388 



POPULATIONS 



The moths have a spring flight in the mid- 

 May to early July interval. They then ovi- 

 posit from late May to mid-July. These 

 eggs hatch into larvae, the earUest of 

 which appear about the first of June. These 

 larvae result from the early moth flights 

 with their subsequent oviposition. The lar- 

 vae continue to hatch through July, and 

 pupae are present from early July until 

 early September. The larvae that hatch 

 late in the season do not pupate, but over- 

 winter in the larval stage to become pupae 

 during late April through mid-June of the 

 following year. The first batch of pupae, 

 however, pass through their development 

 rapidly and provide the new generation of 

 moths during the mid-July to mid-Septem- 

 ber interval (year I). These also have a 

 flight and lay second generation eggs, 

 which complete hatching by mid-Septem- 

 ber. All the second generation larvae over- 

 winter, which means that there are no 

 second generation pupae formed during 

 the first year. Pupation of the second gen- 

 eration larvae, along with the late, first 

 generation residue, occurs in the late April 

 to mid-June period of the following spring. 

 The eclosion of these pupae produces the 

 adult moths that start the new year's re- 

 productive cycle. 



Barber classifies the factors that limit the 

 seasonal abundance of New England corn 

 borer populations into these major groups: 

 weather, enemies, agricultural practice and 

 control, overpopulation, and change in 

 "preferred" host plant availabiUty. Of these, 

 the first group is primarily density-inde- 

 pendent; the second and fourth are pri- 

 marily density-dependent; and the third 

 and fifth have elements of both. The list 

 follows: 



WEATHER: 



1. Heavy winds and rains destroying moths 



2. Heavy winds and rains preventing moths 

 from depositing a normal number of eggs 



3. Unfavorable temperature causing moths 

 to deposit a smaller number of eggs 



4 Heavy drought resulting in lowered egg 

 fertility 



5. Heavy rains resulting in the destruction 

 of newly hatched larvae before they 

 have had time to bore in 



6. The seasonal proportion of individuals 

 that have only a single generation 



7. The abundance of larvae that fail to 

 become full grown before feeding is 

 discontinued because of cold weather 



8. Winter mortality of overwintering larvae 



ENEMIES : 



Birds: 

 9. Feeding on moths 



10. Removing the insect from burrows in 

 the growing plant 



11. Feeding on the overwintering larvae 

 Mammals: 



12. Mice feeding on the overwintering larvae 

 Insect Predators: 



13. Heteroptera and other predaceous insects 

 feeding on larvae 



14. Asilidae (robber flies) and other pred- 

 ators feeding on adult moths 



15. Miscellaneous egg predators 

 Insect Parasites: 



16. Trichogramma minutum (chalcid fly) 

 parasitic on eggs 



17. Miscellaneous native parasites feeding on 

 larvae and pupae 



18. Disease of the larvae 



AGRICULTURAL PRACTICE AND CONTROL 



MEASURES: 



19. Burning of crop refuse containing larvae 



20. Plowing under infested corn stubble 

 in the fall 



21. Feeding infested corn stalks to cattle 



22. Early planting of trap crops 



23. Delayed planting of corn 



24. OVERPOPITLATION 



25. CHANGE IN "preferred" OR OPTIMAL 

 HOST PLANT AVAILABILITY 



The meaning of the majority of these 

 twenty-five items is self-evident, but a few 

 comments are appropriate. Item 3 refers to 

 the fact that if night temperatures get 

 much below the optimum range (66° to 

 68° F.) during the period of flight, as 

 happened to the midsummer brood of 1923 

 when the mean temperature was 57.6° F., 

 fecundity is lowered. Item 5 refers to in- 

 stances in which first instar larvae drown 

 or are washed away by rainfall before they 

 can tunnel into the com stalks and there 

 gain protection. Item 6 refers to the fact 

 that in certain years there is a greater pro- 

 portion of single than of double genera- 

 tions for one reason or another, and this 

 contributes to decUne. 



Item 22 describes a com borer control 

 practice in which strips of corn are pur- 

 posely planted early in the year. These 

 particular plants function as oviposition 

 sites for moths during their first flight 

 period, and thus they become a reservoir 

 for a considerable portion of the eggs of 

 the local population. After this oviposition 

 the corn is destroyed. If the time of plant- 

 ing the main corn crop is delayed (Item 

 23) so that the plants appear above the 

 surface after many of the moths have 



