242 NEW YORK STATE MUSEUM 



females develop. These latter forsake the original, usually by this time 

 crowded, food plant and either fly to similar ones in the neighborhood or, 

 as in the case of some species, betake themselves to entirely different plants, 

 where another series of wingless agamic or asexual generations are brought 

 forth. This may continue for sometime and after a certain number of 

 generations the plants again become crowded, winged females are produced 

 and there may be a return migration to the original food plant, where one 

 or more generations may be produced and ultimately perfect males and 

 females, which latter pair and deposit eggs in crevices of the bark or other 

 shelters, as stated above, and remain unhatched over winter. 



Economic importance. Aphids or plant lice are of considerable eco- 

 nomic importance and the literature of the suiiject abounds with notices of 

 considerable injury inflicted on important crops by these comparatively 

 insignificant, defenseless creatures. Our grains are occasionally seriously 

 injured by species of aphids or plant lice, while the damage inflicted by 

 those occurring on apple and cherrytrees is well known to almost every one 

 at all familiar with plants. The species affecting appletrees are sometimes 

 so abundant and injurious in western New York .State as to necessitate the 

 carrying over by nurserymen of thousands of young appletrees stunted in 

 growth by these little pests, which are at rarer intervals numerous enough 

 to seriously injure larger trees. Recent years have witnessed grave losses 

 on our pea crop, particularly in Maryland and adjacent Atlantic states, 

 caused by a species which has been so injurious as to lead to the almost 

 entire abandonment of late varieties and has compelled growers to raise 

 the earlier peas or go out of business. The species occurring on the hop is 

 another familiar form. It occasions considerable loss from year to year and 

 is sometimes e.xceedingly destructive, thereby having a material effect on 

 the value of this important crop of central New York. 



Natural checks. Aphids or plant lice are defenseless creatures, subject 

 to a number of adverse agencies of one kind or another, which reduce their 

 numbers very materially and prevent what would otherwise be exceedingly 

 serious outbreaks. Among the most effective natural agencies are cold 



