136 MASS. EXPERIMENT STATION BULLETIN 177. 



pearance in injurious numbers, when damage by these insects need be 

 feared. During this brief period potato fields showed injury varying 

 from little to the complete destruction of the plants. Some patches were 

 completely free from infestation, while others near by, apparently no 

 more attractive, were badly injured or destroyed before insecticidal treat- 

 ment could be applied. 



The gradual disappearance of the plant lice from the potato plants, 

 usually about a month after infestation becomes evident, has, in many 

 cases, been the salvation of the crop. This disappearance is due mainly 

 to natural controlling factors, such as parasitic and predatory enemies, 

 weather conditions and disease, all of which contribute to the destruction 

 of myriads of these insects, and to a natural migration of the plant lice 

 from potato plants to other host plants during the latter part of July 

 and August. These factors will be discussed at greater length later. 



Description of Potato Plant Lice. 

 Potato lice are yellowish or greenish in color, with an occasional pink 

 form. Some are furnished with wings and can fly readily, while others 

 are wingless and have to depend upon crawling for getting about. When 

 full grown these insects are no larger than a pin head, and because of their 

 color and small size, and the fact that they occur for the most part upon 

 the underside of the leaves, plants may be badly infested and considerable 

 injury result before their presence is noticed. 



Man^ter of Feeding and Nature op Injury. 



Plant lice are sucking insects and obtain their food by inserting a 

 bristle-like beak into the host plant, from which the juices are extracted. 

 Thus all feeding is done beneath the surface and within the tissues of the 

 plant. On plants badly attacked the leaves begin to turn yellow, curl up, 

 gradually turn brown and die. Under conditions favoring their growth, 

 an attack by plant lice of a week or two will suffice to kill a large portion 

 of the top of a potato plant, and the development of the tubers must 

 necessarily be affected on plants thus injured. When a leaf or stem 

 becomes too dry to afford suitable feeding ground the plant lice crawl to 

 a fresh leaf, or migrate to other plants and continue their injury. 



Where plant lice are abundant enough to cause apprehension, the 

 underside of the leaves, stems and blossom stalks will be covered with 

 these tiny creatures, and the plants become covered with honey dew, a 

 sticky substance excreted by these insects. This honey dew is often 

 attacked by a black fungus, which, together with the molted skins ad- 

 hermg to the sticky surface, gives the plants an unhealthy appearance 

 and midoubtedly interferes with proper functioning. 



In spite of its mumtcness, the beak of the plant louse makes a wound 

 which furnishes a suitable entrance for disease, and even if the infesta- 

 tion with plant lice is insufficient to injure the plants, the infection with 

 disease thus caused may entirely ruin the crop. 



