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. H. 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 



