DISEASE IN CONTROL OF MOTHS. 101 



It is very subject to a " wilt " disease, and no colony has been observed 

 which has reached such proportions as to threaten the defoliation of 

 street trees in which the disease has not appeared. In one instance 

 the disease was so prevalent as to destroy practically all of the cater- 

 pillars, and, as in the case of the gipsy moth, the scattering caterpillars 

 which hatched from the eggs deposited by the few survivors were 

 seriously affected the following year, notwithstanding the presence of 

 an abundance of food. Furthermore, caterpillars hatching from 

 eggs collected in this and similar colonies removed to the country 

 where the few native caterpillars to be found have always been remark- 

 ably healthy, perished through the " wilt " exactly as though they had 

 hatched in the city. 



Nevertheless, the white-marked tussock moth has been for long, is 

 now, and probably will remain the worst defoliating insect enemy of 

 such trees as the horse chestnut, maple, sycamore, etc., in strictly urban 

 communities in the Eastern States generally. It does not appear un- 

 reasonable to suppose that the gipsy moth may similarly continue to be 

 a pest in spite of the disease. As a matter of fact, every observation 

 which has been made upon either the fungous disease of the brown- 

 tail moth caterpillars, the wilt disease of the gipsy-moth caterpillars, 

 or diseases of other defoliating caterpillars, such as that of the white- 

 marked tussock moth, the tent caterpillar, and the " pine tussock moth," 

 has tended to confirm the conclusion that such insect epidemics rarely 

 play more than the one role in the economy of nature. They do not 

 prevent an insect from increasing to an extent which renders it a pest, 

 but they may, and frequently do, render very efficient service in effect- 

 ing a wholesale reduction in the abundance of such insects when other 

 agencies fail. When the insect in question is ordinarily controlled 

 by parasites, as appears to be the case with the white-marked 

 tussock moth, the "pine tussock moth," etc., it is probable that a 

 long time will elapse before it will again encounter the combination of 

 favorable circumstances which make possible abnormal increase. 



When, as with the white-marked tussock moth in cities, the tent cat- 

 erpillar in southeastern New England, or the brown-tail moth and gipsy 

 moth in America, adequate control by parasites is lacking, reduction 

 in numbers through disease is not likely to result in more than tem- 

 porary relief. The more complete the destruction wrought by the 

 insect the longer the period which must necessarily elapse before it 

 again reaches the state of destructive abundance and, looking at it 

 from this standpoint, it is not unlikely that the gipsy moth is much 

 more abundant at the present time than it would have been had it 

 not been for the prevalence of disease. There are, each year, an 

 abundance of localities where the destruction of a great majority of 

 the caterpillars by disease has been the only thing which has saved the 

 whole race from complete extinction in that locality through con- 



