1923 ENTOMOLOGICAL SOCIETY. 23 



hymenopterous or dipterous parasites. It also leaves them exposed to preda- 

 ceous insects such as Chrysopid larvae and adults of the pink, 12-spotted lady- 

 bird beetle (Megilla maculata). The squeezing of a stem or breaking of it at 

 the point where a borer is working, will cause the latter to hurriedly crawl out 

 and to lower itself down by a silken thread. The action of wind may some- 

 times do this. 



Except on the occasions mentioned, borers remain feeding in the stems 

 they originally entered, until they reach the fifth and sometimes the sixth instar, 

 and then they develop a sort of wanderlust which causes them to leave the stems 

 where they have been feeding, and to migrate to near-by hills of corn or to weeds 

 growing among or alongside the corn. Inspection by night of badly infested 

 corn at the fifth or early sixth instar periods of the larvae, reveals many of them 

 wandering around. By this migration, and subsequent eating of new holes in 

 cornstalks, the damage in a field is materially increased towards harvest time. 

 The new entrance holes are generally low down and frequently occur among 

 the adventitious or prop roots; from this position, the borers often work down 

 in late autumn right into the tap roots, especially in the case of flint corn. 



By the end of July, or at any rate by the middle of August, by which time 

 the borers are full grown, this migration ceases, as was proved several times in 

 experiments by placing absolutely untouched hills of corn alongside heavily 

 infested hills, and then cutting up both series at various times to determine 

 the lateness of migration. Each test was guarded by recovery traps and tangle- 

 foot barriers. Just how far these migrating larvae will wander was not deter- 

 mined, but it is probably not beyond the range of the nearest corn plants. 



This migration, which ceases by the middle of August, of mature or nearly 

 mature larvae, cannot account for the fact that the larval population of a field 

 diminishes more and more the later the season advances. For instance: In 

 1922, in a small patch of sweet corn of some 1,000 stalks, sown May 13th, there 

 was by the end of July a carefully estimated and re-checked average infestation 

 of 4.6 larvae per stalk throughout the plot. By the middle of August, two 

 weeks later, this infestation had dropped to a fraction over three larvae per stalk 

 through the plot, and had not increased in corn growing near-by nor in the 

 few weeds among the corn. By the middle of September the infestation was 

 reduced to just two larvae per stalk. 



This same peculiarity was noted in 1921 in flint corn, which at time of 

 cutting and stooking contained approximately four larvae per stalk, but only just 

 over one larvae per stalk in one month's time. Also in stalks that were stooked 

 outside the laboratory in the first week of October, 1921, the larval population 

 was greatly reduced when counts were made in spring, 1922, and no trace of 

 the larvae could be found in grass or weed stems or pieces of stalk around the 

 pile. 



A secondary movement of larvae is noticeable under certain conditions of 

 stooking and storing cornstalks. Thus borers will leave the inner stalks of a 

 stook within a week of the stooking and will migrate to the outer stalks. The 

 rapidity of this movement depends upon the dryness of the stalks and upon the 

 closeness of piling; the drier the stalks the less the migration, and the closer 

 the stalks are piled the more rapid the migration. Also in thoroughly dry 

 stalks piled in a mow in a barn the larvae will remain in their burrows without 

 migration. This was the case in one instance even where the stalks were covered 

 later by a layer of hay. Such larvae in very dry material have been found to 

 pupate very slowly throughout the season, and to emerge as moths as late as 

 the first week in September. 



