INSECTS INJURIOUS TO BEANS AND PEAS 295 



By midsummer, with the harvesting of the peas, most of the 

 aphids upon them have been destroyed by predaceous and para- 

 sitic insects and disease, and they are not observed during late sum- 

 mer unless they have been subsisting on clover throughout the 

 season, when they sometimes destroy the crop in August, as has 

 been observed in Illinois. In early fall they often become com- 

 mon again on late garden peas, and late in October they migrate 

 to clover. Fewer young are born as the weather gets colder in 

 the fall, and the aphids never become numerous enough to do any 

 injury at that season. Late in October and early November in 

 the Middle States as the aphids are migrating to clover, winged 

 males appear, and some of the wingless females developing on 

 clover produce eggs. The winged males are similar in size and color 

 to the migratory females, though slightly darker, and have three or 

 four dark spots along the sides of the abdomen and a deep brown 

 dash on either side of the back of each abdominal segment. The 

 oval eggs are about one-fiftieth inch long, jet black, and are 

 deposited on the lower leaves or stems of clover, and hatch as it 

 commences to grow in the spring. In central Illinois they were 

 observed to hatch March 23d, and the young became full grown 

 and commenced reproduction on April 5th, living until May 12th. 

 In southern Maryland and farther south many of the viviparous 

 females live over winter on the clover and commence to reproduce 

 again in the spring, no eggs having been observed in that latitude, 

 but in central Illinois and northward, the females are probably 

 entirely destroyed by the cold and only the eggs survive. 



Natural Enemies. From 5 to 10 per cent of the aphids are 

 normally destroyed by little wasp-like flies of the genus Aphidius 

 whose larvae live within the aphids. A number of the more com- 

 mon ladybird-beetles,* syrphus-flies, f and lace- winged flies, t 

 which commonly prey upon aphids, destroy large numbers of the 

 pests, but their work comes so late in the season that the peas are 

 seriously injured long before the aphids are checked by them, 

 though they might prevent a reappearance the next year. 



The most important enemy of the pea-aphis is a fungous dis- 

 ease (Empusa aphidis) which is undoubtedly the principal factor 

 in its natural control. The most probable explanation of the 



* Family Coccinellidce. 



f Family Syrphidce. 



j Family Chrysopidce. Concerning these predaceous insects, see Chap. I. 



