison between first and second generation damage...... 109
39. Effect of first generation infestation on infestation by second generation borers ...... 110
k-0. Yield by treatment within hybrids and dates of planting...... Ill
kl. Yield for both hybrids and planting dates. . . . 11^-
Comparison between reduction in yield of treat ments 3 , 6 , and 8 of both hybrids of the early planting...... 116
vii LIST OF'APPENDIX. TABLES
Table Page
1 . Borers, midsummer dissection. Analysis of variance• 1956•• • • • • .. • • ... .. 120
2 . Burrows, midsummer dissection. Analysis of variance. 1956...... 121
3 . Leaf lesions, midsummer dissection. Analysis of variance. 1 9 5 6 ...... • • • •...... 122
k. Borers, midsummer dissection. Analysis of variance. 1957* • • • • • • • • • • • • • • • • • • 123
5 . Burrows, midsummer dissection. Analysis of variance. 1957** • • • • • • • •• * •• . . 12^
6 . Leaf lesions, midsummer dissection. Analysis of variance. 1957 • • • • • ...... 125
7. Larvae, fall dissection. Analysis of variance. 1956...... 126
8 . Burrows, fall dissection. Analysis of variance. 1956...... • 127
9. Larvae, fall dissection. Analysis of variance. 1957...... 128
10. .Burrows, fall dissection.. Analysis.of variance. 1957...... 129
11. Yield. Analysis of variance. 1956...... 130
12. Yield. Analysis of variance. 1957* * ...... 131
viii LIST OF FIGURES
Figure Page
1. First generation natural oviposition on hybrids within dates of planting...... 5^
2 . Effect of resistance and susceptibility on larval establishment. Treatments 2 and A- ...... 60
3. First, generation natural egg deposition on early and late plantings within hybrids ...... 73
k. Larval establishment on early and late, plantings. Treatments 2 and ...... 75
5» First generation population in hybrids and dates of planting...... 91
6 . Fall population. No. of larvae by hybrids and dates, of planting...... 9%
7. Second generation population by hybrids and dates of planting...... 97
8 . First generation borers, burrows, and lesions in hybrids of each planting date.Treatments...... 103
9. First generation borers, burrows, and lesions in hybrids of each planting date. Treatment 4...... 104
ix INTRODUCTION
The great importance of corn production in many parts of the world has made the ultimate economic effect of the European corn borer, Pyrausta nubilalis (Ebn.), of great concern to the growers of this crop. Differences in the responses of plant varieties to insect attack and the interrelations of plants and insects have been on record for more than a hundred years. Con siderable variation was found in the populations of corn borer on different varieties planted on different dates. Apart from the effect of corn hybrids and their planting dates, a complex of Other factors has its effect on borer population as well as other biological populations. These factors are:
1. Physical factors of the environment which could be changes in weather.
2. The genetic characteristics of the host, morphological or physiological, the host abundance, and the agronomic procedu res such as time of planting (synchronization), and soil con ditions.
3. Biotic factors which include parasites, predators, and diseases, though perhaps failing to produce mortality, may ad versely affect the borer's reproductive efficiency.
k. Borer constitution as a genetic factor which includes the borer reproductive potential, its survival, and voltinism.
1 These factors in their effect on population fluctuations either, operate singly or in an inter-related fashion, thus forming a complex. Therefore it seemed almost impossible in a limited period of two years to study and analyze such a complex in order to determine the net result of these factors operating, together, on the European corn borer population. As a result the present work has been limited to the following objectives:
1. To study the effect of resistance and susceptibility of two widely different hybrids of field corn on both first and second generation borer populations.
2. To study the effect of planting date of both corn hybrids on both first and second generation borer accumulation.
3. To measure the damage done by the borer.
(a) first generation borers,
(b) second generation borers,
(c) total borers, under different levels of infestation and for the two hybrids and to determine the relationship between the fluctuation in borer population and the yield of corn.
The work reported here has been done at the Ohio Agricultural
Experiment Station, Wooster, Ohio, U.S.A., in co-operation with the North Central States Regional Project (NC-20) during the two successive seasons of the years 1956 and 1957* REVIEW OF LITERATURE
The following discussion is concerned with all factors which appear in the literature as factors affecting the Euro pean corn borer population. The literature is full of work done by numerous investigators in different parts of the world on different aspects of the borer problem. As late as 1898 the borer population and the different factors which are thought to be affecting its fluctuation have been subject to immense and detailed study. Factors which are the main objectives of this study might be mentioned in this chapter; however, they will be handled in the following chapters separately and in detail as far as the available references enabled the writer.
Factors Influencing European Corn Borer Populations
Mayers et al. (1937) referred to a publication of Jablo- nowski in 1898, stating that the early varieties of corn suf fered more infestation than the later, i.e. the borer popula tion was higher among the early varieties.
Huber and Neiswander (l927)jin their study of the European corn borer and its ecological habitats in Ohio, showed that the most rapid accumulation of the corn borer has occurred rather uniformly in regions where the Swamp Forest type of vegetation is dominant and where Beech - Maple is the upland type. On the other hand the least rapid accumulation has occurred on the
■ . .3, . . ' higher lands where the Oak - Hickory or Beech - Maple -Pine ty pes are dominant. The heaviest infestation in Ohio occurs in an area that consists largely of reclaimed swamp lands.
Huber and Neiswander - (1928)jin their work on the correlation between soil fertility and European corn borer accumulation,re ported that in as far as soil fertility favorably or unfavorably influences corn development, it can be correlated with the rate of accumulation of the corn borer. Any general region which habitually produces late, slow developing or poor corn will have a lower borer population than a region which habitually produces early, rapid developing or good corn. Also, corn in any field, part of a field, or in any local area that has not grown well, within a region highly favorable to accumulation, may have a less than normal borer population because the condition of corn at the period of moth flight influences the biotic potential.
The same authors stated that there is sufficient evidence
to warrant a correlation between corn development and soil ferti lity. Evidence indicates that the nature of the response of the
corn borer adults to corn depends to a very great degree upon
the condition of the corn or the stage of development attained by the plants at the time the moths are in flight. It follows
then that any condition which influences the development of corn will indirectly influence the degree of infestation by the corn
borer. Therefore, in so far as soil fertility, used in the
widest sense, influences the growth and development of corn, it
also indirectly influences the accumulation of the insect. The evidence supporting the foregoing statement seems ra ther conclusive. Over large areas, such as western Ohio (un
derlain by limestone), and eastern Ohio (where some limestones are found but the formations consist mostly of sandstones and
shales), data indicate the ratio of borers per given unit of
stalks in western Ohio as compared with eastern Ohio is about
three to one.
Neiswander and Huber (l929)>after their work on height and
silking as factors influencing European corn borer populations^
stated that the factors of the environment that "causes” fluc tuations in population of the borer is the stage of development of the corn during the period of moth flight and larval feeding.
For several years their experimental plots at Bono, Ohio, have shown a higher population on early varieties of a given planting date than on late ones, while succeeding planting dates have shown a significant reduction in the borer population of all varieties.
Two of the factors which are known to have a significant
influence on borer populations are height and date of silking.
These two factors may worlc either together or independently. In
the event they work together, as in the case of early and late
planting dates, wide fluctuations in borer population would oc
cur, for under such conditions oviposition and establishment
would be maximum in the former instance and minimum in the later.
Neiswander and Huber mentioned that -the 1927 moth flight
over the variety and date series of plantings-, indicated a close 6 relationship between the height of the corn in the various series of plantings and the number of moths seen flying above the res pective series. From June 27 to July 12'a total of 9^7 moths were observed. For each inch reduction in height in these plantings there was a reduction of 8.8 moths. They stated that
this record does not throw light upon oviposition except in so far as it may be inferred that the moths would deposit eggs in the various local areas of the field in which they were observed.
In two years experiments, 1927? 1928, it was found by
Neiswander and Huber that the greater the average height of corn in the plot the more eggs were deposited. They presented data showing the actual egg deposition on two varieties of corn planted on three different dates. They demonstrated that for
each reduction in height there was a reduction of 14.5 in the number of egg masses deposited.
Concerning the relation between silking and egg deposition,
the writers stated that there is a high correlation between moth flight and date of silking. This correlation might lead to
the conclusion that date of silkingjas such, greatly influences
the moth and consequently the number of eggs deposited.
The same writers, in their experiments at Oak Harbor during
1928, showed the effect of planting date of four varieties of
corn on borer survival. The early planting date was May 10, the
late planting date was June 10. In all four varieties there was
an average reduction of 50.^3 per cent in borer survival in the
June 10 planting. They recorded the results of Mr. E. ,0. ■" 7 Kelshcimer's experiments of 1926 in which he artificially in
fested each of 18 stalks of corn, nine being planted on May 10
and nine on June 10. The corn was all of the same variety planted in adjoining plots. Out of 19^- eggs placed on corn planted May 10, 1J.9 larvae were recovered, whereas out of 19^
eggs placed on June 10 plants only k,l larvae were recovered.
The average number of larvae recovered in the early planting was
10.16 , whereas the average in the late planting was 5*72 with a
difference of k.kh.
Patch (1929), in his experiments with six varieties of corn
near Sandusky, Ohio, during the summers of 192? and 1928, showed
that a high positive correlation was found between the height of
corn and the number of egg clusters laid. He stated that the
earliest plantings of all corn fields usually bear more egg
clusters because of the greater height of such corn. One year's
results indicate that the corn borer moth shows a preference of
corn of very early maturity and of about average height or taller
Next in preference is the variety or type that is tallest through
out the oviposition period of the moth, although such variety is
rather late in maturity. Short varieties, although early in
maturity, and varieties late in maturity and that are relatively
short during the early part of the oviposition period received
the fewest egg clusters.
Savage (1930) reported that the habitat offers a reasonable
explanation of differences in corn borer populations within the
infested area of Ohio. A relatively high correlation between . . 8 types of habitats and corn borer populations occurs even when definite limiting factors are known to vary widely. In any of the 23 northern Ohio counties considered, the corn borer popu lation is correlated significantly with the respective amount of swamp-forest within the county, i.e., where the greatest percentage of swamp-forest occurred the greatest increase in corn borer should occur.
Kozhanchikov (1938) in writing on the geographical distri bution and physiological characters of the borer indicated that it prefers humid biotopes both in Eurasia and North America; re gions having in the period from June to August a mean temperature above 20°C (68°F) and 200 - 300 mm. precipitation being parti cularly favourable. Mass propagation takes place, however, only in limited and widely separated territories, namely, eastern
United States and the adjoining parts of southern Canada, central and south-eastern Europe and south-eastern Asia. The northern limits of its occurence are determined by the sum of ’warm tem peratures in the summer necessary for the completion of one ge neration, the low winter temperatures being less important owing to the high degree of cold-resistance of the hibernating larvae.
Only one generation is produced in northern regions, whereas two or more occur in central Europe, the northern Caucasus, the Rus sian Far East and the eastern United States. Natural Enemies and the Borer Populations
Arthropods.- The literature contains a number of referen ces to the destruction of European corn borer eggs by arthropod predators and parasites,
Hergula (1930) reported that young of the mite Trombidium fuliginosum Koch, destroyed 16 per cent of the eggs in fields under observation. ZwBlfer (1928) also reported this species, and Paillot (1928) reported Allthrombium fuligonsum Herm. as feeding on corn borer eggs. Both Hergula and Paillot, as well as Everly (1938b) and Poos (192?), reported Chrysopids feeding of the eggs. In addition, ZwBlfer also reported the feeding of
Coccinella septempunctata L., and Crawford and Spencer (1922)
Caffrey and Worthley (1927) and Everly (1938a) all reported the feeding of Ceratomegilla fuscilabris (Muls.) on corn borer eggs.
Investigations were carried out at Lafayette, Indiana, in
19^6 by Bartholomai to attempt to determine the proportion of eggs destroyed by predators and the species and numbers of predators present. He stated in 193^ that the most abundant predator species was Ceratomegilla fuscilabris, Hippodamia con- vergens Guer. was present in much smaller numbers, and there was a scattering of Orius insidiosus, mostly in the nymphal stage.
Ladybird larvae, larval and adult lacewings, and Hippo damia
tredecimpunctata (L.) adults were rather rare. Three or fewer
of the following were found: 1 0 :
Coccinella novemnotata Hbst..,
Cycloneda sanguinea (L.),
Adalia bipunctata L.,
Thrips, Reduviids, Nabids and mites.
Of 10,076 eggs laid during the season, Bartholomai stated that 1 ,166, or 11.6 per cent, were destroyed by predators. The first generation borer combined a large number of eggs (9 »06l), a small number of predators (212), and a large number (785) but small per cent (8 .7 ) of eggs was destroyed. In the second generation a small number of eggs (1 ,009) was associated with a large number of predators (1,445) and a small number (381) but high per cent (37*8) of eggs were destroyed.
He concluded that in years when there is a large second generation of the European corn borer, predators might destroy a considerable number of eggs.
Baker et_ al. (19^9) pointed out that the parasitic species known to have become established in the United States are
Lydella stabulans grisescens H.D., Horogenes punctorius (Roman),
Macrocentrus gifuensis Ashm., Eulophus viridulus Thoms., Chelonus annulipes Wesm., and Phaeogenes nigridens V/esm. Twenty-nine species of insects indigenous to the area infested by the corn bo rer in this country have been observed to parasitize the borer but none of them have been sufficiently numerous to have any appreciable effect on borer abundance. 11
Enemies other than arthropods.- Barber (1925) reported that evidence of feeding by birds on larvae of the European corn borer has been found during spring for several years in New England.,
Such work was of two distinct types; the work of woodpeckers, par-- ticularly the downy woodpecker, which drills holes into standing stalks in order to reach the overwintering larvae; and work by grackles, blackbirds, starlings, and other species which shred stalks that have fallen over and devour the hiding larvae. For the most part, he stated, such feeding by birds has been confined to localities that have been heavily infested for several years, the extent of such feeding having been found to vary greatly, but counts have shown that in some fields of sweet corn 90 per cent of the overwintering larvae have been removed from stalks. Although the importance of the feeding by birds on overwintering larvae was not great, in small areas the importance of their feeding must be considerable, since these small areas are often heavily infested.
Beauveria bassiana (Bals), stated Baker et_ al. (19^9), is the only disease organism that has been observed in the United States to kill the corn borer in the field.
Effect of Corn Resistance or Susceptibility on Borer Populations
The variability of European corn borer populations occurring in different strains or varieties of corn has been studied over a period of years with the object of determining the presence of resistance and of selecting a strain or strains showing a rela 12 ' tively high degree of resistance which might be of use in future corn borer work. In addition, some attempts have been made to measure the character or characters existing in the plant which might contribute to resistance.
The variability of corn strains to the number of eggs they receive, their comparative rates of borer survival and their final borer populations have been demonstrated by several workers. Patch (1929.) indicated that greater corn borer popu lation in the earliest plantings of both field and sweet corn are due to the preference of corn borer moth for the tallest corn.
Neiswander and Huber (1929) stated that the. size of the corn borer population is fundamentally the result of either the number of eggs deposited or the rate of larval establishment or both, and that height and silking date are closely correlated with oviposition and establishment. They have shown also that a sig nificant difference exists in the rate of survival on a given strain or variety of corn when in different stages of develop ment during the establishment period of the borer, the later planted and less mature corn having a lower rate of survival than the earlier and more mature corn.;
Ficht (1936) indicated that since the variability of corn strains to oviposition is largely relative to their height during the period of oviposition in the plantings being studied by him, the less vigorous, slower growing and shorter strains appear from their borer populations to have a degree of resistance which might not occur in the absence of taller and more rapidly growing strains. ' ■ . . 15
For this reason, he concluded, the percentage of borers which
"establish themselves" within the plants and become full gro\-/n is probably the more important measurement indicating resistance.
The survival rate, which expresses the percentage of full grown larvae resulting from an estimated number of eggs deposited naturally on a given number of plants, is the factor chiefly con
sidered in the discussion of strains which he tested in his ex periments during the four years 1932-1935 inclusive. He found
that Clement's White Cap, a midseason variety, as would be ex pected from its relative earliness as compared to the other
strains he used, showed a significant lack of resistance as ex pressed by 12.97 per cent corn borer survival, whereas the hybrid
TR X 66 had a survival of k.37 per cent. He tested several
strains every year, of which some strains stood out as indicating
superiority over the remaining corns in having lower borer sur vival. Among the group of strains which have appeared to show a greater degree of resistance, the single-cross hybrid B2 X TR appeared to be outstanding. He stated that it seems probable
that lateness of maturity may play a part, at least in giving
this strain a high rating among those tested.
Patch (1937) mentioned that differences in the numbers of
larvae of the European corn borer surviving from a given number
of eggs laid on various strains of corn have been observed by
several investigators. Most of such differences have been
associated with the earliness and lateness of the strains, but ' ' 14 a few differences have been observed between strains of about the same stage of development during the time when the young borers were becoming established on the plants. In general, the early strains have favored a higher rate of borer survival than the late or midseason strains.
