INSECT ENEMIES AND THEIR CONTROL 119 
only means which have been found to be economical and 
satisfactory in destroying the pests are thorough steriliza- 
tion with steam and hot water. (See Chapter VI.) After 
the beds have been planted and the crop is found to be 
infested, nothing can be done until the plants are removed 
and the soil sterilized. 
Aphis.—-Various species of the aphis feed on the differ- 
ent vegetable forcing crops. They are commonly called 
plant lice and green and black flies. While there is con- 
siderable variation in the structure of the different species 
as well as in their life histories, all have sucking mouth 
wi 
GERDA EGAN! 
a 
Fig. 40.—White fly (Aleyrodes vaporariorum); a, egg; b, young larva; c, pupa, top 
view; d, pupa, side view; e, adult—c, d, e, about 25 times natural size; a, b, still 
more enlarged; (a—d, after Morrill, Tech. Bul., Mass. Exp. Sta.; e, original.) 
parts. They are extremely persistent on some crops, 
such, for example, as the green fly on lettuce. The 
young are brought forth alive, and they reach maturity 
in seven to ten days and begin to produce young, so that 
innumerable insects may appear from a few parents 
within a remarkably short time, unless preventive 
measures are taken. 
