FOREST INSECTS 



73 



markable to note the great regularity with which these out- 

 breaks have occurred (respectively 30, 33, 43, 34 and 38 years 

 apart), particularly when it is remembered that they have in 

 no way been coincident geographically. It must be remem- 

 bered, of course, that at many intermediate dates there have 

 been subsidiary outbreaks of lesser severity, and more re- 

 stricted range, but the periodicity of this insect, and that of 



FIG. 33. The gipsy moth (Porthetria dispar). The female moth is shown above 

 and at the left. The dark colored specimen is the male. 



the nun moth mentioned on a previous page, are undoubtedly 

 manifestations of some principle which influences the de- 

 structiveness of many forest insects. 



In connection with agricultural insects we have already 

 called attention to the importance of entomophagous para- 

 sites, and the regulatory effect exercised by these is fully as 

 great in the case of forest insects. In fact some of our most 

 accurate knowledge of the economic value of parasitic insects 

 has been gained through a study of the parasites of the gipsy 

 moth, which was made on an elaborate scale by the Federal 

 Bureau of Entomology in its well-directed efforts toward 

 preventing the spread, and curbing the destructive activities 



