120 VEGETABLE FORCING 



Inasmuch as plant lice have sucking instead of biting 

 mouth parts, they cannot be killed by stomach poisons 

 such as arsenate of lead, but must be destroyed by con- 

 tact insecticides or by suffocating fumigants. Tobacco 

 extract and soap solutions are the most commonly 

 applied of the liquids, and tobacco fumigation is uni- 

 versally regarded as the most desirable means of con- 

 trolling the green fly, especially on lettuce. See page 224. 



White fly (Aleyrodes vaporariorum). The green- 

 house white fly is widely distributed among establish- 

 ments devoted to vegetable forcing. It is universally 

 regarded as one of the worst foes of greenhouse crops, 

 and must be combated in an intelligent and vigorous 

 manner in order to prevent the most serious ravages. 

 Tomatoes and cucumbers suffer most from the depreda- 

 tions of the white fly. Lettuce, eggplant, bean, melon 

 and many floral crops are subject to attack. 



A complete description and life history of the white 

 fly (Fig. 40), and remarks on the appearance of infested 

 plants are given as follows in Circular 57, U. S. Bureau 

 of Entomology: 



"The mature white flies of both sexes are four-winged insects 

 scarcely more than l^ millimeters or three-fiftieths of an inch in 

 length. The adult white flies, as well as the scalelike larvae, are 

 provided with sucking mouth parts. In a short time after the 

 emergence of the adult from the pupa case, the body, legs and wings 

 become covered with a white, waxy substance which gives this, as 

 well as other species of the genus, a characteristic floury appearance. 

 The adults feed nearly continuously during their existence. If de- 

 prived of food, they will rarely live for a longer period than three 

 days under ordinary temperature conditions. The longest recorded 

 length of life of one of these insects in the adult condition is 36 

 days, but it seems probable that the average length of adult life is 

 much greater than this would indicate. The largest number of eggs 

 which an adult white fly is positively known to have deposited is 

 129, but this number is probably below the average. Indeed, the 

 specimen which produced this number of eggs with little doubt de- 

 posited over 50 others which were not recorded. The number of 