In this work, he tried to record the comparative borer re sistance of two single-cross hybrid strains of corn, 111.R4 X
Ill.Hy., and 111. A X Ind. Tr. The two strains require about the same length of season to attain given stages of plant The number of borers reaching maturity in ten plantings of strain R4 X Hy. made in 1936 was equal to 46.6 per cent of the number in strain A X Tr. Strain Rh X Hy. also showed promise as a high yielding strain. Over a period of five years this strain gave a yield of nine bushels per acre more than the mean yield of all the strains in the tests. The same author stated in 1942 that he carried out investi gations on the control of Pyrausta nubilalis Hbn., on corn in Ohio in 1930-39j by the use of resistant strains, to show the relative.resistance to borer survival contributed by inbred lines of field corn to hybrid combinations. The relative resistance 15. was measured as the percentage deviation of the observed popula tion of borers from the predicted population for the date of silk ing of the strain. In the earlier years, when 2^-36 strains were tested each year at several levels of borer population, certain hybrids showed a decided resistance to borer survival and others a marked susceptibility. It was found that the inbred lines varied in their inherent resistance to the survival of the borer and that the factors responsible for this resistance are trans mitted to the hybrids into which these inbred strains enter as parents. He stated that it appears that resistance is the result of an undetermined number of multiple factors, that strains showing the greatest degree of resistance contain the largest number of these factors, whether dominant or recessive, and, conversely, the inbred lines showing susceptibility or the least degree of resistance to the borer contain the smallest number of these factors, and that between these two extremes there is a wide range of inbred strains exhibiting various degrees of resis tance. Patch (19^3) again, in his tests in Ohio between 1932 and 1939 inclusive, with two field corn hybrids, of which one was highly resistant and the other highly susceptible to infestation by European corn borer, showed that survival of the larvae was materially greater on the latter. This was at first attributed to the greater availability of the tassel buds for larval feed ing, since in the resistant hybrid the upper leaves are wrapped round the tassel almost until the pollen is ripe. Most of the 16 differentiation in survival occurred within the first few days of hatching and thereafter it was relatively the same in both hybrids. The larvae developed more slowly in the resistant one, and further indications of its unsuitability as food were given by feeding characteristics, such as the shape and size of the feeding scars and the texture and color of the excrement, and the relative weights of larvae fed on the two hybrids. Although the greater availability of the tassel buds in the susceptible hybrids appeared to contribute to the more rapid growth of the larvae, the general unsuitability of the resistant hybrid as food is evidently a major factor in survival and probably in decreasing the rate of growth. Differences in survival on the two hybrids became less as the plants were near the pollen-shed ding stage when the larvae hatched. Further investigations on the development of strains of dent corn to first-generation larvae of the European corn bo rer were carried out in Ohio in 1938-^fl inclusive by Patch and Everly, the plants being artificially infested with eggs at the rate of about 120 per plant. The authors stated in 19^5 that in comparing inbred lines of previously established resistance or susceptibility and their single-cross hybrids in adjacent hills the relative numbers of larvae in the inbred parents and their hybrids were estimated to be, respectively, as follows: resistant X- resistant, 2.71 and 0.70; resistant X partly re sistant, 3.20 and 1.27; resistant X susceptible, 3*69 and 1.83; partly resistant X susceptible, *f.l8 and 2.40; and susceptible X susceptible, 4.67 and 2.96. The hybrids contained a nearly constant average of 1.86 fewer larvae per plant than the inbred lines from. ;which they were derived, although their average date of silking was 7*1 days earlier. The same authors stated in 1948 after further experiments in Ohio that from a low population of borers in single crosses or double crosses involving resistant lines, the number of bo rers per plant increased by geometrical progression in the cros ses involving successively more susceptible combinations. On the basis of the progressions there were 38.3 per cent as many borers in the resistant as in the susceptible combinations of single crosses, as compared with 25.7 per cent as many in the resistant as in the susceptible double cross. A smaller reduc tion of borers occurred in the presence of a higher infestation in the single crosses than in the double crosses. These were their results on the resistance contributed by inbred lines of dent corn to hybrid combinations. The average effect of parent inbred lines of dent corn on the survival of larvae of the early summer generation of the corn borer in single-cross Combinations in 1939 was compared quantitatively with their effect in double- cross combinations in 1941. Neiswander (1945) indicated that the appearance of large numbers of second generation borers has complicated the investi gation of corn resistance of susceptibility to corn borer infes tation in that many strains that have shown resistance to the 18 first generation do not exhibit the same resistance to the se cond generation. He stated that: i!It appears now that corn borer resistance or susceptibility in corn strain is not a speci fic character that is peculiar to certain strains at all stages of growth, but rather it is a relationship between the corn plant and the insect that varies with certain phy siologic changes in the plant as it progres ses towards maturity.''* Patch (19^8) tested the resistance of different dent corn hybrids to the survival of larvae of the June generation by having egg masses, placed on the plants by hand, hatched: at different times with respect to the development attained:by the plants. Plant development at time of infestation was va ried by planting on different dates and by infesting the plants with eggs at different times. . Eggs hatched late in June and early in August before and after the hybrids silked. A group of hybrids known to be resistant to the survival of June generation of borers averaged nearly as many borers per plant as a group of susceptible hybrids when the eggs hatched within eight days after most of the hybrids silked. Borer-resistant single cross hybrid Ins. P8 X L317 contained about 51 per cent as many borers as susceptible double-cross Ind. 6lOB when the eggs hatched in June from 32 to ^1 days prior to the silking of the plants. Hy brid P8 X L317 contained 72.5 per cent as many borers when the eggs hatched in August from one to seven days before the plant silked, but both hybrids averaged 9*9 borers per plant when the eggs hatched from three to eight days after the plants silked. ' . 19 This indicates that resistance is lost when the plants reach the silking stage. Turner and Beard (1950) tried to detect the effect of .stage.growth of field corn inbreds on oviposition and survival of the European corn borer. Their data show that growth stages known to be susceptible to oviposition and conductive to high survival occurred during the period of high natural oviposition* They stated , that the evidence was strong that the resistance and susceptibility of these inbreds have been caused by relation between the occurrence of favorable stages of growth and time of infestation by the corn borer. If this is true, they reported, then the implications are plain. It may be necessary to develop different resistant varieties for areas with different ecological conditions. Resistant varieties may also loose their resistance in a given area in seasons when climatic factors cause oviposition much later than usual. Effect of Planting Date on Borer Populations The effect of maturity of corn on infestation and damage done by the European corn borer was observed many years ago. Mayers et al. (1937) referred to a publication of Jablonowski in. 1398 stating that the early varieties of corn suffered more than the later. Huber e_t ad. (1S28) found that the earliest silking stalks had a higher borer population than those in the same plot that silked five days later. In the same experiments early silking varieties had a larger number of larvae than those ' ' • • ■ ■ ■ 20 which silked later. They stated that infestation correlates with the planting date as shown by the following table 1 . TABLE 1 Showing borer population per hundred stalks for all varieties and each planting date, 1926 and 1927* Year May 10 May 20 May 30 June 5 June 10 1926 108 98 79 30 27 3.927 318 176 85 43 12 Average 213 137 82 36 19 Harvey and Hartsell (1931)' indicated that, in general, early planted sweet corn is more heavily infested than that planted later and the differences become greater as the dates of planting are more widely separated. During the season of 1930 at Sheridan, New York, 105 borers were found in 139 Golden Bantam stalks planted on May 12,'whereas 27 and 12 borers were found in plants planted on June third and eleventh respectively. In another experiment at Brockport, 156 plants planted on May 3 contained 148 borers, whereas the same number of plants planted on June 12 contained only three borers. Ficht (1931) in a report of some of the progress made in three seasons of study pertaining to the relation of the plant ing date of corn to corn borer infestations and populations 2 1 stated that, other conditions being equal, greatly reduced in festations and subsequent borer populations were evident in the later plantings. In natural field plantings where such,factors as soil, variety, fertility, planting rate, etc., were variable, the planting date was not always an index to the borer infes tation or population. The result of delayed planting on the stalk infestations and borer population has been well marked over the three year period under discussion. When all the varieties were classified on the basis of planting date, a decrease in the stalk infes tation and subsequent borer load occurred in all of the delayed plantings. It appears from his data that the May 10 and 20 plantings contained 2,466 and 2,082 eggs and 4l0 and 358 live larvae per hundred plants, whereas; the June 1 and 10 plantings of the same season contained 516 and 84 eggs, and 101 and 18 live larvae per hundred plants respectively. He stated that a significant positive correlation existed between the average heights of the plantings during the oviposition period, the average heights of the plantings at the peak of egg-laying, and the number of eggs received. The comparative freedom from eggs of the delayed plantings was due to the fact that they did not receive eggs in the presence of the taller plantings until they: had reached what might be termed an "attractive height." The later plantings therefore received eggs only during the latter part of the egg laying period. As"was previously pointed out by Patch (1929) and Neiswan der and Huber (1929) height appeared to be an index to the most important factors governing egg laying, and the required grow ing period of the varieties as an index to the factor or factors chiefly responsible for variations in the rate of larval survival. The average date of silking of the plantings as indicative of their comparative maturity when compared to larval survival showed the same significant correlation as. height did when com pared to egg deposition. Harry and Anderson (1936) carried out tests in Virginia in order to secure information on the relationship between planting date and the abundance of the European corn borer. Plantings of corn were made on May lb and June 1, 10 and 27, 1935* Four lots of 21 stalks were examined on October 9 Tor each planting date and the first five infested stalks in each lot were dissected to determine the borer population. They stated that there was not much difference between the percentage of stalks infested for the different planting dates. However, there was a decided dif ference in the number of borers per stalk between the corn plant ed on May l^f and June 27» as compared to the corn planted on June 1 and 10 (^8 and ^5 for May 14 and June 27, and 9^ and 117 for June 1 and 10). Schlosberg and Mathes carried out some experiments in northern Ohio during 193^, 1933 and 1936 to determine the prac ticability of planting sweet corn late to reduce infestation by the European corn borer, all seasonal factors including injury ...... 23 by other pests, being taken into account. Batches of sweet corn were planted each: year at weekly intervals from the first week in May to the first week in July. A single generation of the borer occurred in 193^ and 1935 and two in 1936. The authors stated that the peak of oviposition by the overwintered gener ation in the three years was on 3rd, 12th and 9th July re spectively, and that of oviposition by the first generation in 1936 on the 13th of August. Of the total number of eggs recorded in 1936, 69.67 per cent were of the first generation. The earlier plantings which were taller and in the early tassel stage received most of the first generation eggs and the late planting received most of the second generation egg masses. During oviposition by second generation adults all corn was about the same height, but that planted later vras at a more attractive stage of tassel development and therefore received nearly all the eggs. The first five lots of sweet corn to be planted contained all the eggs in 193^1 92.6 per cent in 19351 and 97.2 per cent of the first generation eggs and 68.9 per cent of the total in 1936. Larval distribution corresponded to that of eggs. They concluded that it appeared, therefore, that under single generation conditions sweet corn planted in June or early July contains relatively few larvae as a result of its escaping, heavy oviposition. Cheu (19^0) reported that significant differences in degree of infestation were found on plants planted at intervals of two weeks from March to August. Corn planted in May was the most . . ■ . . . 24 ■- heavily infested. That planted earlier received fewer egg- masses owing to the scarcity of moths, whereas that planted later received as many egg-masses, but had fewer larvae, possibly be cause of parasitism of the eggs of Trichogramma st>. in July and August or because of the hot dry weather in the fall which is unfavourable for the hatching and establishment of the larvae. Cory £t al. (1941) stated that in experiments in which 12 corn hybrids were planted on five different dates between 13th of May and June 10, some gave good yields under conditions of moderate corn borer infestation and the later plantings appeared to be less infested than the earlier ones.. In a study of the relationship between date of planting and corn borer accumulation at Van Wert, Ohio, Neiswander (1946) indicated that borers in the field planted May 16 were nearly all of the first generation, whereas those of the May 25 and June 5 plantings were largely, if not entirely, second genera tion borers. In (1947) the same author pointed out that late planted ; fields carried more borers than early planted fields. In an area west of Springfield, Ohio, a number of relatively late, planted fields were observed by him in which the borer popula tion varied from 640 to 1,220 borers per 100 plants. Practi cally all of these larvae were second generation borers. The same author stated in 1948 that delayed planting is no longer the optimum corn borer control measure that it was under single-generation corn borer behavior. Borer population ... . 2 5 ■ data on corn planted at different dates and in different locali ties over the State of Ohio ■showed--..'that relatively high infes tations occurred in the very earliest plantings and again in the moderately late plantings, although not necessarily the latest. The earliest plantings were infested by first generation borers and the late plantings by second generation borers. The author stated that: The optimum corn planting time for corn borer control is the same as before the corn borer occurred, that is, a medium ' date. Neiswander (19^9)> in discussing the relation between corn borer infestation and date of corn planting, pointed out that the optimum date of planting for corn borer control is not the same for all latitudes across the State of Ohio. The fall population records from 5^ fields in different counties showed that the lowest populations prevailed in the northern counties (Wayne and Hancock) in fields planted near May 20 while in the southern county (Clark) high, populations developed in fields planted near this date. ; The same author indicated in 1951 that greater borer in festation and stalk damage occurred in early plantings of borer susceptible hybrids and in late plantings of all hybrids, v/hether considered resistant or susceptible. The Damage Done by Borer Huber et al. (1928) stated that damage by the European corn borer may be either direct or indirect or both. When damage is the result of the attack of the borer on any part of the corn plant it is considered direct. In severe infesta tion the midribs of leaves are entered, causing the leaves to break down, thus depriving the plants of a part of their power to manufacture food. If the stalk is badly tunneled translo cation of the depleted food supply results and the stalk may or may not collapse. The location of the tunnels may be more im portant than their number. Under any of these conditions of infestation the proper development of the ear is affected. More over, a large percentage of the cobs may be not only entered but the developing grain may be directly damaged to a certain but lesser extent. Indirect damage from borer injury may result from the en trance of bacterial and fungous organisms, which cause the stalks and ears to rot, thus greatly increasing the loss. Their data show that the short-season variety, Wisconsin 25, carried the heaviest borer load and also had the highest percentage of broken stalks. They also stated that it seems that differences in breakage and damage were the results of differences in popu lation, maturity, and size of stalk, or some combination of these conditions. 26 • • . . ..'