3 
PAG ee ee 
~_.-- arP 
49 
capable of rapid and enormous increase when any of these checks are 
temporarily weakened to any considerable degree. As they affect the 
plant by abstracting the elaborated sap upon which its vital activity de- 
pends, the injury done is usually general, and especially is this true if the 
root be the part infested. Some species, however, in addition to the 
general drain upon the life of the plant, cause a “distinet local defor- 
mity to root or leaf in the nature of a gall, which protects them at the 
same time that it secures them food. ‘Any crop lable to their attack 
in force is never long free from danger, but, on the other hand, a seem- 
ingly irresistible outbreak may disappear as quickly as it came, a slight 
and almost imperceptible change of conditions often taking tremendous 
effect on these delicate insects.* 
Economically, plant lice may be divided, according to the peculiari- 
ties of their life histories, into several groups or classes. Some genera- 
tion, or some part of some generation, may grow wings, fitting them for 
rapid dissemination, or the species may be “without winged epresenta- 
tives. They, may live through the whole season of their active life 
above ground, on exposed parts of the plant; they may spend the whole 
season under ground, upon the roots; or they may alternate, spreading 
each year from roots to stalk and leaves and back again. Whatever part 
of the piant they infest, they may live on a single host species, they may 
spread indefinitely from one to several others, or they may migrate 
definitely, by means of a fixed generation, from one species to another, 
requiring thus for their continuance two plant species often extremely 
unlike. t | 
Finally, the sexual, oviparous generation (commonly the last to ap- 
pear in fall) may leave its eggs on the exposed parts of the plant last 
infested, or it may deposit them in the earth among or on the roots of 
its host. In the former case the destruction of the plant, or of its re- 
mains, will destroy the lice; in the latter, the eggs rest like a seed in the 
earth to stock the ground the following spring ‘with a horde of young, 
ready to infest the “succeeding erop if suited to their tastes and habits. 
All the plant lice of our present list of species infesting the roots 
of corn are, so far as known,{ subterranean only, producing no galls, 
but leaving their eggs in the earth over winter. They infest more than 
one plant, spreading from one to another species in an indefinite man- 
ner, not definitely migrating. The corn and grass root lice (Aphis 
— maidiradicis and Schizoneura panicola) develop early in the season 
winged forms by which they easily spread from field to field. 
* A marked illustration of this fact is afforded by the somewhat recent history 
of the grain louse (Siphonophora avene) in Illinois. (See Seventeenth Report State 
Entomologist of Illinois, p. X.) 
+ The apple louse (Aphis mali), for example, passes from the apple to Waren! 
grasses in midsummer, returning to the apple leaf in fall, and leaving upon the twig 
eggs from which young hatch the following spring. The hop louse (Phorodon ae 
alternates in a similar manner between the hop and the plum; and a grass root louse 
(Schizoneura corni) between the grasses and the dogwoods. 
4 It has long been a general opinion that the corn root louse, presently to be 
discussed, is a subterranean form of the leaf louse (Aphis maidis), a surmise which 
I have failed to verify after many efforts extending through several years, and so 
have declined to accept. 
