CICADAS, PLANT LICE, AND SCALE INSECTS 37 



Early in the history of the invasion of these scale insect 

 pests it was thought possible either to exterminate them or 

 at least to check their spread over the country. This has 

 proved to be impossible. Orchard and shade trees in many 



Fig. 22. Oyster-shell scale insect 



a, female, from beneath, showing eggs protected by scale (X 24) ; b, female from 

 above (x 24) ; c, female scale on branch (natural size) ; d, male scale (x 12) ; 

 e, male scales on twig (natural size). (After Howard, Yearbook, United States 



Department of Agriculture, 1894) 



parts of the country are kept alive only by periodic spray- 

 ing to kill the scales. Since these are sucking insects, get- 

 ting their food directly from the sap of the plant, poisons 

 are not available against them. The sprays, to be effective, 

 must on contact kill the insects under the scales. 