.27 • : Neiswander and Herr (1930) found that within the variety there was a direct correlation between borer population and reduction in yields and that for the three varieties which were under observation there was an inverse correlation bet\\reen per cent reduction in yield and length of season between planting and silking. Patch (19^2 in.his work'carried out in Ohio in 1929-3^1 determined the degree of reduction in yield of field corn re sulting from definite levels of corn borer population by mannually infesting plants with various numbers of egg masses. He pointed out that reduction in yield within fields was shown to be proportional to the number of borers present up to 22 per plant. Within the range from 28 to 85 bushels per acre the fields that would have produced greater normal yields in the absence of borers had smaller proportions ox their crops destroyed by a given number of borers than the fields with lower yields. Data from plantings of the Clarage variety in various localities in northwestern Ohio in 1929-33 showed that .the rates of yield reduction were 2.68 and ^.86 per cent borer per plant When the normal yields were 85 and 28 bushels per acre respectively. He stated that the i±age of plant development at the time of infestation is an important factor. Plants infested early in their development suffered greater rates of yield reduction owing to the longer duration of borer feeding before the critical ■ ' ' ' ' 28 period of ear production and the subsequent weaker conditions of the plants, and to the larger average size of the borers during the period of ear production. The average normal yield of 85 bushels per acre over a four-year period, for corn planted on the average date 9th May, was reduced 2.85 per cent per borer per plant as compared with 4.71 per cent for the same hybrids giving about the same yield, but planted 23 days later. Deay et al. (1949) reported that single-cross hybrids with potential yields ranging from 45.8 to 88.3 bushels per acre were tested in 1944 and another lot of single and double crosses with potential yields ranging from 66.8 to 92.9 bushels per acre were tested in 1945. As an average of the two lots •> hybrids with potential yields of 66.8 bushels per acre lost 1.21 bushels or 1.81 per cent of their yield per borer per plant and this loss increased by linear regression to 2.61 bushels or 2.95 per cent as the potential yield increased to 88.5 bushels. The writers stated that, in general, the hybrids having eggs which hatch the earliest in the development of the plants suffered the greatest reduction in yield. Since the borers in these hybrids had a longer time to feed before the critical period of ear production and were therefore larger in size during the period of ear produc tion, these late silking hybrids would be expected to suffer a greater reduction in yield from this cause alone. Patch et al. (1951) investigated the stalk-breakage of dent corn infested with the August generation of the European corn borer. They stated that borer tunnels present favorable 2 9 conditions' for stalk rot infections, and that corn strains sus ceptible to stalk rot would be expected to show the most break age under borer infestation. Moreover, the larvae tunnel in the stalks and ear shanks thus weakening them to the extent that they break and interfere with the proper development of the ears and their harvest by a mechanical corn picker. Their experiments showed that, in 19^ , the borer caused an increase of 15*3 per cent in the plants broken below the ear, and 7 per cent of this increase had ears regarded as unmarketable because they were in contact with the ground. Also one per cent of the ears on un broken plants were on the ground. Therefore, there was an in crease of 1.9 per cent in the number of plants that bore unhar- vestable ears. The borers caused a decrease of 18.9 per1 cent in the yield by reduction in size of ears, including the ears on the ground. If the unmarketable ears on the ground are considered a total loss, the loss in yield through reduction in size of ears was ten times as much as the loss due to unmarket able ears. Chiang jet al. (I95t) carried out experiments at Waseca, Minnesota, during the years 19^8 to 1952 inclusive, to study: 1. The effect of second-generation borers on the,yield, of corn. The data indicate that even though second-generation ; borer population in sprayed plots was 83 per.cent less than in the checks (60 as compared to 36^- borers per hundred plants), the yield in these was not increased over that in the unsprayed plots where a higher borer population was present. The results ■ ■ ■ ' ■ ■ '30' indicate that the presence of the additional 83 per cent (2kh more borers per hundred plants) of the second-generation borer population did not significantly affect the yield.: 2. The effect of second-generation borers on stalk break age and ear dropping. In their study they found that the amount of' stalk breakage and ear dropping was not apparently affected by the size of the first-generation borer population,- For the second-generation borer population they pointed out that the amount of stalk breakage is not as well correlated with the number.of second-generation egg masses as with the number of second generation larvae. This is primarily due to the highly variable mortality which botheggs and larvae may suffer during different seasons. They stated that although in their study there was a higher correlation between the amount of stalk breakage and the number of mature second generation borers, one must realize, that this relationship may not always be as pronounced due to : other factors such as the following: With the same degree of borer infestation the amount of stalk breakage and ear dropping increased with a prolonged weathering of the stalks as Chiang anu Hudson showed in 1950. This relationship also varies with the variety of corn ac cording to Patch et al. (I95l)» As a conclusion the writers stated that the second gener ation borers have little effect on the ear growth but are re sponsible for stalk breakage and ear dropping...... ' ' 31 Holdaway (1955) in an unpublished report about the results of the.Central Regional Project (NC-20) experiments which were carried out during the two growing seasons of 1953 and 195^ stated that the effect of resistance to the borer infestation on yield is seen particularly when a comparison is made bet ween the yields of the borer-free plants and those infested with six first-generation egg masses per plant in both the early planted (Table 2) and the late planted varieties (Table 3)* TABLE 2 Yield of susceptible and resistant varieties early planted- treatment 4 compared with treatment 1. Yield (Bu/acre) Change in ES (1) * ER (1) ES (4) ER (4) yield due to borer attack (Per cent) State Year ES ... ER Iowa 1953 105.0 98.7 99.3 99.3 - 5.4 0.6 195^ 102.7 97-6 79.2 84.4 -22.9 -13.5 Minn. 1953 104.3 96.3 101.3 92.6 - 2.9 - 3*8 195k 91.4 78.2 74.2 8i.8 -2.8.8 4.6 Ohio 1953 72.1 78.2 59.9 70.1 -16.9 -10.4 1954 69.1 70.6 65.O 66.0 - 5-9 - 6.5 * ES Early planted susceptible ER Early planted resistant (l) Borer-free plants (4) 6 first generation-egg masses per plant TABLE 3 Yield of susceptible and resistant varieties late planted - treatment 4 compared with treatment 1. Yield (Bu/acre) Change in LS (1) * LR Cl) LS (4) LR (4) yield due to borer attack (Per cent) State Year LS - LR Iowa 1953 99.1 100.6 93-0 95.0 - 6.2 - 5.6 1954 111.2 102.1 78.2 95.5 -29.7 6.5 Minn. 1953 104.4 84.0 91.5 76.7 -12.4 S'. 7 1954 ?8.6 74.8 65.7 64.0 -16.4 -14.4 Ohio 1953 73.1 70.6 : 69.1 71.1 - 5.5 0.7 1954 68.1 65.0 62.0 62.0 - 9.0 4.6 * LS Late planted susceptible LR Late planted resistant (l) Borer-free plants (4) 6 first generation egg masses per plant Agronomists consider that little significance can be at tached to differences in yield of less than five bushels per acre. In general the resistant variety is reduced in yield less by infestation than is the susceptible variety. The data also bring out the fact that, in general, while the resistant: variety is better able to maintain yields in spite of borer infestation, the expression of resistance is not constant but may vary with the time of planting, season, and location. He also reported that the effect of borer injury on growth varies with the variety, resistance, planting date, and treat ment. It is most marked in the susceptible variety with a high- 32- ' ■ ■ 33 er degree of borer infestation. The effect of injury on growth of the early planted susceptible variety is given in Table 4. TABLE 4 Reduction in growth of susceptible plants as a result of borer infestation — early planting* ; ■■ T a s s e T ' ( l n c h e s y ' :"m,l''T:'"~ ■' Reduction Borer-free Six 1st brood in height plants egg masses Inches Per cent (Treatment 1) per plant State Year (Treatment 4) Iowa 1933 97.5 91.0 6.5 6.7 1954 83.O 69.8 13.2 15.9 Minn. 1953 90.4 83.0 7-4 8.2 1954 91.3 79.8 11.5 12.6 Ohio 1953 89.1 84.V . 4.7 5.3 1954 53.8 53.0 G .8 1.5 He stated that reduction in yield through injury by borer, even by the uniform infestations developed in the experiments, varies with planting date and from year to year and from place to place. Reduction in yield was most marked in the early planting of the susceptible variety. Yields from borer-free plants and plants artificially infested xd-th six egg masses per plant of the early^planted susceptible variety are given in Table 5» TABLE 5 Reduction in yield of susceptible plants, by borer attack - early planting. Yield (Bu/acre) Reduction Borer-free Six 1st brood in yield plants egg masses Bu/acre Per cent (Treatment l) per plant (Treatment 4) State Year Iowa 1953 105.0 99.3 5.7 5.4 1954 102.7 79.2 25-5 22.9 Minn. 1953 104.5 101.5 5.0 2.9 1954 91.4 74.2 17.2 18.8 Ohio 1953 72.1 59.9 12.2 16.9 1954 69.1 65.° 4.1 ; 5-9 34 PROCEDURES AND TECHNIQUE The following steps are an outline of the procedures set up in the order of their application. Field preparation. Plot layout. Planting and maintaining the corn plants. Artificial infestation. Sampling and data recorded. Preparing the Field of Experiment The experiment took place in a 2.2 acres field at the State Hospital Farm, Apple Creek, five miles east of the Ohio Agricul tural Experiment Station, Wooster, Ohio. After the necessary agricultural processes were undertaken, i.e.,ploughing and disk ing, the field was sprayed with toxaphane at a rate of 1.5 lb., per acre to control cutworms. The fertilizer 5-10-10 ( 5 per cent nitrogen + 10 per cent phosphate + 10 per cent potassium) was added to the soil at a rate of 3 0 0 lb. per acre in rows fourty inches apart. With a field marker crosswise rows were marked to form with the fertilized rows ^-O" x hOu squares at which corners the corn was planted with a hand planter in hills at double rate of seeding in order to obtain a perfect stand of three plants per hill. 35 Plot Layout As has been mentioned beforey the experiment was designed to provide information on borer population and yield of field corn under conditions of susceptible and resistant hybrids, early and late plantings, and various egg loads of both borer generations. Therefore a split-split plot design was considered most appropriate to help in obtaining the desired information. Accordingly the field was divided into six replicates, each of which was split into two plots, the plot into two split-plots and each of the latter into eight split-split plots. Dates of planting appeared in the: whole plot, corn hybrids in the split- plot and treatments in the split-split plot. Each split-split plot consisted of seven rows deep and six rows wide (42 hills with 126 corn plants), and was subject to a particular treatment according to the following scheme: Split-Split Plot Treatment 1 Insect free plants throughout the season. The plants were sprayed with EPN wettable powder at a rate of half pound toxicant per acre for first brood borers and with DDT wettable powder at a rate of one pound toxicant per acre for second brood borers. Spraying was repeated at 5-day in tervals as long as twenty or more egg masses per hundred plants were encountered. 2 Natural infestation by both first and second borer generations. 3 Natural infestation by both generations plus three first generation egg masses (60 eggs) per plant. (continued on next page) 36 Split-Split Plot' Treatment (continued) h Natural infestation by both generations plus six first generation egg masses (120 eggs) per plant. 5 Natural second generation infestation only. Plants v/ere sprayed with EPN wettable powder at a rate of half pound toxicant per acre to eliminate first generation borers. 6 Natural second generation infestation plus three second generation egg masses (60 eggs) per plant. Plants were sprayed with EPN wettable powder at a rate of half pound toxicant per acre to eliminate first generation borers. 7 Natural second generation infestation plus six second generation egg masses (120 eggs) per plant. First generation borers were eliminated by EPN sprays as in treatment N° 6. 8 Natural infestation by first generation borers plus three first generation egg masses (60 eggs) per plant. To eliminate second generation borers, plants were sprayed with DDT wettable powder at a rate of one pound toxicant per acre at 5-day intervals and continued as long as egg masses were encountered on the corn plants at a rate of twenty or more per hun dred plants. Diagram N° 1 shows a field plan, and Diagram N° 2 shows a single split-split plot'. 37 Diagram 1* : Field Plan. Early Planting Late Planting Rows 2k 6 6 6 6 28 7 S2 SL R8 R3 R4 R5 S8 S7 7 S6 S8 R5 R2 R7 R2 S3 S5 Reolicate I 7 SI S5 r 4 R7 R8 ;R1 Sk s6 7 S3 S7 r6 R1 R6 R3 S1 S2 Replicate II Late Planting Early Planting Dates, corn hybrids, and treatments are randomized within the : replicates. .. S = Susceptible corn hybrid. R = Resistant corn hybrid. Plot area: 20' x 23.31 = *r66 square feet. Replicate area: 80' x 190* =15,200 square feet. Experimental area at kO inch row spacing = 190' x 500' = 95,000 square feet = 2.2 acres. 38 Diagram 2. Single Split-Split Plot, Row Nc 1 2 3 4 5 x X 7 6 5 4 3 2 1 X X X XXX Plot area: 20' x 23.3* = 466 square feet. The 2 x 5 hill center portion of rows N° 3 and 4 was undisturbed and used for yield determination. 39 Planting and Maintaining the Corn Plants Wfq x Ml*r and 0HV5 x OH51A were the two adapted, full sea son, single cross hybrids of uniform relative maturity selected for the experiment. Wfq x Ml4 is a susceptible hybrid and 0H^3 x 0H51A is a hybrid resistant to corn borer infestation. There were two plantings each season, one early in the normal planting period for the area (May 2k, 1956 and May 28, 195?)> and one nine to twelve days later (June 6 in both years). The corn was planted in hills forty inches apart at double rate of seeding three inches apart with three kernels per hill using hand planters. The stand, at about six to eight inches normal height, was thinned and transplanting took place i«rithin one hy brid and date of planting to maintain three plants per hill. Artificial Infestation Hand application The hand application of first generation egg masses was initiated when plants of the early planting date were approxi mately 35 inches in extended height and plants were receiving natural egg masses. In the late planting^egg masses were ap plied on the same date as in the early planting in plots scheduled to receive hand applied egg masses. In treatments receiving three egg masses, the eggs were applied one mass per day on three days spaced approximately four days apart. In treatments receiving six egg masses, they were applied two masses per day on,three days/spaced approxima tely four days apart. Three egg masses approximated sixty. kO ' 41 ■■ eggs, six egg masses approximated one hundred and twenty eggs. For the first generation infestation egg masses were placed inside the whorl. Second generation egg masses were pinned on Leaf 1 the under side of the 1st egg raid rib of three adja Leaf 2 mass 2nd egg cent leaves. The first mass ear egg mass was pinned to Leaf 3 3rd egg leaf 1, the second to mass leaf 2, and the third to leaf 3 as shown in Diagram 3* Diagram 3• For the first generation all the rows in the treatment were infested; but only the two center rows (2x7 hills) were in fested by second generation borers due to a shortage of egg masses. , ' - Number of first generation egg masses required for infest ing all plants in all rows of treatments so scheduled: 6 x 7 = b2 hills (three plants per hill) 42 x 3 =126 plants per treatment 126 x 6 =756 egg masses for treatment #4 infested with 6 egg masses per plant 126 x 3 =378 egg masses for treatment #3 infested with 3 egg masses per plant 126 x 3 =378 egg masses for treatment #8 infested with 3 egg masses per plant 756 +378 + 378 = 1,512 1,512 x 2 hybrids = 3)024 3,024 x 2 plantings = 6,048 6,048 x 6 replicates = 36,288 Total egg masses required. ■■ k2 Number of second generation egg masses required for in festing yield rows (treatments #6 and .#?■) s lk hills x 3 plants = k2 plants per treatment k2 x 3 =126 egg masses for treatment #6 in fested with three egg masses per plant k2 x 6 =252 egg masses for treatment #7 in fested with six egg masses per plant 126 + 252 = 378 378 x 2 hybrids = 756 756 x 2 plantings = 1,512 1,512 x 6 replicates = 9.072 Total egg masses required. Laboratory production of corn borer egg masses In order to obtain the large number of egg masses required for infesting the plants artificially, approximately five acres of heavily infested corn stalks were placed during the fall in the 102' x 16* x 7' screened moth emergence cage. The moths, emerging from the stalks during the night and congregating on the screen, were collected early in the morning throughout the emergence period; otherwise they seek shelter from the hot sun in the shocks of stalks. One hundred females and fifty males were collected in each cone shaped screened bottom container. The moths from each collecting cone were placed in a screened oviposition cage 36” x 36” x 12” , the top of which is made of quarter inch mesh hardware cloth. Two sheets of waxed paper were placed side by side on top of the hardware cloth and held firmly in place with a felt pad. Oviposition took place between the openings in the hardware cloth onto the waxed paper. The sheets of waxed paper were removed and replaced with new ones each day. The oviposition room was operated at a temperature of 80°F and 90 per cent relative humidity. A fan was used to prevent the layering of air of different temperatures. Under the above adverse conditions of temperature and relative humidity, the incidence of the fungus Beauveria bassiana was expected to be high. Therefore all cages, pads, and oviposition room were dis infected with k per cent formaldehyde solution after being used each season. Disks of waxed paper (one-half inch diameter), each con taining one corn borer egg mass, were cut out with a specially designed machine. The waxed paper disks were pinned onto ' 8M x 10" Celotex boards (200 disks per board) and placed in an incubator room operating at a temperature of 80°F and high humidity until they assumed the black color which preceeds hatching and hence were ready to be put on the plants. For the second generation approximately one acre of heavily infested (green) sweet corn stalks was cut and hauled from the State Hospital Farm, Toledo, Ohio,to Wooster. To prevent a high mortality of larvae and pupae from high temperatures pro duced within the load by the green stalks during the trip to Wboster, the bundles of stalks were placed in the truck in layers with crushed ice spread over each layer. These stalks were placed in the moth emergence cage to obtain second brood moths for egg production. Because of the heavy rnaindtf&Ll during the planting period of 1957 throughout most of the State of Ohio, both sweet and field corn were planted late and corn borer infestation was low. Therefore second brood egg masses necessary for the experiment were secured from the European Corn Borer Research Laboratory, Ankeny, Iowa. Sampling and Data Recorded 1. The 2 x 5 hill center portion of the split-split plot (rows#3 and Diagram 2.) were kept undisturbed and were used only for yield determination. The corn plants of the two outside rows were used for plant measurements and dissections for larval development records. The second and fifth rows were regarded as guard rows for the two yield rows, therefore kept undisturbed. As a rule no more than one plant had been dis sected from any one hill. For records on plant and larval developments and for first generation borer census, only plants of the borer free, the first generation naturally infested treatments, and treatments which received hand.application of first generation egg masses were measured and dissected. 