INSECTS INJURIOUS TO ORCHARD TREES 443 



SCALE-INSECTS 



Although most of the Coccidce, or scale-insects, are so small 

 or obscure that they are usually overlooked by the ordinary 

 observer they are economically the most important group in 

 all the insect class. The scale-insects attack almost all kinds of 

 trees and shrubs, but it is difficult to make anything like a reason- 

 ably accurate estimate of the amount of damage done by them 

 because so many factors are involved. Some trees may harbor 

 many scale-insects and yet show but little injury, others when 

 only slightly infested by certain species suffer severely. The 

 injury may be only temporary, causing the leaves or fruit 

 to drop, or it may be permanent, retarding, dwarfing or even 

 killing the tree. In discussing the development and life history 

 of the scale-insects it will be convenient to group them into 

 three more or less well-defined groups. The first group includes 

 the mealy-bugs, which are common in almost all greenhouses, 

 and a few others, most of which move about over the plant 

 freely until ready to lay their eggs. In these the segmentation 

 of the body remains distinct, and the legs and antennae are func- 

 tional throughout the life of the female. The second group 

 includes the genus Lecanium and others, such as the common 

 black scale and the cottony maple-scale. The young insects of 

 this group wander about freely for a while, but before they are 

 half grown they insert their long projecting mouth-parts into 

 the tissues of the plant and remain stationary for the rest 

 of their life. The body-wall becomes dark and hard, often 

 very convex or hemispherical, and usually all resemblance 

 to an insect is lost. The third group includes those species 

 that have the body concealed by a scaly covering made up of 

 a waxy secretion in which is imbedded the molted skins of 

 the insect. The San Jose scale, the oyster-shell scale, the 

 rose-scale, and others, are examples of this group. With 

 the second molt the female loses her eyes, antennae and legs, 

 and becomes a sac-like creature covered over with its protect- 

 ing scale and with its long beak thrust into the plant tissue. 



In the first, or mealy-bug, group, the eggs of many species 

 issue from the body before they are hatched, the female often 



