110 
PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 
abundant, the number of eggs to a mass averaged considerably less 
than 300. In egg masses from outlying districts where the infesta- 
tion was new, and where the moth had never reached its maximum 
abundance, the average in a few masses counted was slightly in 
excess of 500. The number is, however, very variable, and the 
character of food and the meteorological conditions during the feed- 
ing period of the caterpillars are doubtless important features. Hot 
weather during June forces the development of the caterpillars and 
they do not become large. Small moths deposit fewer eggs rather 
than smaller eggs. It is possible that there is actually a decrease in 
the fecundity of moths brought about by our short and ardent sum- 
mers, but for the present it is not proved, and it is believed that 
whenever abundance of the insect is sufficiently reduced the original 
rate of multiplication will prevail. The point is one well worthy of 
further investigation, but for the present the potential rate of 
increase, provided no controlling factors whatever are operative, will 
be considered as 250-fold annually. 
The best information available as to the rate of increase of the 
gipsy moth actually prevailing in Massachusetts is contained in the 
report entitled "The Gypsy Moth," by Mr. Edward H. Forbush and 
Dr. C. H. Fernald, which was published under the direction of the 
board of agriculture. These authorities, in their discussion of the 
matter, say as follows: 
The study of the increase and dissemination of the gipsy moth in Massachusetts 
is most interesting. Perhaps there never has been a case where the origin and 
advance of an insect could be more readily traced. As the moth appears to be con- 
fined as yet to a comparatively small area, and as the region has been examined 
more or less thoroughly for five successive years, the opportunities offered for the 
study of the multiplication and distribution of the insect have been unequaled. 
When it is considered that the number of eggs deposited by the female averages 
from 450 to 600, that 1,000 caterpillars have been seen to hatch from a single egg 
cluster, and that at least one egg cluster has been found containing over 1.400 eggs, 
there can be no doubt that the reproductive powers of the moth are enormous. Mr. 
A. II. Kirkland has made calculations which show that in eight years the unre- 
stricted increase of a single pair of gipsy moths would be sufficient to devour all 
vegetation in the United States. This, of course, could never occur in nature, and 
is mentioned here merely to give an idea of the reproductive capacity of the insect. 
It seems remarkable at first sight that an insect of such reproductive powers, which 
had been in existence in the State for 20 years, unrestrained by any organized effort 
on the part of man, did not spread over a greater territory than 30 townships, or about 
220 square miles. Some of the causes which at first checked its increase and lim- 
ited its diffusion in Medford have already been set forth. Most of the checks which 
at first served to prevent the excessive multiplication of the gipsy moth in Medford 
operate effectively to-day wherever the species is isolated. True, it has now become 
acclimated. But any small isolated moth colony still suffers greatly from the attacks 
of its natural enemies and from the struggle with other adverse influences which 
encompass it. The normal rate of increase in such isolated colonies as are found 
to-day in the outer towns of the infested district seems to be small. The annual 
increase can be readily ascertained by noting the relative number of egg clusters laid 