2. One plant in each plot of each planting and each hy brid was measured and dissected at four-day intervals from the time the plants reached approximately 2h inches in extended height until full height was attained. Corn height, natural and extended, tassel emergence and silking dates, and the number of borer larvae by instar were recorded. .. . . 45 : Number of plants measured and dissected every four days was: 1 (plant per treatment) x 4 (number of treatments) x 2 (number of hybrids) x 2 (number of planting dates) x 6 (num ber of replicates) = 9 6 (the number of plants measured and dissected). 3 . At four-day intervals egg mass counts were taken on one randomly selected plant out of each of the four treatments. The development of the eggs was classified as white (fresh), yellow, blackhead, and hatched. The number of egg masses per hundred plants was used in determining the need of toxicant sprays of the treatments so scheduled. Both first and second generation egg masses were checked for and counted if present. 4. For the summer borer population census six plants se lected at random from each of the 96 treatments were dissected at the beginning of pupation of the first generation (in 1957 it was undertaken when most of the larvae were in the fifth instar•)• Records were made of the number of borers and stage of their development, the injury done by the borers as measured by the number of leaf lesions, and the number of burrows %■ inch deep or more in the main stalk. 5. For the fall population census ten plants selected at random from the yield rows of each of the 192 treatments com posing the experiment were dissected. Number and instar of larvae above ear, below ear, in ear and in ear shank, and the number of burrows inch deep or more were recorded. In some KG cases when there was sign of borer activity in the ear, the latter was dissected and the pieces were assembled and saved by the hill to be added to the yield. 6. Procedures for talcing yield data: Yield was taken only from the 2 x 5 hills of the two center rows (Diagram2.). The following '.'two requirements were fulfilled by each of the yield hills: (1) All |2 x #5 hills should contain three plants each. (2) The hills adjacent to those used should contain at least two plants. If any of the yield hills failed to meet one or both of these requirements it was replaced by the adjacent hill or by the first or the last hill of the yield rows provided that the latter hills were artificially infested if so scheduled. Ten hills meeting the above requirements were counted and the following procedures were followed. 1. The ears were picked by hand and weighed to obtain the "field v/eight" for yield per treatment. Both the first and second ears were harvested; suckers (usually having a deformed ear at the position of the tassel) were not picked. 2. Ears from one treatment were placed in a cloth bag properly labeled. 3* At harvesting time the moisture content of the ears, roughly estimated by the appearance of the kernels and the hardness of the cob, was more than 30 Per cent. Therefore the ears were dried for one week in an air drier. . . . ■ ■ ' ■■ . v ? 4. Twelve ears were picked at random from each bag, shelled, and the kernels were mixed. 5. Approximately one pint of the kernels from each sample was kept in a properly labeled plastic bag for 48 hours to al low them to come to equilibrium as regards moisture content at room temperature. 6. After equilibrium was reached, the temperature of the kernels was measured and their percentage moisture content was determined by using a moisture meter. The dial readings were recorded and converted to percentage moisture then adjusted to the temperature of the kernels and the yield of each treatment was computed at 15 per cent moisture. All data recorded were compiled and analysed statistically using the analysis of variance of the split-split plot design procedures according to the following scheme. Statistical Analysis of the Split - Split Plot Design R 6 Replicates D 2 Dates H 2 Hybrids T 8 Treatments d.f. Degrees of freedom E.M.S. Estimated Mean Square Source of Variation d.f. E.M.S. Whole Plot Replicates (R) 5 8x6x2 (D)‘ Dates (D) 1 1 2 8x2 Split Plot Hybrids (H) 6x8x2 i (H)‘ 1 . H x D (interaction) 1 6x8 $ (HD)" 1 Error B 10 Split-Split Plot Treatment (T) 7 6x2x2 j (T) 7 T x D (interaction) 7 6x2 i (TD) 7 T x H (interaction) 7 6x2 f(TH) 7 T x D x H (interaction) 7 6 ■((THD)‘ 7 Error C l40 ■cr.C T o t a 1 191 48 : RESULTS The experiment, as has been previously mentioned, was carried out in two successive seasons to study the following three objectives: 1. The effect of resistance and susceptibility of two field corn hybrids on the first - and second - generation borer population. 2. The effect of early and late planting on the borer ■ ■■■■■■■■■■■■ * accumulation, 3. To measure the damage done by the borer. The study of the borer population aimed toward studying its fluctuation. It must not be borneiinniiirid that the objectivee was to study year to year fluctuation in European corn borer population, but the fluctuations within a year on two particular corn hybrids, one susceptible and another resistant to borer infestation, and two dates of planting, one early starting within the regular planting period in the region and the other almost ten days later. Since Ohio has a two-generation borer "strain" the infest ation will be discussed under tv/o categories. The first gener ation infestation which develops from the overwintering larvae, and the succeeding second generation infestation which develops from midsummer to early fall and overwinters as larvae in the fifth instar until the next year. The two populations under con' sideration in this experiment have developed from two sources: 1. A natural infestation resulting from natural oviposi tion. 2. ; A higher level of infestation developed from natural : oviposition and a superimposed egg load by artificial infest ation. It was hoped that artificial infestation would, give two different population levels other than the natural infest ation level for each generation, but this was not always the case as will be seen later in this discussion. The effect of resistance, susceptibility, and date of planting on the population of each generation and of the two generations together will be discussed. Wherever it seems nec essary, data achieved from the experiment with their statisti cal interpretation will appear in the text in the form of tables and graphs. The abbreviations used in the tables to designate resistant and susceptible hybrids, dates of planting, ana their combinations will be as follows: H = hybrid D = date of planting R = resistant S = susceptible E = early L = late ES = early susceptible ER = early resistant LS = late susceptible LR = late resistant Effect of Resistance and Susceptibility on European Corn Borer Populations First Generation Population The population of the first generation borers was measured in terms off (a) number of egg masses found on the plants. (b) number of larvae found in or/and on plants dissected every four days starting with the first application of artifi cial infestation until most of the larvae reached the fifth instar. (c) number of borers found in midseason when pupation started. Natural ovioosition: Moths which layed eggs in June were those which emerged from the overwintering larvae of the pre- ceeding year. Natural egg production of the first generation in 1957 was so low that no egg masses were encountered when 192 plants were checked every four days. Therefore, only data of- 1956 natural oviposition will be presented. Table 6 shows the number of egg masses per hundred plants for each hybrid and planting date and their relationship to the average normal and extended heights in inches of the corn plants on which they were encountered. 51 TABLE 6 First generation egg masses per 100 plants by hybrid x planting date vs. average normal and extended height of corn plants. Date D x H No. egg Av. normal Av. extended masses height (inches) height (inches) 6-26-56 ES 70 21.7 33.0 ER 60 20.if 31.7 LS 25 18.0 23.5 LR 5 13.8 23.8 6-30-56 ES 85 I9 .I 29.6 ER 70 18.3 28.9 LS 20 12.8 20.3 LR ■:■■■ 5. /: 12.1 21.if 7-5-56 ES 60 31.6 if3.if ER 4o 31.5 if if .1 LS 15 23.7 33.1 LR 15 21.0 33.3 7-10-56 ES 15 35.5 50.3 ER 0 ifl.3 5 W 3 LS " 5 28.5 if 3 .0 LR 5 .26.3 if3.0 It is evident from the above data that in both early and late planting the moths have shown preference for oviposition on the susceptible over the resistant hybrid. Examining the relationship'between the number of egg masses and the average normal and extended height of the plants on which eggs were found it appears that during the first two counts the heights of the early susceptible plants slightly exceeded that of the early resistant. During the last two egg counts the height of the early resistant was either equal to or exceeded the 52 • ■ ■ 53 height of the early susceptible. Regarding the late planting of both hybrids, the susceptible has exceeded the resistant in normal height during all counts. When the numbers of egg masses found on one hybrid, regardless of the date of planting, were added^the number of egg masses on the susceptible hybrid was found to exceed that on the resistant hybrid by 32. per cent as shown in Table 7* The resistant had 25 egg masses whereas the susceptible hybrid had 37 per 100 plants. TABLE 7 First generation egg masses in hybrids vri.thin date of planting. Date No. egg masses/200 plants S R 6-26-56 95 65 6-30-56 105 75 7-5-56 75 55 ... 7-10-56 20 / 5. V. Total 295 200 Av. No./lOO plants 37 25 Figure 1 shows that although natural oviposition followed the same trend on both hybrids, it was less on the resistant than on the susceptible. 1.75 cn \~ 2 < 150 _1 Q. O o cvJ tr 125 UJ q l U) UJ U) Susceptible If) 100 Resistant < o cp UJ 2 o fe o: uj 2 50 UJ o J- C/) cr U. U. O O 2 June 2 5 3 0 July 5 1 DATE Figure 1 First generation natural oviposition on hybrids within dates of planting. : 55.. Larval establishment and rate of developmenti The devel opment of the first generation borer larvae was carefully studied in the two' corn hybrids and dates of planting by dis sections made every four days. One plant was chosen at random from eacih of the naturally infested plots and from'those'which received different loads of egg masses by hand application. The number and instar of the larvae found on the plants are re corded in tables 8 and 9 : the first represents data taken from naturally infested plots (treatment 2); the second represents data taken from treatment If which received natural infestation plus six egg masses (approximately 120 eggs) by hand application. Several workers have reported that the reduction in corn borer population is to a great extent due to the death of the larvae right after hatching. Ceaser (1925) recorded an average mortality of 78.2 per cent or a survival of 21.8 per cent and that 76.2 per cent of the mortality occurred in the first two larval instars when the larvae were less than ten days old. Painter and Fitch (1925) found the average mortality for three types of corn to be 80 per cent and;that 75 per cent of the; mortality occurred in the first, two larval instars "when the . larvae were attempting to establish themselves." Caffrey (1927) reported, on work carried out at Sandusky, Ohio, in 1924, that the larval mortality was 91.6 per cent and that most of the mortality took place during the first two instars. Huber et al. (1928) reported a larval mortality of as high as 56 78.7 per cent with 65.6 per cent in the first and second in:-* stars. The latter authors also stated that the "death rate is always highest while the young borers are first beginning to feed." It has also been shown by Neiswander and Huber (1931) that "other things being equal there is an inverse relation ship between the number of eggs deposited and the rate of lar val survival." According to the data presented,this decrease in larval survival begins at a point somewhere between 100 and 250 eggs per plant, but up to about 100 eggs per plant there is an increase in the survival with each increase in the number of eggs. They indicated that this factor of competition has not been studied in relation to resistant and susceptible strains of corn but the results suggest that a level of infest ation may be reached in which no more increase is possible on susceptible strains while resistant ones may still give an in crease in survival. The level at which this occurs appears to be found rarely in natural infestations but may have occur red in some artificially infested corn. A marked decline in the larval population was also noticed in the present experiment with apparent evidence that resistance and susceptibility have different effect on larval survival and rate of development both in naturally infested plants and in those which received natural plus artificial first generation infestations. Two examples are given in tables 8 and 9 whereby " . ■ 57 ' the condition of the larval population is illustrated covering the period which elapsed from the time the eggs hatched until two weeks prior to the midsummer dissection. If appears from the two tables that - .1. Larval establishment in both natural and natural plus ar tificial infestations was greater in 195^ than in 1957 and that nat ural population of 1957 was too low to give dependable data. 2. The natural infestation record shows three oviposition peaks; the first occurred before June 50, the second before July 5 and the third before July 19 as shown in Figure 2. 5* On July 10.the larval population of both treatments suf fered high degree of mortality. k. The two hybrids showed different effect on the larval es tablishment. In naturally infested plots of 1956 the susceptible plants lost ^6.7 per cent of their population on July 10 whereas the number of larvae on the resistant plants remained unchanged. Later on July l^f the population on both hybrids decreased by ^6.9 per cent mortality on the susceptible and 66.7 per cent on the resistant. The data taken from treatment k (natural infestation + 6 egg masses) give a clearer picture of the effect of the two hybrids on the larval survival. It must be mentioned that in festing each plant artificially with six egg masses was completed within a seven day period owing to the vast number of egg masses required to infest in one day all the plants in all plots so scheduled to receive artificial infestation. TABLE 8 Larval establishment and rate of development in hybrids within dates of planting. Natural first generation infestation (treatment 2). No,, larvae *7 instar 1956 No. larvae by instar 1957 Per * Per * * Per * Per*" cent cent cent cent Date Sybrid 1 2 3 4 5 Total . 1 2 3 4 5 Total _+ 6-30 S 8 2 10 20 R 8 1 9 11.1 7-5 S 52 19 9 6 o +500 15 R 12 3 0 15 + 66.7 0 7-10 S 7 13 12 32 -46.7 37.-5- 0 3 0 0 0 3 R 8 6 1 15 0 6.7 0 0 0 0 0 0 7-14,15 S 0 4 7 4 2 17 - 46.9 35-3 0 2 2 - 33.3 R 1 6 2 : 9 - 66.7 0 2 0 0 7-19 S 1 6 18 7 13 45 +164.7 28.9 0 0 1 4 5 +150 80 R 0 3 5 4 2 14 + 55.6 14.3 0 0 2 0 2 '.I +200 0 7-24,25 S 0 0 16 19 16 51 + 13.3 31.4 0 0 1 5 3 9 + 80 33-3 R 0 0 10 17 3 30 +114.2 10 0 0 0 0 l l - 50 100 * ' per cent decrease or increase in larval population. * * per cent of the latest developing instar in each dissection. TABLE 9 Larval establishment and rate of development in hybrids within dates of planting. Natural first generation + j5 egg masses (treatment 4). No. larvae by instar 1956 No larvae by instar 1957 Per * Pe r ** Per * Per** cent cent cent cent Date Hybrid 1 2 3 4 5 Total + 1 2 3 4 5 Total + 6-30 S 104 3 107 2.8 E l4? 0 14? 0 7-3 S 238 ho 278 +159.8 0 E 214 13 5 232 + 57.8 2.2 7-10 S 81 68 22 1 172 - 38.1 13.4' 79 29 1 109 E 70 28 6 104 - 55.2 5.8 48 7 55 7-14,15 S 30 58 33 6 127 - 26.2 4.7 9 59 2 1 71 - 34.9 4.2 E 7 24 18 4 53 - 49.0 7.5 9 27 36 - 34.5 0 7-3-9 S 1 46 46 24 6 123 - 3.1 4.9 0 4 33 23 60 - I5.5 38. E 2 31 24 12 0 69 + 30.2 0 0 2 14 11 27 - 25 .O 4o. 7-24;, 25 S 0 7 34 48 24 113 - 8.1 21.2 0 0 19 36 9 64 + 6.7 14.3 E 0 6 27 21 12 66 - 4.3 18.2 0 0 14 17 2 33 + 22.2 6.1 per cent decrease or increase in larval population. per cent of the latest developing instar in each dissection. Susceptible Resistant 3 0 0 cn CL 2 5 0 a . 200 < I 150 100 O Treatment 4 h- 50 Treatment 2 June 25 DATE Figure 2 Effect of resistance and susceptibility on larval establishment. Treatment 2. (1st generation natural infestation.) Treatment: 4. (1st generation natural infestation + 6 1st generation egg masses.) 