122 VEGETABLE FORCING 



useful in distinguishing this from nearly allied species of the 

 white fly. 



"The entire stage from the insect's third molt to the emergence 

 of the adult form lasts from 12 to 16 days in the laboratory and 

 greenhouse. The adult emerges from a T-like opening, leaving the 

 glistening white pupa case attached to the leaf. At first the. wings 

 of the adult are crumpled close to the body, giving them a peculiar 

 appearance. In the course of a few hours the wings unfold and the 

 insect has then completed its development, which has extended over 

 nearly five weeks, if under the ordinary temperature conditions of 

 a greenhouse. 



"Appearance of infested plants. As already stated, the upper 

 leaves of a plant are preferred by the adult females for the deposi- 

 tion of their eggs. Thus there is a slow but continuous migration 

 of adults upward to keep pace with the unfolding of the leaf buds. 

 On thoroughly infested plants we find on the uppermost leaves only 

 adults and freshlv laid eggs; a little lower on the plants we find 

 eggs in the process of hatching; and, finally on the lowermost parts 

 of the plants we find discolored, shriveled leaves with many pupae 

 and emerging adults and few, if any, unhatched eggs or young 

 larvae. The larvae and pupae secrete little globules of honey-dew, 

 so named after the material of a like nature secreted by plant lice. 

 These globules usually either drop or are forcibly ejected, and fall- 

 ing on the upper surface of leaves directly below, give them a 

 glazed appearance. This is frequently followed by the growth of a 

 sooty fungus which hastens the complete destruction of the leaf. 



"When overcrowding of the young occurs, this fungous growth 

 finds favorable conditions for its development on the under surface 

 of the leaf, resulting in the destruction of many of the immature 

 insects. Owing to the interference with the respiratory processes 

 of the leaf, both by the bodies of the insects themselves and by the 

 fungous growths due to them, badly infested plants have a tendency 

 to wilt when exposed to the sun's rays. In seriously infested green- 

 houses the leaves of the plants gradually die, the lower leaves first, 

 and if unchecked the insects greatly impair the value and vitality 

 of the plants, even though they do not actually cause their total 

 destruction." 



In the control of the white fly in greenhouses, preven- 

 tive measures should be taken as much as possible. The 

 pests are so minute that they may easily be introduced 



