138 MASS. EXPERIMENT STATION BULLETIN 177. 



lice, since these insects pierce beneath the poison before feeding is begun. 

 Accordingly, a contact insecticide, a material which kills by contact with 

 the body, is required to deal effectively with these sucking insects, and 

 satisfactory results with an insecticide of this nature can be expected only 

 when apphcation is absolutely thorough. Each insect must be hit by the 

 spray in order to be killed. Careless work will merely lead to a waste of 

 material, time and energy and to a continuation of the infestation. Such 

 carelessness, frequently due to ignorance of the essentials of apphcation 

 rather than intent, is often the source of complaint that material recom- 

 mended for the control of plant hce is ineffective. Almost invariably 

 unsatisfactory results with standard contact insecticides are attributable 

 to improper apphcation. Since potato Uce confine their feeding almost 

 wholly to the underside of the leaves, care must be taken to direct the 

 spray upward so that the underside of each leaf will be well covered. 



To apply such a spray before the infestation reaches the distinctly 

 dangerous stage, while it might kill manj^ of the scattered plant Uce, 

 might, on the other hand, be merely a waste of energy, for the amount of 

 injury which the plant hce are going to inflict is purely problematical, so 

 many elements of uncertainty enter in. For instance, weather conditions 

 play an important part in the welfare of the plant hce. Heavy rains 

 wash these deUcate insects from the plants, and cold weather retards 

 their increase. Warm, damp weather is favorable to a parasitic fungous 

 disease which may destroy the plant Uce over large areas. Parasitic and 

 predatory enemies, when conditions are favorable, often destroy such 

 numbers of the plant lice, even after considerable injury to the plants is 

 evident, that control measures are superfluous. Then, too, the natural 

 migration of the plant Uce from potato plants to the winter hosts is an 

 element of some uncertainty. The greater amount of injury may be com- 

 pleted and the plant lice soon be ready to leave the potato plants for the 

 winter hosts before injury to the vines is extensive enough to become 

 particularly noticeable. At tliis time, if the fact were known, it would 

 hardly appeal to the average grower as an economical proposition to insti- 

 tute control measures. 



AU of these factors combine to make the matter of the dcsirabiUty or 

 necessity of artificial control measures for potato plant lice often a diffi- 

 cult one to determine. Furthermore, it has been the obsei-vation of the 

 writer that in many cases where control measures have been carried out, 

 particularly where improper apphcation made several sprayings necessary, 

 more actual injury was done the plants by the handling and trampUng 

 incidental to such work with a contact insecticide than, it is probable in 

 most cases, the plant lice would have inflicted had the infestation been 

 aUowed to run its course. 



One application with the proper material, properly appUed to the under- 

 side of the foliage, when the infestation is severe enough to cause evident 

 wilting of the leaves, can in most cases be made economically and to 

 advantage, especially if injury is noticeable before the early part of 