60 61 ■ Therefore, to give all the plants equal opportunity to develop infestation, all the plots were infested on the same day with equal numbers of egg masses. In the season of 1956 the increase in the number of larvae: of treatment 4 was noticed on the two hybrids on July 5» table 9. Since the last two egg masses were put on the plants on July 5 . and 6 another increase in the larval population was expected in the dissection of July 10 and/or l4. The reverse was true and a striking decrease in the number of larvae was noticed on"both hybrids. On July 10 and 14 the larval population on the resistant plants suffered a mortality of 55*2 and 49 per cent respectively compared to 38.1 and 26.2 per cent mortality on the susceptible plants. In the dissection of July 19 the larval population on the susceptible plants showed 3.1 per cent mortality, whereas the population on the resistant plants gained 30.2 per cent increase. This increase in the population was'parallel to the increase in natural population shown in table 8 on the same date and hybrid. Later in the last dissection larval populations of both hybrids showed a decrease in number (8.1 for the susceptible and 4.3 for the resistant). Figure 2 shows that the fluctuation in the lar val population followed the same pattern in both hybrids except for the aforementioned increase in the population of the resist ant’. hybrid which occurred on July 19. 62 In 1957 the per cent decrease in the population of the two hybrids was about equal on July 15, whereas on July 19 the de crease on the resistant hybrid surpassed that on the susceptible by 10.5 per cent. On July 25 the per cent survival was higher on the resistant than on the susceptible hybrid. A look at the number of larvae by instar in the two dis sections of July 10 and 14 shows that most of the first and second instar larvae on both hybrids failed to survive. To compare the effect of the two hybrids on the rate of the larval development the percentage of the larvae which reached the last developing instar in each dissection was computed. Da ta of the two treatments indicate that the rate of larval devel opment'.' was consistently faster in suscejitible plants than in the resistant plants except for July l^f, 1956 and July 19i 1957* It seems, by comparing data of the two treatments under consider ation, that the rate of the larval development was higher in the naturally infested plots than in those which received the extra load of egg masses artificially. The slower rate of larval de velopment in treatment k may be attributed to the density fac tor. Summer population■; The sampling of the first generation borers was made in the .midseason period when most of the larvae were in the fifth instar and pupation was underway. This mid summer dissection covered the first.four treatments and treat ment 8. Treatment 1 was kept relatively free of borers by EPN sprays. Treatment 2 was left to be naturally infested. Treat- ' . ' 6? / ments 3i and 8, besides natural infestation, were given extra loads of egg masses artificially, three egg masses for each plant of treatments 3 and 8, and six egg masses for treatment k* The results of the midsummer dissection which covered number of bor- :e-rs, burrows, and leaf lesions for each hybrid and date of planting are given in table 33* The data have been statistical ly analyzed each year according to split-split plot design pro cedures. The levels of significant differences are indicated by the usual characters in table 10, TABLE 10 Midsummer dissection. Analysis of variance showing levels of significant difference s • 1956 1937 Borers Burrows Lesions Borers Burrows Lesions Dates ITS * * * NS NS * * Hybrids *# * * ** * * # * H x D NS NS, NS * NSNS Treatments * * ♦ ♦ * * * * ** * * T x D * ♦ * * NS ** NS. T x H * * ★ * ** * * * * * T x D x H NS NS . NS ; NS NSNS NS = not significant, * =: significant at the 5 per cent level. ** = significant at the one per cent level. The effect of resistance and susceptibility within dates of planting on borer population is shown in table 11. In the two years resistant plants have been consistent in reducing borer populations and accordingly they showed fewer burrows and leaf lesions than the susceptible plants in all treatments under con sideration. The average reduction in borer population on the resistant strain as compared with populations on the susceptible strain was over 50 per cent. TABLE 11 Number of first generation borers in hybrids within dates of planting. Year Treatment Borers in Per cent decrease S R by R 1956 2 127 59 55.5 3 558 155 62.3 b 578 158 58.2 1957 • 2 8 k 50.0 '■■■ 5. ' ' 129 54 58.I 4 175 96 d4.5 Average 2 67.5 51.5 55.5 243.5 9^-5 61.2 4 275.5 127.0 55.9 There has been a treatment - hybrid interaction which showed high significance for borers, burrows, and leaf lesions in both years as shown in table 10. This indicates that the two hybrids did not react in both years in the same way to the different treatments, i.e. source of infestation or different population levels. ' : ■ 65 It has been shown previously how the resistant plants de creased the rate of the larval development. A similar effect appeared to operate on the rate of pupation. Regardless of date of planting the resistant plants harbored fewer pupae than did the susceptible plants. Number of pupae found in the suscep tible plants surpassed that on the resistant plants by 58 per cent in some treatments as shown in table 12. TABLE 12 First generation pupae in hybrids within date of planting. No . pupae 7 100 plants 1956 1957 Treatment s R S R " '.2 28 8 0 0 3 58 0 11 0 4 67 3-7 19 0 8 47 0 25 0 It must be pointed out that the number of pupae found during the midsummer dissection does not help in distinguishing between the univoltine and multivoltine borer populations. These pupae after emergence represented but a portion of the second generation moth population in as much as some of the larvae might have been on their way toward pupation. 66 Fall population: The sampling of the fall population was carried out at the end of the season when the ears were ready for harvesting. All eight treatments were included in this dis section and there were four levels of population. The first was in treatment 2 which was left to be naturally infested by both first and second generation borers. The second level'was in treatments 3 and k which, besides being naturally infested by both generations, were given extra loads of first generation egg masses. Treatment 3 was kept first generation borer free by EFPf sprays, thus harbored natural second generation borers only. The fourth level type was in treatments 6 and 7 which were kept first generation borer free and were given two dif ferent levels of second generation egg masses in addition to the natural infestation. In order to study the first generation bor er population which resulted from natural and artificial in festations in 1956 treatment 8 was sprayed with DDT to eliminate second generation borers. Therefore the borers of the fall pop ulation were assorted by the different treatments to give the following types ox population: 1. First generation population only in treatment 8 (in 1956)• 2,. Second generation population only in treatments 5» 8, and 7» 3* First and second generation populations in treatments 2, 3, and V, and treatment 8 of 1937* 67 The results of the fall dissection which covered the number of borers and burrows are given in tables 26, 36, and 37* The data have been analysed statistically each year according to split-split plot design procedures and the levels of significant differences are indicated by the usual characters in table 13 . TABLE 13 Fall population. Analysis of variance showing levels of signi ficant differences. 1956 1957 Larvae Burrows Larvae Burrows Dates ■ HS * * NS Hybrids * * * * * * * # H x D NS ** NS Treatments * * ♦ * * * T x D NS * NS NS T x H * * *.* NS * * T x D x H NSNSNSNS Second generation natural oviposition was so low that it did not give dependable data in both years. In 1956 only two egg masses were encountered on 96 plants checked on August 21; six days later four egg masses were found and finally two egg masses on August 31. In 1957 no egg masses' were found until August 20 when 72 plants were checked and nine egg masses were encountered. Four days later two egg masses were found and none during two later counts. 68 The resistant hybrid has consistently reduced the borer pop ulation'-. in both years and in all eight treatments (except a slight increase in treatment 5 of 1956)* Comparing the popula tions of both hybrids it appears that in some cases the borer population in the susceptible plants exceeded that in the re sistant plants by almost 7° per cent, Table 14. Accordingly, as may be expected, the damage done by the larvae as measured by the number of burrows was less in the resistant than in the susceptible hybrid. TABLE 14 Fall dissection. Borer population in hybrids within planting dates. Treatment No. Year Hybrid 1 2 3 4 5 6 7 8 1956 S 9. 94 217 287 17 116 203 179 E 6 39 85 ■87 19 105 179 59 Per cent decrease 33.3 58.5 60.8 65.7 10.5 9-5 61.1 67.0 1957 S 51 36 98 92 56 2k0 274 87 E 23 15 55 57 17 123 169 7° Per cent decrease 54.9 58.3 43,9 38.0 69.6 48.7 38.3 19.5 Average of two years S'. 30 65 157.5 189.5 36.5 178 238.5 133 E 14.5 27 70 72 18 114 174 64.5 Per cent decrease 51.6 58.5 55.6 b2 .0 50.7 27.4 27.4 51.5 The above data show that although resistance reduced the lar val population in all treatments the per cent decrease followed different patterns in the two years. In 1956 the effect of re sistance was more severe in plots which received extra load of " V 69 egg masses ofNeither first or second generation (except treat ment 6) than in plots which harbored natural infestation only. This phenomenon was reversed in the season of 1957 when the ef fect of resistance proved to be less in treatments which received extra egg masses artificially than in naturally infested treat ments. The average of the two years data shows that the effect of resistance in decreasing the borer population was low if this factor was operating against second generation borers only (treat ments 5, 6, and 7)jwhereas it proved to be more effective against borers of the two generations together (treatments 2, 3V and 8). This latter observation may be accounted for as the factor of resistance was more effective in reducing the first generation population when the plants were in an earlier stage of develop ment. This effectiveness appears to be more obvious when a com parison is made between the two treatments 6 and 8 of 195&. .la as much as treatment 8 received first generation infestation only (natural + three egg masses), treatment 6 received second generation infestation only (natural + three egg masses), we might consider the plants of the two treatments to have carried approximately the same number of egg masses although of different generations. The factor of resistance in plants of treatment 8 therefore had the chance to operate against the first generation larvae early in the season and reduced the population 67 per cent as compared with the population in the susceptible plants of the same treatment (table Ik). On the other hand the factor of re sistance in the plants of treatment 6 had the chance to operate . . ■ ■ 70 . against second generation larvae later in the season when the plants were in a more advanced stage of development, hence the per cent decrease in population was only 9'f5*- If this was true, the hybrid which showed resistance to the first generation did not exhibit the same degree of resistance to the second gener ation. The best explanation seems to be the one Neiswander (19^5) gave in a similar case when he stated that: corn-borer resistance or susceptibility in corn strains is not a specific character that is peculiar to certain strains at all stages of growth, but rather it is a rela tionship between tha corn plant and the in sect that varies with certain physiologic changes in the plant as it progresses to wards maturity.*' Effect of Date of Planting on European Corn Borer Population First Generation Population The date of planting affected the first generation borer population in three ways:- , ITatural oviposition: Egg deposition initiates the popula tion of the pest on its host. Therefore the' population of the pest on the field crop may be reduced to a minimum if a more at tractive host is available for oviposition. A host may be at tractive to the pest either because of its quality, abundance, genetic, morphological or chemical characters, synchronisation of its development and the flight period of the insect or be cause of its being in the most suitable development stage during the oviposition period. The effect of quality', genetic and mor phological characters, i.e.jthe effect of resistance and suscep tibility,; of the corn plants on the natural oviposition of the European corn borer moths was discussed in a previous section. Synchronization and nonsynchronization of the host de velopment and the moth flight is affected by different factors, one of them, which was under consideration in the present work, is planting date in relation to natural oviposition. Table 6 shows the numbers of egg masses per hundred plants for each hybrid and planting date and their relationship to the average normal and extended height of the plants on which eggs were deposited. It is evident from the data that in both hybrids the moths have shown preference for oviposition on the early over the late planting. Examining the relationship between the number . . . 71 ' ' 7 2 ; of egg masses encountered and the height of the plants it appears that the moths were more vigorously attracted to the higher corn plants. When the numbers of egg masses found on one planting date regardless of the corn hybrid were added the early planting was found to have received in some cases six times as many egg masses as have been received by plants which were planted eight days- later as shown in table 15 and figure 3* TABLE 15 First generation egg masses in date of planting within hybrids. Ho, ♦...?££ masses / 200 plants Date E ' L, • 6-26-56 ; 130 50 6-30-56 155 25 7-5-56 100 30 7-10-56 15 1° Total 4oo 95 Larval establishment and rate of development: The rate of larval development was observed to be different on plants of different planting date. To explain this observation data of two treatments are recorded in tables 16 and 17; the first represents data of treatment 2 (natural first generation infestation), the second represents data of treatment 4 with natural first gener- 'ation infestation plus six egg masses per plant. Number of larvae by instar found in each dissection in each of the early and late plantings within hybrids were recorded. The percentage 175 Eorly Late 150 125 100 75 5 0 if) cr: .June* 25 3 0 July 5 DATE Figure J First generation natural egg deposition on early and late plantings within hybrids. 73 74 of the larvae which, reached the last developing instar in each dissection was computed. These data show that the rate of lar val development was consistently faster in early than in late planting except in the last two dissections of the artificially infested plot of 1956 when the larvae of the two populations showed equal rate.of: development. It appears by comparing data of the two tables that the rate of larval development was higher in the naturally infested plots than in those which re ceived extra loads of egg masses artificially. The effect of V the late planting in retarding the rate, of development of the larvae was more pronounced in treatments which received natural infestation only than in those which received extra egg masses artificially. The course of larval establishment in the two treatments 2 and 4 is shown in figure 4. It is obvious that, except In one case, the early planted corn of 1956 in both treatments always harbored more larvae than the late planted corn. In naturally infested plots the populations of the two planting dates fol lowed the same pattern in their establishment except during the five-day:, period which elapsed between July 5 and 10. During that period the level of the larval population in the late planting remained stable, whereas the population in the early planting suffered a great loss in number possibly due to the density of the larvae on the plants. Early Late 3 0 0 Z50 ZOO 150 UJ 100 Treatment 4 (/) 5 0 Treatment Z ZO DATE Figure k Larval establishment on early and late plantings. Treatment 2. (1st generation natural infestation.) Treatment k. (1st generation natural infestation + 6 1st generation egg masses.) * 75 TABLE 16 Rate of larval development on early and late plants within hybrids. Treatment 2 (natural first generation, 1956). Dissection Planting No. of larvae by instar Date Date 1st 2nd 3rd 4th 5th • Total Per cent ** instar 6-30-56 E 15 3 18 16.7 L 1 0 1 0 7-5-56 E 30 19 7 * 56 12.5 L 14 3 2 19 10.5 7-10-56 E 6 13 9 28 32.1 L 9 6 4 19 21.1 7-14-56 E 0 4 7 4 2 17 11.8 L 1 6 2 9 0 7-19-56 E 1 3 17 9 12 42 28.6 L 0 6 6 2 3 17 17.6 7-24-56 E 0 0 13 25 17 55 30.9 L 0 0 13 11 : 2 26 7.7 * Natural infestation in 1957 was too low to provide dependable data. * * Per cent of the latest developing instar in each dissection. TABLE 17 Sate of larval development on early and late plants within hybrids. Treatment 4 (natural first generation + 6 egg masses). No . larvae by instar No. larvae by in star Dissection Planting 1956 Total Per * 1957 Total Per * Date Date 1 2 3 4 5 cent 1 2 3 4 5 cent 6-30 E 152 3 155 ■1.9" L 99 99 0 7-5 E 254 37 295 1.4 L 198 16 1 215 0.5 7-10 E 100 71 16 1 188 0.5 60 19 1 80 1.3 -o L 51 25 12 88 0 67 17 0 84 0 -v3 7-14,15 E 11 35 20 6 72 8.3 5 36 1 1 43 2.3 L 26 47 31 4 108 3.7 50 1 64 0 7-19 E 1 40 33 21 3 98 3.06 0 1 8 22 31 71 L 2 37 37 15 3 94 3.2 0 5 39 12 56 21.4 7-24,25 E 0 7 28 ' 4i 19 95 20 0 0 5 24 8 37 21.6 L 0 6 33 28 W;. 