Vegetable Plant Lice 13 



because while the lice can reproduce and increase in numbers, their 

 small but most efficient natural enemies (very small four-winged 

 bee-like insects, which sting their eggs into the louse's body and un- 

 dergo their immature development inside the louse's body) cannot 

 reproduce as rapidly as the lice. The only natural enemies that could 

 be effective at such temperatures (the lady bird beetles) are rarely 

 sufficiently abundant to eat the lice as fast as they can reproduce. 



If at any time after the lice have become established upon a 

 vegetable crop, there come hard dashing rains, the plants are likely 

 to be washed nearly, if not completely, free from lice, and the out- 

 break stopped at that point. 



CLEAN CULTURE 



The fact that many of the vegetable-infesting plant lice produce 

 their spring and early summer generations upon weeds, indicates 

 that the practice of keeping down weeds should be followed. The 

 fact that the cabbage aphis over-winters as an egg upon the remains 

 of the cabbage and related plants indicates that the remnants of the 

 crop should be removed from the ground and destroyed. 



On the other hand, the good results of clean culture may largely 

 be annulled when one is so unfortunate as to live among neighbors 

 who cannot be induced to take up the practice. 



DIRECT ARTIFICIAL CONTROL 



The method of direct artificial control depends on the growth- 

 habit and stage of growth of the crop infested, the size of the area 

 concerned and whether grown in the open or under glass. For the 

 sake of definiteness, the writer will discuss the treatment of lice on 

 each important kind of vegetable crop with which he has had ex- 

 perience or concerning which he has been able to obtain reliable data 

 from others. 



On the Nightshade Family 



Members of the nightshade family — potatoes, tomatoes, and egg- 

 plant have an erect growth habit until they approach maturity when 

 they in some cases become recumbent. The solution of the louse 

 problem on this type of plant involves: (1) discovering the lice 

 shortly after they arrive and before they have had time to injure the 

 crop; (2) adoption and practice of measures that will effect their 

 prompt destruction. 



The first requisite can be fulfilled only by keeping a sharp watch 

 of the field, examining at frequent intervals — say twice a week — the 



