No. 4.] GREENHOUSE PESTS. 385 



comes too crowded, they move away to other parts of the plant and 

 continue their feeding. 



The mother louse (in one species at least) produces from four to 

 sixteen young every two or three days until about two hundred have 

 thus appeared, and when these young have fed till full grown they in 

 turn begin to produce young. After a few generations of this nature 

 another generation is produced, the members of which develop wings 

 and are thus able to fly to other plants and start new colonies. In 

 this way an entire greenhouse may in a short time become infested 

 from a single originally infested plant. 



With most kinds of plant lice a generation finally appears which 

 consists of males, and females which lay eggs, these hatching and 

 producing females which start new colonies; but the egg-laying 

 generation has not been observed in all cases. 



The White Fly. {Aleyrodes vaporariorum Westw.) 

 This pest has been a very serious one in Massachusetts during the 

 last ten years, often destroying entire crops of tomatoes, cucumbers, 

 etc. In one case seen by the writer a crop of tomatoes valued at 

 $4,000 was entirely lost, the vines drying up completely just as the 

 fruit began to ripen. 



The adult fly is a tiny, white-winged insect, with a yellow body, 

 flying quite freely when disturbed, and resting on the plants, chiefly 

 on the under side of the leaves. The eggs, which are too small to be 

 seen without a magnifying glass, are laid there, and the young suck 

 the juices from the leaves. The young are very small, almost trans- 

 parent, lie flat against the surface of the leaf, and are very incon- 

 spicuous. As they feed, the leaves gradually turn brown, and wither 

 when the insects are abundant. 



From the time the eggs are laid till adults from them appear is 

 about forty days, so that there is plenty of time in most cases for 

 several generations to develop while a single crop is being grown. 



Thrips. 



Several kinds of thrips are found in greenhouses, but the most 

 common species is the onion thrips, which is often a serious pest to 

 field onions. It is a slender insect, about one-twentieth of an inch 

 long, yellowish brown, and winged. The eggs are probably laid in 

 the tissues of the stems or leaves of the plants. The young which 

 hatch from the eggs resemble the adult, but are smaller and without 

 wings. 



These insects, both as young and adults, suck the juices of the plant 

 they are on, the damage they cause being in proportion to their abun- 

 dance. Each female probably lays about fifty eggs, and the adults 

 from these eggs appear in about three weeks, thus enabling this pest 