84 20.2 0 0 28 29 3 60 5 * Per cent of the latest developing instar in each dissection. .7 8 . Summer population: The effect of early and late planting on borer population is shown in table 18. TABLE 18 Number of first generation borers in dates of planting within hybrids.. Year Treatment No. Borers in Per cent E L difference 1956 2 131 : 53 - 58.0 3 250 243 - 2.8 4 274 232 - 15.3 8 248 213 - 14.1 1957 ' ' 2' " 4 8 + 50.0 3 87 96 + 9.4 4 137 132 - 5.6 8 72 90 + 20.0 In 1956 the late planting resulted in fewer borers than the early planting in both naturally infested plots and in plots which received an additional load of egg masses by hand applica tion. Although the per cent difference between borer populations was as high as 58 per cent in the naturally infested plots of 1956jthe difference between the populations of the two planting dates proved,to be nonsignificant in both years as shown in table 10. In both years there was a marked difference in borer popu lation between naturally infested plots and those which received extra egg loads artificially, - In both early and late planted corn the addition of six egg masses on plants of treatment 4 did not give twice as many borers as was found on plants of " ' . ' 79 treatment 3 which received only three additional egg masses per plant. There has been a significant treatment-date interaction for borers of 1956 only which indicates that the different treat ments reacted differently to the two dates of planting in 1956. It has been shown previously how late planting retarded the rate of larval development. A similar effect appeared to operate i on the rate of pupation. Regardless of corn hybrid the early planted corn harbored more pupae than the late planted corn in both years as shown in table 19• TABLE 19 First generation pupae in dates of planting within hybrids'. Ho. pupae / 100 plants 1956 1957 Treatment E L e l . " 2 ■ 22 14 0 0 3 44 25 6 6 4 42 42 17 3 8 42 14 17 8 Fall •population: The fall population consisted of a mix ture of the borers of the two generations in some treatment (2, 3i and 4), whereas in treatments 5 j 6, and 7 only second generation borers were present. In 1956 the number of borers found in five of the treatments of early planted corn slightly exceeded that which was found in late planted corn. In general the difference between the number of borers of the two planting dates proved to be nonsignificant. In 1957 the late planted corn harbored more second generation larvae than the early planted corn as shown by ■ ■ ' ■ ■ ■ '80 the number of larvae in treatments 5» 6, and 7 (table 20). Also treatments 4 and 8 which were given extra loads of first genera tion egg masses proved to have had more borers in the late plant ed corn by the end of the season than in the early planted one. The difference in number of borers between early and late plant ed corn proved to be significant at the 3 per cent level in 1957* There has been no significant treatment-date interaction in both years. TABLE 20 Fall population in dates of planting within hybrids. Treatment ITo. Year 2 3 4 - 5 6 7 8 1956 E 98 158 195 18 124 189 1^0 L 35 lM 179 18 97 193 98 Per cent 64.3 8.9 8.2 0 21.8 2.1 30.0 difference 1957 E 25 81 69 33 142 175 66 L 26 72 80 40 221 268 91 Percent 3.8 11.1 13.8 17.5 35-7 3^.7 27.4 difference Combined Effect of Corn Hybrids and Planting Dates on European Corn Borer Populations The foregoing discussion dealt with resistance and suscepti bility, early and late planting, as independent factors which in fluence the corn borer populations. Since the resistant and sus ceptible corn hybrids were planted early and late each season with ten days between the two plantings, corn hybrid and date of plant ing were expected to work simultaneously as two factors influ encing both first and second generation borer populations with re gard to the different treatments which took place in this experi ment. . ■ ' First generation population The corn borer population is fundamentally a result of either the number of egg masses deposited, or the rate of larval estab lishment, or both. Therefore any factor or factors which in fluence the adult female to lay her maximum number of fertilised eggs or promote the larval establishment on the host are- to be considered harmful to our crops from the economical stand point. It is evident from data of table 6 that the early planting re ceived more eggs than the late planting whether considered re sistant or susceptible. Within one date of planting the sus ceptible plants had more eggs than the resistant plants. Within hybrids and dates of planting the early susceptible corn plants received the highest number of egg massesuhile the late resistant plants had the lowest number. The number of egg masses found on ■ ■ -: 8l ■ ■■ . . TABLE 21 Bate of larval development in hybrids and planting date. No. per 6 plants - 1956* Treatment 2 Treatment 3 Treatment 4 Date DxH Ist2nd3rd4th3th Total Per * Ist2nd3rd4th5th Total Per * Ist2nd3rd4th5th Total Per* 1956 cent cent cent 6-30 ES 8 2 10 2.0 38 2 4o 5.0 61 3 64 4.7 ER 7 1 8 12.5 31 0 31 0 91 0 91 0 LS 0 0 0 0 33 0 33 0 43 0 43 0 LR 1 0 1 0 17 0 17 0 36 0 36 0 7-3 ES 18 16 7 41 17.1 112 27 10 149 6.7 119 27 0 146 0 EE 12 3 0 .15 0 90 9 2 101 2.0 135 10 4 149 2.9 LS 14 3 2 19 10.3 73 12 1 86 1.2 119 13 0 132 0 LR 0 0 0 0 0 45 5 1 51 2.0 79 3 1 83 1.2 7-10 ES if 9 8 21 38.1 33 31 21 1 106 0.9 51 51 13 1 116 0.9 ER 2 if l 7 14.3 20 5 2 0 27 0 49 20 3 0 72 0 LS 3 4 4 11 36.4 28 20 6 0 54 0 30 17 9 0 56 0 LR 6 2 0 8 0 14 10 1 0 25 0 21 8 3 0 32 0 7-1 if ES 0 4 6 4 2 16 12.5 13 42 19 1 1 76 1.3 9 20 13 3 43 6.7 ER 0 0 1 0 0 1 0 3 5 2 0 0 10 0 2 15 7 3 27 11.1 LS 0 0 1 0 0 1 0 11 19 19 5 0 54 0 21 38 20 3 82 3.7 LR 1 6 1 0 0 8 0 7 21 6 1 0 35 0 5 9 11 1 26 3.8 (Continued) TABLE 21 (cont'd) Treatment 2 Treatment 3 Treatment 4 Date DxH Ist2nd3rd4th5th Total Per * Ist2nd3rd4th3th Total Per * Ist2nd3rd4th5th Total Per* 1956 cent cent cent 7--19 ES 1 3 14 6 10 34 29.4 1 27 23 15 3 69 4 .3 0 25 22 17 3 67 4. 5 ES 0 0 3 3 2 8 2 5 .O 1 13 7 4 0 25 0 1 15 11 4 0 31 O' LS 0 3 4 1 3 11 27.3 4 19 23 9 3 38 5 .2 1 21 24 7 3 56 5 .4 LR 0 3 2 1 0 6 0 2 12 10 3 1 28 3 .6 1 16 13 8 0 38 0 7-■25 ES 0 0 8 14 14 36 38.9 0 6 17 18 18 39 30 .5 0 4 1.9 29 i4 66 21. 2 EE 0 0 5 11 3 19 15.8 0 2 6 8 1 17 5 .9 0 3 9 12 5 29 17. 2 LS 0 0 8 5 2 15 13.3 0 3 19 14 21 57 36 .8 0 3 15 19 10 47 21. 3 LR 0 0 3 6 0 11 0 0 1 10 8 2 21 9 .5 0 3 18 9 7 37 1 8 .9 * Per cent larvae which reached the latest developing instar in each dissection • • . . . .. , 8^ the early resistant plants exceeded that on the late susceptible and resistant plants. It is also evident from data of the same table that the.early plants/were higher than the late plants during the moth flight, a factor which, according to Neiswander and Huber (1929) and Patch (1929), was responsible for the higher number of egg masses received by the early plants. But it is worth mentioning that although higher in natural and extended height during July 5, 1° the early resistant plants received fev/er egg masses than the early susceptible plants. The larval population showed different response in its establishment and rate of development for the different treat ments as well as for the different hybrids and dates of planting. It is evident from data of table 21 that the larvae of treat ment 2 (which received natural infestation only) showed faster rate of development, as measured by the per cent larvae which reached the latest developing instar, than those of the arti ficially infested treatments. Also, except for the last dis section, the effect of resistance and susceptibility,, early and late planting, on the rate of larval development appeared to be greater in plants of treatment 2 than in treatments 3 and This last effect was measured in terms of the difference between the per cent larvae which reached the latest developing instar in the resistant and susceptible hybrids, the early and late plantings. With- few exceptions the fastest rate of larval . . . . g5 .. development was proved to be in the early susceptible plants. In most of the dissections the records shov/ed superiority .in the rate of larval development in the late susceptible plants over the early resistant plants. The late resistant plants were the most effective in retarding the development of their larvae. The larvae had better chance to "establish themselves" on the plants of some treatments as well as on a particular hybrid with regard to its date of planting’. Table 22 reveals the fact that a better establishment was achieved when the infestation was at the lowest level. TABLE 22 Comparison of larval establishment in plants of treatment 2 , 3» and 4. Treatment No. ■ ■ 2 . 3 ■ : 4 Date HxD 7-5-56 7-25-56 Per* 7-5-56 7.-25-56 Per* 7-5-56 7-25-56 Per* cent cent cent ES 4l 36 -12.2 149 59 -60.4 146 66 -54.8 ER 15 19 +26.7 101 17 -83.2 149 29 -80.5 LS 19 15 -21.1 86 57 -33.-7. 132 4? -64.4 LR 0 11 51 21 -58.8 83 37 -54.5 * Per cent difference in larval populations. The above data, which were taken from table 2 1 , shows the per cent difference in the larval populations of both hybrids of the two planting dates.in three treatments. These percentages . ' . 86 were calculated, on the basis of the difference between the larval populations of each treatment in two dates, namely, July 5 when the population reached its peak and July 25 when the last dissec tion took place. It is evident that the per cent mortality in the larval population was at its minimum in the naturally infested treatment 2 where larvae were less crowded. Decrease in population was the dominant phenomenon among the artificially infested plants inhere the individuals were more dense, hence as high as 83.2 per cent of the larvae perished on the early resistant plants of treatment 3* Populations of treatments 3 andsuffered the highest percentage of larval perishment on the early resistant plants. Better .establishment took place on the late susceptible of treatment 3 and the early susceptible and^late resistant of treatment The early susceptible plants of treatment;2 with a mortality of only 12.2 per cent of their larval j>opulation proved to be less deleterious to larval establishment than the late susceptible plants. It is noteworthy that the hybrid and planting date combina tions did not respond in affecting the establishment of the larvae or their rate of development in a different way when plants of treatment 4 were infested with twice as many egg masses as was given to plants of treatment 3« The final sampling of the first generation borers was made in the midseason period (August) when most of the borers were in the fifth instar and pupation was underway. This midsummer 87 dissection covered only the first four treatments and treatment 8. In 1956 the high natural oviposition resulted in marked difference between the borer populations of tke; borer free plants of treat ment 1 and that of treatment 2 which were left to be naturally in fested (table 2k), This difference was insignificant in the sea son of 1957 due to the low natural oviposition. In both years there was no difference in the borer populations of treatments 3, and 8. There was a marked difference between the first two treatments and the last three treatments;, a result which was expected due to the extra load of egg masses received by the lat ter. The two levels of artificial infestation did not give two levels of borer populations on treatments 3 and k, Figure The difference in the number of borers among treatments proved to be highly significant in both seasons. Within one planting date the susceptible hybrid had more borers per hundred plants than the resistant hybrid in all treatments under consideration (table 23) and the difference was highly significant in both seasons. Within one hybrid the early planting harbored more borers per hundred plants than the late planting in 1956 and the situation was reversed in 1957; but the difference in both years was not significant. TABLE 23 First generation population by hybrids and planting dates per 100 plants. ■ Hybrid Planting Date S R E ' ' ' L '' 1956 Borers 676 261 503 433 1957 Borers 244 118 172 190 Average 46o 189.5 337.5 311.5 In the season of 1956 the highest population was found in the early susceptible and the second highest population was on the late susceptible plants, Figure 5. With two exceptions the early resistant harbored more borers than the late resi stant plants which had the lowest populations in all treatments under consideration, table. 24. There was a significant hybridI - date interaction for borers of 1957 only. TABLE 2k First generation population by treatment in both hybrids and dates of planting'. Borers; in Year Treatment ES ER . LS LR Total 1956 1 2 . 0 6 1= 9 2 94 37 33 22 186 3 189 61 169 74 493 k 193 81 185 77 536 8 194 5k ' 151 62 461 1957 1 6 4 8 8 26 ■. 2 3 1 5 3 12 3 53 3k 76 20 183 4 82 55 91 41 269 8 51 21 65 25 162 Total . 867 348 789 333 2,337 Average 86.7 34.8 78.9 33.3 233.7 88 ■ ■ 89 The: effect of the factors .under consideration in this ex periment on the rate of larval development appears to have ex tended also to an influence on the rate of pupation. The num ber of pupae per hundred plants varied with the different hybrids and planting dates (table 25). The early susceptible plants which proved to have had better establishment and faster rate of development in their larval populations harbored the maximum num ber of pupae (133) found in all combinations of hybrids and d,ates of planting in both seasons. : On the other hand, the late resist ant plants which retarded the rate of development, of their lar val population and had a poorer larval establishment harbored but eight pupae in all eight hundred plants. The late susceptible had more pupae than the early resistant plants but fewer pupae than the early susceptible had. Factors which affect the rate of pupation of the first borer generation in the Midwestern States are not as yet fully understood. These factors, if fully investigated, would not only be of interest from the physiological view point but also of the utmost importance from the economic aspect. TABLE 25 First generation pupae per hundred plants in hybrids and date of planting. . ; Treatment No. Total Average Year D x H .: •' 2 3 4 8 1956 ES 14 44 33 42 133 33 ER 8 0 8 0 16 4 LS 14 25 33 14 86 21 LR 0 0 8 0 " 8 ■ 2 1957 ES 0 6 17 17 40 10 ER 0 0 o 0 0 0 LSG 6 3 8 17 4 LR 0 0 0 0 0 0 Total 36 81 102 81 300 Average 4.5 10.1 12.8 10.1 37.5 Fall population and second generation The population samples in the fall were actually two popu lations: the larvae which went into diapause in midsummert and those which developed from egg masses layed by first:generation adult females. The former are usually called the: univoltine population; the latter are usually referred to as multivoltine populations. Therefore, the borer population at the end of the season will be referred to, in the present work, as fall popu lation rather than second generation population except where the population is specifically designated as "second generationl1" The populations of treatments 5» 6 , and 7 were all of the mul tivoltine type since first generation borers were eliminated by EPrT sprays. The populations of treatments 2, 3i and 4 were "■ 90, ■ ■ ■ ■ ■ - is gnrto pplto nbt yrd ad ae of dates and hybrids both in population generation First planting. NO. OF FIRST GENERATION LARVAE 200 iue 5 Figure NO. T N E M T A E R T ■ i Early Susceptible Susceptible Early i ■ SggJ lj Lt Resistant Late llljl La te S usce ptib le le ptib usce S te La E arly R esistant esistant R arly E 08 92 mixture of univoltine and multivoltine types. The population of treatment 8 of 1956 was all of the univoltine borers since the second generation borers were eliminated by DDT sprays. All treatments were investigated for fall population sampling and the number of borers found per 60 plants by hybrid and plant ing date is shown in table 26. TABLE 26 Fall population. Number of larvae by hybrids and planting dates per 60 plants. Treatment No.1 '■ ■ ■ Year DxH 1 2 3 k 5 6 7 8 Total 1956 ES 4 68 108 143 9 55 81 97 565 ER 4 30 50 52 9 69 108 43 365 LS 5 26 109 144 8 61 122 82 557 LR 2 9 35 ; 35 10 36 71 16 214 1957 ES 21 15 46 45 21 8.7 103 32 370 ER 12 10 35 24 12 55 72 34 254 LS 30 21 52 47 35 153 171 55 564 LR ll 5 20 33 5 , 68 97 36 275 Total 89 184 455 523 109 584 825 395 3,164 Average : 11.0 23.0 56.9 6 5 .4 13.6 73.0 103.1 The number of larvae differed within treatments. The natu rally infested treatments 2 and 5 harbored fewer larvae than the rest of the treatments which were infested by extra load of either first or second generation egg masses (table 27 and Figure 6). The difference in the number of borers between treatments was highly significant in the fall dissection. TABLE 2? Fall population. No. of larvae within hybrids and dates of plant ing. Data transformed by Vobs.+0.5 Treatment L.S.D. Year 1 2 3 4 5 6 7 8 0.05 0.01 1956 0.74 O .96 1.24 1.32 O .78 1.10 1.32 1.13 0.40 0.53 1957 1.79 1.55 2.51 2.54 1.73 3.81 4.19 2.57 0.4 0.5 The borer resistant plants (table 28) had fewer larvae than the susceptible plants; the difference between the two populations was highly significant. The late planting had fewer larvae in 1956 than the early planting, but the reverse was true in 1957 when the difference between populations was significant at the 5 per cent level. The average of the'two years (table 28) showed slight increase in the number of larvae in the late planting over the early planting. TABLE 28 Fall population. No. of larvae by hybrids and planting dates. .Hybrid Planting Date Year S R E L 1956 1122 579 950 771 1957 934 529 624 839 Average 1028 554 777 805 93 Early Susceptible m r Early Resistant Late Susceptible Late Resistant VO 4=- TREATMENT NQ Figure 6 Fall population. No. of larvae in hybrids and dates of planting. . ■ ■ ...... 95' In 1957 the number of borers found in the fall in the late susceptible plants surpassed what was found in the other three combinations of hybrid and planting date. In 1956 the populations on the early and late susceptible plants were equal in number (table 29). The late resistant harbored the minimum number of larvae found in the fall of 1956 but its population exceeded that of the early resistant in 1957* The population of the early susceptible has always exceeded in number that of the early resistant. TABLE 29 Fall population. ,No. of larvae in hybrids of each planting date in all treatments. Hybrid x Date Year ES ER: LS LR 1956 565 365 557 214 1957 370 2 5 V . 564 275 Average, 467.5 309.5 560.5 244.5 In a comparison between treatments which received only second generation borer infestation, either naturally or: by hand application to supplement, the natural infestation, it was evident that the late planting was more susceptible to the second gener ation borer infestation than the early planting (Figure 7). The data, in table 30 indicate that (l) The late susceptible plants harbored, more second generation borers than any other hybrid - date combination. (2 ) Although the late resistant plants harbored '■ 96 fewer larvae than was found in the early resistant plants the difference between the two populations was not significant. TABLE 30: Second generation larval populations. H x D Treatment No. 5 6 / 7 Total 1956 + 1957 ES 50 142 .184' 556 ER 21 124 180 325 LS 43 214 295 55 0 LR 1 5 102 168 285 Averages ES 15 J 71 92 178 ER 10.5 62 90 162.5 LS 21.5 107 146.5 275 .■ LR 7.5 51 84 142.5 The above data also show that resistant and susceptible plants when planted early or late did not seem to show different effects on second generation borer populations. This was evident even when one treatment (7 ) received twice as r/iany egg masses as the other treatment 6. Evidence that in the fall dissection both univoltine and multivoltine population existed in treatments which received first and second generation infestations is clearly shown in a comparison between treatments 3 and 8 for 1956. Both treatments received natural first generation infestation plus three egg masses (approximately 60 eggs) by hand application. The Ll I 200 Early Susceptible Sffi Early Resistant Late Susceptible Late Resistant Ll I 120 vO -O TREATMENT NO. Figure 7 Second generation population in hybrids and dates of planting. 