242 



NEW YORK STATE ML'SEL'M 



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

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

 as in the case of s(jme 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 crowtled, 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 tlejxjsit eggs in crevices of the bark or other 

 shelters, as stated above, and remain un hatched over winter. 



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

 nomic importance and the literature of ilu' sidjject abounds with notices of 

 considerable injury inllicted on iini)orlant crops by these comparatively 

 insignificant, defenseless creatures. Our grains are occasionally seriously 

 injured by species of aphids or ])lant lice, while the damage inflicted by 

 those occurring on apple and chirr) trees is well known to almost every one 

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

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

 carrying over by nurser\inen of thousands of young appletrees stunted in 

 growth l)y lliese little jiests, which are at rarer intervals numerous enough 

 to seriously injure larger trees. RectMit \i-ars ha\ e witnessed grave losses 

 on our i)c;a croj), particularly in Maryland antl adjacent Atlantic states, 

 caused by a species which has \>cvn so injurious as to leatl to the almost 

 entire abandonment of late \arieiies 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 consiilerable loss from \ear to year and 

 is sonu'limes exci'edingly destructive, thereby ha\ing a material effect on 

 the \alue of this important crop of central New \ ork. 



Natural checks. Aphids or i)lain lice- are ilefenseless creatures, subject 

 to a nmuIxT of atlverse agencies of one kintl or another, which reduce their 

 numbers very materially and ]>revent what would otherwise be exceedingly 

 serious outbreaks. Among the most effective natural agencies are cold 



