INSECTS INJURIOUS TO BEANS AND PEAS 325 



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 further 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 aphides are 

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

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

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

 commonly prey upon aphides, destroy large numbers of the pests, 

 but their work comes so late in the season that the peas are seri- 

 ously injured long before the aphides 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 

 remarkable outbreak of the pea-aphis in 1899 and 1900 seems to be 

 that, due to two exceptionally dry springs, the fungus was unable 

 to develop, as it propagates best in damp weather, and the aphides 

 increased unchecked. Though occasional individual aphides were 

 found killed by the fungus early in the season, not until June 11, 

 1900, were diseased aphides found in any quantity, but after that 

 so swiftly did the disease destroy them that a week later but few 

 aphides were to be found and almost all were diseased. Probably 

 this fungus usually destroys the aphides on clover before they have 

 - become excessively numerous or have migrated to peas. 



* Family Coccinellidce. f Family Syrphidce. 



J Family Chrysopidce. Concerning these predaceous insects, see p. 9. 