9 8 . difference between the two. populations in that year was that treat ment 8 was sprayed with DDT to eliminate second generation borers. The two populations were sampled after three days and again two. weeks after spraying. No significant difference was detected in the number of the first generation borers on the two treatments (table 31)j■a result which indicates that DDT sprays did not reduce the fifth instar larval population. TABLE 31 ■' First generation borer population in treatments 3 and 8 , (8 was sprayed with DDT). Date Treatment Hybrid x Date. ES ER LS LR 7-8-56 3 92 22 62 25 8 93 2^ 66 22 8-7-56 3 75 20 70 22 8 77 17 52 17 The population at the fall dissection in treatment 8 there- fore represented the univoltine borers which went into diapause in summer. The difference between the populations of the two treatments at the fall dissection is an estimate of the number of second generation larvae which developed on treatment 3 through natural infestation (table 32 ). ; TABLE 32 Estimate of 1956 second generation larvae on treatment 5* H x D Treatment Difference 3 8 ES i 108 97 11 EE 50 1 7 LS 109 82 27 LS 35 16 19 It appears from the above data that both hybrids of the late planting of treatment 5 had more second generation lar vae than the early planting. Within dates of planting the sus ceptible harbored more second generation larvae than the re sistant hybrid. 99 Damage Done by the European Corn Borer The total Injury done by the borer appears as a reduction in yield. This may be the result of an increase in the number of barren plants, a decrease in the size of the ears, and an increase in the number of unmarketable ears.which fall on the ground and are not picked up by mechanical pickers and are eventually spoiled by contact with the soil. Another type of injury is the breakage of plants because of the tunnelling of the borer. If this breakage occurs below the ear, harvesting is interferred with and the loss of corn. from fungi and rodents increases. The European corn borer does several other types of damage: feeding on and making holes in leaves; feeding on the tassel, frequently causing breakage of its stalk and bran ches; tunnelling and causing breakage in the midrib and near the point of junction of the leaf and leaf sheath; tunnelling in the stalk; and tunnelling in the ear shank and in the ear itself. Loss in yield is due to the feeding activity of the borer on the plant chiefly in the production of lesions and cavities. The leaf lesions reduce the leaf area which can produce the carbohydrates necessary to insure yield. The leaf lesions and cavities inhibit the transportation of these products through out the corn plant. 100 '1 0 1 ' Burrows and leaf lesions It has been pointed out that regardless of the date of plant ing the resistant plants harbored fewer first generation borers than the susceptible plants. It is evident from table 33 and figures 8 and 9 that the damage done by the first generation borers as measured by the number of burrows and leaf lesions is correlated-with the number of larvae found on the plants. The number of burrows and leaf lesions showed significant differences among treatments. The naturally infested plants (treatment 2) had fewer burrows and leaf lesions than treatments 3 and. k which received artificial infestations. In both years the resistant plants suffered less damage from the first generation borers than the susceptible plants, with a highly significant difference between the number of burrows and leaf lesions found on. the plants of the two hybrids (table 3^)• The average of the t\^o years shows more burrows and leaf lesions per plant on the susceptible plants than the resistant plants. 7 2 91 8 0 31 56 137 1 1 0 571 3.1 2 1 36 1 1 1 1 2 9 1 8 2 137 1 8 5 8 8 0 1 . 8 96 96 ER LS67 LR 53 58 1 1 33 79 59 115 Lesions in 529 8 3.2 95 67 76 2 0 8 115 9 1 8 1 9 0 4 2 27 LR ES 32 52 51 157 113 159 6 139 575 - 88 83 74 14 2 0 1 246 113 1 0 2 9 .4 .4 3. 2 65 89 49 1 1 2 2 1 0 1 0 1 1 6 2 125 705 ER LS Burrows in 7 6 2 4.' 97 96 277 291 158 138 3 0 0 ES 1363 3 1 . 1 6 2 77 LR 22 74 2 0 41 2 3 324 TABLE 33 5 2.7 76 33 65 151 1 8 5 1 6 9 775 1 1 . 2 ER LS Borers in 81 37 61 54 34 55 91 2 1 344 3 3 94 82 ES 53 51 194 193 189 859 8 4 2 8 2 3 3 4 plants. . 3 6 Total Average per plant Number of first generation borers, burrows, and leaf lesions, in hybrids of each planting date 1956 per Year Treatment 1957 102 Burrows MM Lesions gSS Larvae < Q '20 QC'Z Ll1 < J-1 o U_ CD m LS LR IHYBR1D x DATE OF PLANTING ■ : Figure 8 Number of first generation borers, burrows and leaf lesions in hybrids of each planting date - Treatment 2 (natural infestation.) Burrows Lesions Larvae o -p" a&aS HYBRID x DATE OF PLANTING Figure 9 Number of first generation borers, burrows and leaf lesions in hybrids of each planting date, Treatment k (natural infestation + 6 egg masses.) TABLE 34 Number of burrows 'arid leaf lesions in hybrids within:date of planting. First generation damage. Year Treatment Burrows in Lesions in ' ' s ' S S R 1956 2 232 14-0 131 98 : 3 301 264 375 206 537 319 : 390 252 1957 2 20 ; 6 29 18 . • 3 : 165 97 187 109 ■ : ^ • 251 153 244 149 Total 1 706 : 979 1356 832 Average per plant 7.9 4.5 6.3 3-9 There has been significant difference between the number of burrows.and leaf lesions; found o n ;the early and late plantings in 1956 with the early planting suffering more damage than the late planting. In 1957 the late planting showed more lesions than the early planting but the difference in the number of burrows was not significant. The average for the two years (table 3 5 ) shows more burrows per plant on the early than on the late planting, but there was no difference in the number of lesions between the two plantings. 105 TABLE 35 Number of burrows and leaf lesions in dates of planting within hybrids. First generation damage. Year Treatment Burrows in Lesions in E L E L 1956 2 2k7 125 162 67 3 *+25 3*+0 286 295 k *+53 *+03 323 319 1957 2 • 8 18 19 28 3 162 120 120 167 k • 239 165 173 220 Total 153*+ II7I 1092 1096 Average per plant 7.1 5.*+ 5.! 5.! In general the early susceptible plants had the highest number of burrows and leaf lesions caused by the first gener ation borers (table 33 and Figures '3' and 5 1-). The late resistant plants had the lo^^^est number of burrows but few more lesions than the early resistant had. The late susceptible plants had more burrows and leaf lesions than the early resistant. It is apparent that the second generation borers alone were responsible for the damage done to the plants of treatments 5 i 6 , and 7 since the borers of the first generation were eliminated by EPN sprays. It has been shown previously that the resistant plants have consistently reduced the first and second generation borer populations in both years and in all treatments. Accord ingly, as may be expected,.the damage done by the second gener ation larvae as measured by the.number of burrows found by the end of the season was less on the resistant:than on the susceptible 106 ' . . 107 - plants of treatments 5, 6, and 7 (table 36). It is obvious that the difference between the number of burrows in the two hybrids was greater in both years on naturally infested plants (treat- ' ment 3 ) than on those which received artificial infestations. TABLE 36 Number of burrows in hybrids within dates of planting. Second generation damage. Treatment No. : Average Year Hybrid 5 6 7 Total : per plant 1956 s 54 289 318 861 ■ 4.8 R 39 258 444 741 4.1 per cent difference 27.8 10.9 14.3 14.6 1957 s 89 530 631 1250 6.9 R 30 303 378 711 4.0 per cent difference 66.3 42.8 40.1 42.0 Regardless of the corn hybrid the early planting had little more burrows than'the late planting in treatments 6 and 7 (table 37) • In 1957 the late planting had more burrows than the early planting. TABLE 37 Number of burrows.in dates of planting within hybrids. Second generation damage * Year Planting Treatment No. Average Date 3 6 7 Total per plant 1956 E ' 43 296 532 871 4.8 L 30 251 430 731 4.1 1957 E 58 362 431 851 4.7 L 61 471 ; 579 1111 6.2 In comparing the damage done by the larvae of the two gener ations, data from treatments 6 and 8 of 1956 were used. The two treatments received natural infestations plus three egg masses per plant artificially. The first generation borers were elim inated from plants of treatment 6 by EPN sprays; treatment 8 was kept second generation borer free by DDT sprays. This procedure made it possible to provide almost equal number of borers on plants of both treatments and to have only first generation larvae on plants of treatment 8 and second generation larvae on plants of treatment 6, It is evident from data of table. 38 that the first generation larvae inflicted heavier damage on the plants than the second generation larvae. The damage done by the second generation larvae did not differ significantly, among the two hybrids or planting dates of treatment 6, but there was significant difference between the number of burrows made by the first generation borers with the early susceptible plants having the maximum number of burrows and the early re sistant showing the minimum number of burrows. The late suscep- 108 109 tible plants had more burrows than, the. late resistant plants. TABLE 38 ■ Comparison between damage done by first and second generation larvae of 195& ' D x H No, Burrows in Treatment 6 /8 ■' . : ES 153'"'': 470 ER 1^3 188 LS 136 36k LR 115 : r : 207 . Total 5^7 1229 The infestation of plants by first generation borers seemed to have had a certain degree of effect on the infestation of the same plants by second generation borers. Since plants of treat ment 2 were naturally infested by borers of both generations.,the result of subtracting the number of burrows found in the midsum mer dissection from that which was found in the fall dissection gives an estimate of the number of burrows made by second gener ation borers. By comparing this number with the number of bur rows found in the fall in treatment 5>which was naturally in fested by second generation borers only, it appears that regard less of hybrid and date of planting, plants of treatment 2 had fewer burrows made by the second generation borers than plants of treatment 5 (table 39)* TABLE 39 Effect of first generation infestation on infestation by second generation borers. Comparison between damage on treatments 2 and 5 (numbers per 100 plants). D x H 2nd generation burrows on treatment '■ 2 ' ' " ' " 5 ■ ES 27 V? ER 3 25 LS 12 b3 LR 2 : bo This leads to the conclusion that the more the plants vrere attacked by first generation borers the less they were attacked by borers of second generation. Yield The total injury done by the corn borer appears as a re duction in yield. There was a highly significant difference in the yield of the different treatments in both years (table ^fO). The borer free treatment (l) outyielded every other treatment in 1957* The increase in the yield of treatment 5 of 1956 (natural second generation infestation only) over the yield of the borer free treatment 1 of the same year was pos sibly due to experimental error. 110 TABLE 40 Yield in bushels per acre by treatment within hybrids and dates of planting. Year Treatment No • - 1 2 3 4 5 6 7 8 1936 96.1 88.5 89.8 83.7 100.9 86.1 85.3 90.5 1957 90.6 89.5 86.4 84.1 89.1 87.1 84.9 86.8 Average 93.4: 89.1 88.1 83.9 95.0 86.6 85.1 ------— ; Reduction in yield was most marked in treatment 4 (natural first and second generation infestation + 6 first generation egg masses) where the average of the two years showed a reduc tion of 9.5 bushels per acre or 10.2 per cent compared with the yield of the borer free treatment. The naturally infested treatment (2) outyielded treatment 3 (natural + 3 first gener ation egg masses) by only one bushel per acre, a reduction which can be attributed to the first generation three egg masses. Since treatment 2 received natural first and second generation infestations, and treatment 5 received only natural second generation infestation, and since treatment 5 outyielded treatment 2 by 5*9 bushels per acre or 6.2 per cent, it follows that the reduction in the yield of treatment 2 was probably due to the first generation natural infestation. The effect of the extra first generation egg masses received by plants of treat ment 4 resulted in a reduction of 4.2 bushels per acre or 4.8 111 ' ■ ' . . 1 1 2 ': per cent loss in yield as compared with treatment 3* It is evident that the second: generation borers alone were responsible for the yield iosses in treatments 6 and 7 since their plants were kept free of first generation borers by EPN sprays. The two treatments produced the second'lowest yield. The extra three egg masses received by plants of treatment 7 resulted only in 1.7 per cent reduction in yield or 1.5 bushels per acre less than treatment 6. The effect of the borers of each generation on yield can be explained by comparing the yield of treatments 6 and 8 of 1956 (table bo). Plants of treatment 6 were naturally infested by second generation borers plus three egg masses of the same generation and were kept first generation borer free by EPN sprays. Plants of treatment 8 were naturally infested by first generation borers plus three egg masses of the same generation, and were kept second generation borer free by DDT sprays. Al though this procedure made it possible to provide equal loads of borer population on plants of the two treatments at the time of their infestation, treatment 8 outyielded treatment 6 by ,b.b bushels per acre. ; It follows that natural second generation infestation plus three egg masses resulted in 5 Pe^ cent reduc tion in yield as compared with, natural first generation infes tation plus three egg masses. In a comparison between yields of treatments 3 and 8 of 1956, the second generation natural infestation showed little 113 ■■■ effect on the yield of treatment 3. Plants of both treatments received natural first generation infestation plus three egg masses artificially and were found to harbor equal numbers of borers in midsummer dissection. Later in the season treatment 3 was naturally infested by second generation borers which were eliminated from treatment 8 by DDT sprays. Treatment 8 out- yielded treatment 3 by only 0.7 bushels per acre (table 40). The effect of resistance and susceptibility to corn borer infestation on yield appears in a comparison between the yield of the two hybrids at certain levels of infestation and the yield of the same hybrid and the same planting date of the borer free treatment No. 1. In the season of 1956> when a fairly good borer establishment was maintained, the yield of the two hybrids was alike in treatment No. 1 (table 4l)". When the early planting of both hybrids were infested artificially n^ith six first gener- ation egg masses per plant in addition to first and second gener ation natural infestations (treatment 4) the yield loss was 14.7 bushels per acre or 14.6 per cent for the susceptible plants as compared with 9*5 bushels per acre or 9*5 per cent for the resistant plants. The second generation artificial infestation of six egg masses per plant in addition :to natural second gener ation infestation (treatment ?) proved to have caused reduction in yield of both hybrids more than that which was caused by six first generation egg masses.. TABLE 4l Yield’in bushels per acre Cat 15-5 .per cent moisture) for both hybrids and planting dates. Treatment Ko • Year D x H 1 2 5 4 5 6 7 8 Average 1956 ES 100.7 95.6 98.6 86.0 110.2 96.1 83.5 90.6 95*2 Yield reduction* 2.1 14.7 0 4.6 17.2 10.1 ER 100.1 89.6 95L. 6 90.6 104.7 89.1 89.6 101.0 9 5 .I Yield reduction 10.5 4.5 9.5 0 11.0 10.3 0 LS 95.6 81.5 85*5 77-5 97-6 76.3 86.6 88.6 86.2 Yield reduction 14.1 10.1 13.1 0 19.1 9.0 7.0 LR 88.1 8 7 .I 79.5 80.5 91.1 82.5 81.3 81.5 84.0 Yield reduction 1.0 8.6 7.6 0 5.6 6.6 6.6 Yield average 96.1 88.5 89.8 83.7 100.9 86.1 83.3 90.5 1957 ES 104.7 96.6 98.1 100.1 97.6 103.7 95.6 97.6 99-3 Yield reduction 8.1 6.6 4.6 7.1 1.0 9.1 7.1 ER 94.6 95-1 89-1 88.1 94.6 90.6 91.1 92.1 91.9 Yield reduction 0 5-5 5.5 0 4.0 3-3 2.3 LS 81.0 82.0 76.0 72.5 83.0 76.5 80.0 80.0 79.1 Yield reduction 0 5.0 8.5 0 4.5 1.0 1.0 LR 82.0 94.5 82.5 75-5 79.0 80.0 73.0 77.5 79.3 Yield reduction 0 0 6.5 3.0 2.0 9.0 4.3 Yield average 90.6 89.6 86.4 89.1 87.7 84.9 8 6.8 * Reduction in yield was computed in comparisonwith yield of borer free treatment 1. .'ii5 The reduction in yield of treatment 7 was 17.2 and 10.5 bushels per acre for the early susceptible and the early re sistant plants respectively, v/hereas. in treatment 4 the reduc tion was 14.7 bushels per acre in the yield of the early.sus ceptible and 9*5 in the early resistant. The reduction in the yield of the early resistant plants exceeded that of the early susceptible in treatments -which received three first or three . second generation egg masses artificially (treatments 3 and 6 ). The reduction by three first generation egg masses was 4.5 and 2.1 bushels per acre for the resistant and the susceptible : hybrids respectively. The three second generation egg masses caused a reduction of 11.0 bushels per acre in the yield of the susceptible plants. The first generation infestation alone in treatment 3 of 1956 caused a ten per cent reduction or 10,1 bushels per acre in the yield of the early susceptible, while the yield of the early resistant plants did not suffer any reduction. Therefore by comparing the yield of the two hybrids in treatments 3 and 8 (table 42) it becomes evident that the natural second generation infestation was responsible for the yield reduction of the early resistant plants of treatment 3 * Another comparison can be made between the reduction in yield of the resistant and the susceptible plants in treatments 6 and 8 (table 42). The yield of the susceptible plants was re- duced by first generation infestation in treatment 8 , while the 116 yield' of the resistant plants was effectively reduced by second generation infestation in treatment 6. . TABLE h2 : Comparison between reduction in yield of treatments 3> .6 , and 8 of both hybrids of the early planting of 1956. Reduction in yield Bu/acre Treatment 3 Treatment 6 Treatment 8 natural 1st & 2nd natural 2nd + 3 natural 1st + 3 1st generation 2nd generation + 3 1st gener egg masses egg masses ation egg D x H masses ES 2*3. Zf.O 10.1 .