INSECT CONTROL 303 



each of the others, one of the most common of which is 

 that each host supports a variety of parasites, often differ- 

 ing among themselves to a remarkable degree in habit, 

 natural affinities, and methods of attack. The conclusion, 

 therefore, has been drawn that to be effective in the case 

 of insects like these moths, parasite control must come 

 about through a variety of parasites working together 

 harmoniously, rather than through one specific parasite, as 

 in the case with less specialised insects such as the scale 

 insects, having a less well-defined seasonal history. In 

 short, there must be a seasonal sequence of parasites to 

 effectually control the gipsy and brown-tail moths 

 parasites of eggs, larvae, and pupae. 



" In a country where the conditions are settled, each 

 species of insect is subjected to a certain fixed average 

 percentage of parasitism, which, in the vast majority of 

 instances and in connection with numerous other control- 

 ling agencies, results in the maintenance of a perfect balance. 

 The insect neither increases to such abundance as to be 

 affected by disease or checked from further multiplication 

 through lack of food, nor does it become extinct, but 

 throughout maintains a degree of abundance in relation to 

 other species existing in the same vicinity, which, when 

 averaged for a long series of years, is constant." Though 

 storms and other climatic conditions, birds, and even 

 disease all play their part in insect control, it is probable 

 that only through parasites and predators is an insect 

 brought under complete natural control, because their 

 increase numerically is directly affected by the numerical 

 increase of the insects on which they prey. 



After this digression, let us return to the consideration 

 of the gipsy and brown-tail moths and learn what steps 

 have been taken for their actual control. Seeing that 

 more than thirty distinct parasites, and seven kinds of 

 predatory beetle, have been introduced into the United 

 States to wage war on these two moths, it is plain that 



