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 



Fig. 40. White fly (Aleyrodes vaporariorum) ; a, egg; &, 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. Bui., 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. 