■ ER \ 4,5 11.0 0 In the naturally infested treatments No. 2 of 1956 (table ^l) the reduction in yield of the resistant plants was twice as much as the reduction in yield of the susceptible plants. It is evident that this reduction was due to first generation natural infestation rather than second generation natural infestation since the latter did not reduce the yield of treatment 5 (natural second generation infestation). The late susceptible planting suffered more reduction in yield than the early planting. Without exception the reduction in yield of the late susceptible plants of 1956 surpassed the reduction in the yield of the late resistant plants to an ap preciable extent. The loss in yield in the naturally infested treatment 2 was 14.1 bushels per acre for the susceptible ■ ' ' ■' 117 plants and only one bushel per acre for the resistant plants (1^.7 and- one per cent). The first generation infestation in plants of treatment 8 reduced the yield of the two hybrids by seven and 6.6 bushels per acre for the susceptible and the re sistant plants respectively. The effect of the first and second generation infestations in treatment was greater than the ef fect of the second generation infestation in treatment 7 on the yield of both hybrids. The effect of three second generation egg masses (treatment 6) was greater than the effect of three first generation egg masses (treatment 3) on the yield of the late susceptible while the late resistant suffered more reduction in yield in treatment 3 than in treatment 6. In the season of 1957 the reduction in the yield of both hybrids by the corn borer infestation in the different treat ments was neither great nor consistent. • ' SUMMARY Investigations involving the European corn borer were carried on in the two seasons of 1956 and 1957 at the Ohio Agricultural Experiment Station,,. Wooster, Ohio. The objectives were (l) to study the effect of corn plant resistance and susceptibility to borer infestation, when early and late planted. (2) to study the damage done by the .'two generations of the borer under different levels of infestation. The 'two hybrids used were OH4j5xOH51A, the resistant hybrid, and WF9xMl^'j the susceptible hybrid. One planting was made. early and the other late in the'., normal planting season for each year. Different levels of infestation were attained by adding ! egg masses manually. The experiment was set up on a split-split plot design with eight treatments. Dates of planting appeared in the whole plot, hybrids in the subplot and treatments in the split plot. There were six replications of all treatments. The folloxiring results were achieved: 1. The early planted corn received more first generation egg masses than the late planted corn whether considered resistant or susceptible. Within hybrids and planting dates the early sus ceptible plants received the highest number of egg masses -while the late resistant plants had the lowest number. 2. The fastest rate of larval development was proved to be : in the early susceptible plants. The late resistant,plants were the most effective in retarding the development of the larvae. ■ ■ '118 ■ . ■ ■ ■ •' -ii9 ' 3* 'In the season of 1956 the highest first generation popu lation 'was found to be in the early susceptible and the second, highest population was.on the late susceptible plantings. The early resistant harbored more borers than the late resistant plant ings which had the lowest populations in all treatments under con sideration. 4. The second generation borer population of the early sus ceptible plants has always exceeded in number that of the: early resistant. In 1956 the populations on the early and late suscep tible plants .were equal in number. In the fall of 1957 the num ber of borers in the late susceptible plants surpassed what was found in each of the other three combinations of hybrids and planting dates. The late resistant harbored the minimum number of larvae found in the fall of 1956 but its population exceeded that of.the early resistant in 1957* 5. The late planting was more susceptible to the second generation borer infestation than the early planting. 6. In both years the resistant plants suffered less damage done by the first generation borers than the susceptible plants with a highly significant difference between the number of burrows and leaf lesions foundon the plants of both hybrids. 7* The damage done by the second generation borers was less to the resistant than to the susceptible plants. In 1956 the ear-: ly planting had a few more burrows than the late planting,, while in 1957 the late planting had more burrows than the early planting. APPENDIX TABLE 1 Borers, 1956 midsummer dissection. Analysis of Variance. Data transformed by V obs+.$ Source df MS ; ■ F Reps .'■5 .6812 .670 NS Date 1 .8517 .951 NS error a 5 .8953 Hybrids 1 32.5233 59.111 : H x D 1 .6474 1.176 NS error b 10 .5502 * * Treatments 3 45.7165 137.534 * T x D 3 1.1013 3.313 * ★ T x H 3 4.8015 14.444 T x D x H 3 .3419 1.028 NS error c 60 .3324 Total 95 MEANS: Hybrids: Treatments: S = 1.6743 1 = .7393 R =1.1990 2 = 1.2207 3 = 1.8583 Dates: 4 = I .9283 E = 1.4751 L = 1.3982 120 APPENDIX TABLE 2 Burrows, 1956 midsummer dissection. Analysis of Variance. Data transformed by V obs+.5 Source df MS F * Reps 5 1.1010 9.45 * * Date 1 4.8034 41.23 error a 5 .1165 Hybrids 1 26.9447 54.53 ** H x D 1 1.8136 3.670 NS error b 10 .4941 Treatments 5 84.5561 164.987 * * iN T x D 5 I .8086 3.528 ** T x H 5 2.4592 4.798 T x D x H 3 .7529 1.469 NS error c 60 .51.25 Total 95 MEANS: Hybrids:k Treatments: S = 2.000? 1 = .7774 R s 1.5681 2 = 1.6025 3 = 2.3175 Dates: 4 = 2.4403 E = 1.8757 L = 1.6951 121 APPENDIX TABLE 3 Leaf lesions, 1956 midsummer dissections. Analysis of Variance. Data transformed by V obs+.5 Source df MS F Reps 5 .7317 4.732 NS Date 1 1.6889 10.924 * error a 5 .1546 Hybrids 1 11.8479 47.524 ** H x D 1 .5936 2.381 NS error b 10 .2493 Treatments 3 63.5474 223.995 ** T x D 3 2.1613 7.618 *'* T x H 3 2.3577 8.310 ** T x D x H 3 .1216 . 428 NS error c 6o .2837 Total 95 MEANS: Hybrids: Treatments: S - 1.7142 1 = .7383 R = 1.4274 2 = 1.3348 3 = 2.0586 Dates: 4 = 2.1515 E = I.625O L = I.5I67 122 APPENDIX TABLE if Borers, 1957 midsummer dissection. Analysis of Variance. Data transformed by V obs+.5 Source df MS F Heps 5'.-, ' .3120 .883 NS Date 1 .5887 1.1 NS . ^ . error a • 3535 Hybrids 1 13.6344 91.26 ** D x H 1 .8398 5.62 * error b 10 .1494 Treatments > 26.5528 125.01 . ** T x D if .1207 .568 NS T. x H if 1.7463 8.222 ** T x D x H if .3667 1.726 NS error c 80 .2124 Total 119 MEANS: » • Hybrids » Treatments • S = 2.498 1 = 1.172 B = 1.824 2 = • 943 3 = 2.744 Dates: 4 = 3-364 E = 2 .IO3 8 = 2.581 L = 2.218 125 APPENDIX TABLE 5 Burrows, 1957 midsummer dissection. Analysis of Variance. Data transformed by V o'b's+.5 Source df MS F Reps 5 .583 .88 NS Date 1 2.3739 3.6 NS e r r o r a 5 .66 * * Hybrids 1 18.0948 96.5 D x H 1 .3617 1.93 NS e r r o r b 1.0 .1875 * + Treatm ents 4 40.733 106.35 T x D 1.463 3.82 * * T x H 4 1.605 4.19 ** T x D x H 4 .813 2.12 NS e r r o r c 8o .383 T o ta l 119 MEANS: Hybrids: Treatments: s = 3.023 1 = 1.359 R = 2.247 2 = 1.141 3 = 3.413 D ates: 4 - 4.045 E = 2.776 8 = 3.217 L = 2.494 APPENDIX TABLE 6 Leaf lesions, 1957 midsummer dissection. Analysis of Variance. Data transformed by V obs+.5 Source df MS F Reps 5 .56 7.785 * Date 1 if. 20 58.198 ** error a 5 .072 Hybrids 1 9.59 29.619 ** D x H 1 .072 .2286 NS error b 10 .317 Treatments k 35.5.03 75.928 ** T x D k .13^ .2855 NS T x H . k l.l6l 2. if 83 * T x D x H k .978 2.091 NS error c 80 .if 6 8 - Total 119 MEANS: Hybrids: Treatments: s = 2.967 1 = 1.362 R = 2.if07 2 = l.if38 3 = 3*/+9zf- Dates: k = if.010 E = 2.500 8>3.131 L = 2.8?if 125 APPENDIX TABLE 7 Larvae, 1956 fall dissection. Analysis of Variance. Data transformed by V obs+.5 Source df MS F Reps 5 1.63 4.42 NS Dates 1 1.80 4.90 NS error a 5 .368 * * Hybrids 1 20.78 60.095 H x D 1 1.53 4.41 NS error b 10 .346 * * Treatments 7 12.49 25.41 T x D 7 .421 .857 NS * * T x H 7 2.94 5.97 T x D x H 7 .352 .716 NS error c 140 .492 Total 191 MEANS: Hybrids: Treatments: s = 1.1801 1= .7398 5 = .7840 R = .9721 2 = .9640 6 =1.1000 3 = 1.2417 7 =1.3234 Dates: 4 = I .3228 8 = 1.1334 E = 1.1068 L = 1.0455 126 APPENDIX TABLE 8 Burrows.,: 1956 fall dissection. Analysis of Variance. Data transformed by V obs+.3 Source df MS F Reps 5 .889 .925 NS * Dates 1 7.642 7.95 error a 5 .961 Hybrids • 1 75.220 117.62 * * II x D 1 3.568 5.27 * error b 10 .640 Treatments 7 103.834 127.12 * + T x D 7 1.720 2.106 * * * T x H 7 7.672 9.59 T x D x H 7 .794 .971 NS error c 140 .817 Total 191 MEANS: * Hybrids • Treatments: S = 1.9556 1 =■ .7856 5 = .8849 R = 1.5598 2 = 1.6497 6 = 1.5660 3 = 2.3672 7 = 2 .0322 Dates: 4 = 2.5142 8 = 2.2619 E = 1.8208 L = 1.6946 127 APPENDIX TABLE 9 Larvae, 1957 fall dissection. Analysis of Variance. Data transformed by V obs+.5 Source df MS F Reps 5 1.55 5.57 NS * Dates 1 5.59 7.57 e r r o r a 5 0.46 H ybrids 1 26.11 50.2 ** D x H 1 4.50 8.65 * e r r o r b 10 O .52 Treatm ents 7 22.39 52.07 ** T x D 7 0.69 1.60 NS T x H 7 0.57 1.55 : NS T x D x H 7 0.27 0.63 NS e r r o r c 140 0.43 T o ta l 191 MEANS: Hybrids: Treatments: s = 2.95 1 = 1.79 5 = 1.75 R = 2.22 2 =1.55 6 = 3.81 3 = 2.51 7 = 4.19 Dates: 4 = 2.54 8 = 2.57 E = 2.45 L = 2.72 128 APPENDIX TABLE 10 Burrows, 1957 fall dissection. Analysis of Variance. Data transformed by \ o'bs+,5 Source df MS F Reps 5 1.05 2.19 NS Dates 1 O.Oif 0.08 NS error a ■ . '"5 o.if8 Hybrids 1 60.58 81.86 * * D x H 1 1.01 1.36 NS error b 10 0.74 ♦ * Treatments 7 86.15 187.28 T x D 7 1.6 0.35 NS * * T x H 7 1.31 2.85 T x D x H 7 0.53 0,7?' NS error c IbO 0A 6 Total 191 MEANS: Hybrids: ; Treatments: S = if.92 1 s 2.11 5 = 2.18 B = 3.79 2 = 2.05 6 = 5.83 3 = 5.36 7 = 6.39 Dates: if = 5*81 8 = 5.11 E = k.3h L = if.70 1 2 9 APPENDIX TABLE 11 Yield, 1956. Analysis of Variance. Data transformed by V obs+.5 Source df MS F "■ ■ Reps 5 127.53 3.705 NS Dates 1 193-40 5.619 NS error a 5 ; ■ 34.41 Hybrids 1 2.45 .246 NS D x H 1 1.98 .198 NS error b 10 9.96 Treatments 7 32.65 5.58 ** T x D 7 3.35 .571 NS T x H 7 2.65 .453 NS T x D x H 7 6.53 1.116 NS error c 140 5.85 Total 191 MEANS: Hybrids : Treatments: S = 18.02 1 = i9 .ll 5 = 20.08 R = 17.79 2=17.57 6 = 17.10 3 = 17.87 7 =16.95 Dates: 4 = 16.61 8 = 17.97 E = 18.91 L = 16.90 130 APPENDIX TABLE 12 Yield, 1957* Analysis of Variance. Data not transformed by V obs+.5 Source df MS f Reps 5 6.86 0.37 NS * * Date 1 510.6 27.5 ■ error a . ■ ' 5 . . . 18.56 Hybrids i 24.70 9.46 * D x H i 27.50 10.53 error b 10 ■2,61 ** Treatments: 7 ■5.19' 2.90 T x D 7 1.63 0.91 NS : T x H 7 1.04 0.58 NS ' T x D x H . 7 3.86 2.16 * error c 140 1.79'' Total 191 MEANS: Hybrids: Treatments: s = 17.72 1 = 18.02 5 = 17.71 R = 17.00 2 = 17.80 6 = 17.44 3 = 17.18 7 = 16.87 Dates: 4 = 16.68 8 = 17.21 E = 18.99 L =15.73 131 LITERATURE CITED Baker, W.A., W.G. Bradley, and C.A. Clark. 1949. Biological Control of the European Corn Borer in the United States. U.S.D.A. Tech. Bui. 983, pp. 185. Barber, George W. 1925. The efficiency of birds in destroy ing overwintering larvae of the European corn borer in New England. Psyche 32(l):30-4?. Bartholomai, C.W. 1954. Predation of European corn borer by arthropods. Jour. Econ. Ent. 47 5 295-299* Caffrey, D.J., and L.H. Worthley. 1937* A progress report on the investigations of the European corn borer. U.S.D.A. Dept. Bull. 1476. Ceaser, L. 1925. The mortality of the larvae of European corn borer in the early instars in 1924'. Fifty-fifth Ann. Rept. Ent. Soc. Ont. 1924. Cheu, S.P. 1940. Studies on the planting date of corn in re lation to European corn borer infestation, (in Chinese). Kwangsi Agr. 1(6):373—3^3* (with a summary in English). Chiang, H.C., L.K. Cutkomp, and A.C. Hodson. 1954. The effects of the second generation European corn borer. Trans. Penin sula Hort. Soc.: p. 94-96. Crawford, H.G., and G.J. Spencer. 1922. The European corn borer control measures. Jour. Econ. Ent. 15(3)5231-236. Deay, H.O., L.H. Patch, and R.O. Snelling. 1949. Loss in yield of dent corn infested with the August-generation of the European corn borer. Jour. Econ. Ent. 42 5 81-87. Everly, R.T. 1938a. Spiders and insects found associated with sweet corn with notes on the food and habits of some species. 1. Arachnida and Coleoptera. Ohio Jour. Sci. 38 (3 )5136-148. 1938b. Spiders and insects found associated with sweet corn with notes on the food and habits of some spe cies. II. Ephemerida, Lepidoptera, Neuroptera, Odonata, : Orthoptera, Thysanoptera, and Trichoptera. Ohio Jour. Sci. 3 8 (6 ):311-315. Ficht, G.A. I93I. Some observations on the planting date of corn and its relation to European corn borer population. Jour. Econ. Ent. 24:330-386. -132'': 133 Ficht, G.A. 1936. Relative resistance of selected strains of corn,to European corn borer. Jour. Econ. Ent. 29: 687-691. Harry, G.W., L.,D. Anderson, and C.R, Willey. 1936. European, corn borer on the Eastern shore of Virginia. Jour. Econ* Ent. 29:202-204. Hergula, B. 1930. Observations of the corn borer in Jugos lavia. Int’l, Corn Borer Inves., Sci. Repts. 111:142- 147* Hervey, G.E.R., and F.Z. Hartzell. 1931* Influence of planting dates of sweet corn on European corn borer infestation. Jour. Econ. Ent. 24:183-188. Holdaway, E.G. 1955* Unpublished data on the factors influencing corn borer population. Kept, in Tenth Annual Conference, Ent. Soc. Amer., East Lansing, Michigan, \ Mar. 24. Huber, L.L., and C.R. Neiswander. 1927* The European corn borer and ecological habitats. Jour. Econ. Ent. 2 0 : 337-341. ■ • ■ ■ ■ '' . 1928, The correlation between soil fertility and the European corn borer accumulation. Jour. Econ. Ent. 21:118-120. Huber, L.L., C.R. Neiswander, and R.N. Salter* 1928. The European corn borer and its environment. Ohio Agr, Expt. Sta. Bui. 429:pp.196o Kozhanchikov, I.V. 1938. Geographical distribution and . . physiological characters of Pyrausta nubilalis Hub., (In Russian). Zool. Zh. 1 7 (2 ):246-259* (with a summary in English). Meyers, M.T., L.L, Huber, C.R. Neiswander, F.D. Richey, and G.H. Stringfield. 1937* Experiments on breeding corn resistance to the European corn borer. U.S.D.A. Tech. Bui. 583. Neiswander, C.R., J.B. Polivka, and L.L. Huber. 1928. Correlation of corn borer population with corn development. Ohio Agr. Expt. Sta. Bui., 429. Neiswander, C.R. , G.W. Conrey,: and L.L. Huber. 1928. Soil type influences European corn borer accumulation. Bi monthly Bui. Ohio Agr. Expt. Sta., Jan.-Feb. . . ■ ■ . ' 13k : Neiswander, C.R.y and L.L. Huber. 1929. Height and silking as factors influencing European corn borer population. Ann. Ent. Soc. Amer. 22(3):527-542. Neiswander, C.R., and E.A. Herr. 1930. Correlation of corn borer population,with degree of damage. Jour. Econ. Ent. 23:938-9^5. Neiswander, .C.R*,. and L.L. Huber. 1931. Competition as a factor influencing corn borer survival. Ohio Agr. Exot. Sta. Bui. 470:81-82. Neiswander, C.R. 19^5* Insect Control. The European Corn Borer. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 659 :66-67. ______1946. The European Corn Borer. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 665:18-20. • " . 19^7* The European Corn Borer in 1945. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 6?3: 45-^7. . 1948. Late Planting no Longer Controls Corn Borer. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 674, p. :42. . ■ ■■- . 1949. The Relation Between Corn Borer In festation and Date of Corn Planting. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 6955 P* 86. . I95I. Corn Borer Infestation versus Date of Planting. Ohio Agr. Expt. Sta., Farm Science and Practice. Bui. 705:49-50. Painter, R.H., and G.A. Fitch. 1925* A field study of the re duction of European Corn Borer larvae in standing corn. Fifty-fifth Ann. Rept. Ent. Soc. of Ont. 1925. Palliot, A. 1938. On the natural equilibrium of Pyrausta nubilalis. Int'l. Corn Borer Inves., Sci. Repts. 1927-28:77-106. Patch, L.H. 1929. Some factors determining corn borer damage. Jour. Econ. Ent. 22(1):174-183. _____ - 1937. Resistance of a single-cross hybrid strain of field corn to European corn borer. Jour. Econ. Ent. 30:271-278. Patch, L.H., G.W. Still, M. Schlosberg, ana G.T. Bottger. 1942 Factors determining the reduction in yield to field corn by the European corn borer. Jour. Agr. Res. 65:473-82. Patch, L.H., J.R. Holbert, and R.T.Everly. 1942. Strains of field corn resistant to the survival of the European corn borer. U.S.D.A. Tech. Bui. 823. Patch, L.H. 1943. Survival, weight, and location of European :corn borers feeding on resistant and susceptible field ■ corn. Jour. Agr. Res. 66:7-19» Patch, L.H., and R.Tl Everly. 1945*. Resistance of dent corn inbred lines to survival of first-generation European corn borer larvae. U.S.D.A. Tech. Bui. 893* ____ . 1943. Contribution of inbred lines to . the.resistance of hybrid dent corn to larvae of the early summer generation of the European corn borer. Jour. Agr. Res. 76:257-63. Patch, L.H, 1948, : Effect ox plant development on resistance o corn hybrids to European corn borer. Jour. Econ. Ent. 41:766-769. Patch, L.H., H.O. Deay, and S.O. Snelling. 1931. Stalk- breakage of dent corn infested with the August generation, of the European corn borer. Jour. Econ. Ent. 44:534-539. Poos, F.W. 1927. Biology of the European corn borer and two closely related species in northern Ohio. Ohio Jour. Sci. 27:47-88. Savage, J.R. 1930, The relation of the habitat to European corn borer populations. Jour. Econ. Ent. 23:936-938. Schlosberg, M., and Ralph Hatiies. 193?. Egg and larval popu lations of European corn borer in relation to time of planting and yields of, sweet corn. Jour. Econ. Ent. 30:280-287. Turner, K., and Raimon L, Beard. 1950. Effect of stage: of : growth of field corn inbreds on oviposition and survival of the European corn borer. Jour. Econ. Ent. 43:17-22. ZwBlfer, W. 1928. Corn borer controlling factors and measures in southern Germany. Int'l. Corn Borer Inves. Sci. Rept. 1927-28:135-142. AUTOBIOGRAPHY I, Mohamed Tahir Kira, was born in Fariskour, Egypt, April 29, 1921. I received my High School education in Hel- wan Secondary School, Egypt. My undergraduate education was received at the Faculty of Agriculture, University of Cairo, Giza, Egypt, from which I obtained the degree Bachelor of Science in Agriculture, May 19^5• In December, 19^5i I was appointed Demonstrator of Entomology in the Faculty of Agriculture, University of Cairo. I obtained the degree Master of Science in Entomology from the same university in February, 1952. I remained in this teaching position until I was sent by the Egyptian Government to the United States of America for more graduate study in Entomology. Since Sep tember, 195^» I have been associated with the Department of Zoology and Entomology at the Ohio State University, completing the requirements for the degree Doctor of Philosophy. During 1956 and 1957 I conducted my field research at the Ohio Agricultural; Experiment Station, Wooster, Ohio. 156