PARASITISM IN INSECT CONTROL. 105 
wandering characteristics of those of the gipsy moth, but rather the 
opposite, find themselves crowded in excessive numbers upon a 
limited variety of shrubs and trees; complete defoliation of these 
comparatively few host plants quickly follows, and weather condi- 
tions being favorable to the development of disease, wholesale 
destruction is all that intervenes between an unnatural migration or 
starvation. Such reduction is followed by a period of years during 
which the parasites check but do not overcome the tendency to 
increase, and it is only a little while before the process is repeated. 
There were, in certain localities in eastern Massachusetts in the 
summer of 1910, continuous strips of roadside grown up to a variety of 
trees and shrubs, the most of which were defoliated by tent cater- 
pillars, all of which had hatched from eggs deposited upon the 
occasional wild-cherry tree which was present. Several such strips 
were visited at about the time when the caterpillars elsewhere were 
beginning to pupate, and not a single living caterpillar or pupa 
could be found amongst the thousands of dead and decomposing 
remains of the victims of overpopulation. These were but a repeti- 
tion of conditions as observed a few miles north in New Hampshire 
12 years ago. How frequently similar conditions occurred during 
the intervening period is not known. 
In addition to those species mentioned in the preceding pages, 
quite a number of other leaf-feeding Lepidoptera have been more or 
less casually studied in a less comprehensive but at the same time a 
careful manner. 
PARASITISM AS A FACTOR IN INSECT CONTROL. 
In reviewing the results of these studies, the fact is strikifiigly 
evident that parasitism plays a very different part in the economy 
of different hosts. Some habitually support a parasitic fauna both 
abundant and varied, while others are subjected to attack by only 
a limited number of parasites, the most abundant of which is rela- 
tively uncommon. No two of the lepidopterous hosts studied, 
unless they chanced to be congenoric and practically identical in 
habit and life history, were found to be victimized by exactly the 
same species of parasites. Neither are the same species apt to occur 
in connection with the same host in the same relative abundance, 
one to another, year after year in the same locality, nor in two 
different localities the same year. 
At the same time there are certain features in the parasitism of 
each species which are common to each of the others, whether these 
be arctiid, liparid, lasiocampid, tortricid, saturniid, or tineid, one of 
the most common of which is that each host supports a variety of 
parasites, oftentimes differing among themselves to a remarkable 
degree in habit, natural affinities, and methods of attack. Depart- 
