78 INSECTS AND HUMAN WELFARE 



more accurately the nature of the regulatory effect of para- 

 sites in the case of most of our forest insects. One common 

 native insect enemy of deciduous trees, closely related to the 

 gipsy moth, is the white marked tussock moth {Hemerocampa 

 leucostigma). This, species is especially destructive to shade 

 trees in cities. The caterpillars, which are familiar to nearly 

 everyone, are beautiful objects, adorned with pencils of black 

 and scarlet hairs, and with dense tufts of buff-colored hairs 

 on the back. They are periodically abundant in most of our 

 eastern cities, usually increasing in numbers for several years, 

 and then rather suddenly almost disappearing, gradually to 

 regain their previous abundance. 



Some years ago Howard showed that the preparatory 

 stages of the tussock moth are subject to attack by at least 

 twenty-one primary parasites, belonging to several different 

 families of insects. These in turn fall prey to fourteen hyper- 

 parasites which attack the primary ones and thus tend to 

 reduce the numbers of the latter, although they do not pre- 

 vent the destruction of the parasitized caterpillars in any 

 case. There are two or three broods of the tussock moth 

 each season and, as has been noticed in the case of other in- 

 sects having more than one brood each year, the first is less 

 heavily parasitized than the second, or later ones. This is 

 probably due to a greater mortality during hibernation 

 among the parasites than with the host, so that the host 

 species usually shows a lesser degree of parasitism early in the 

 season, but due to a greater prolificacy and rapidity of mul- 

 tiplication, the parasites gain the upper hand later in the 

 season, only to lose most of their advantage during the 

 hibernating months. However, when the host has been 

 especially abundant one year, and thus permitted an unusual 

 increase among its parasites, it may be very highly para- 

 sitized early the next season. Such a condition was noticed 

 with the tussock moth by Howard in Washington, in 1895 

 and 1896. During the season of 1895 this pest had shown an 

 enormous increase, and threatened the destruction of shade 

 trees throughout the city. The third or autumn brood was 



