RATE OF INCREASE OF GIPSY MOTH. 113 



enemies. While it may be true that the parasitic enemies of the moth will also develop 

 rapidly under conditions that hasten the growth of their host, birds and other verte- 

 brate enemies will secure fewer of the moths in 6 or 7 weeks than in 10 or 12. It is 

 believed that dry weather is unfavorable for vegetable parasites of insects, but to 

 what extent the caterpillars are affected by them in a humid season it is impossible 

 to say. 



The past two years have been "cankerworm years" in the infested region. Many 

 of the birds which habitually feed on the caterpillars of the gipsy moth have been 

 largely occupied during May and the early part of June in catching cankerworms, 

 which they seem to prefer, turning their attention to the gipsy-moth caterpillars in 

 the latter part of June and July, when the cankerworms have disappeared. The 

 birds, therefore, have not been as useful in checking the increase of the gipsy moth 

 as in years when the cankerworms were less numerous. 



A few of the restraining influences which have been less active than usual during 

 the past two years have been mentioned, and possibly many others have escaped 

 observation, but those given serve in a measure to explain the unusual increase of the 

 moth. It is during such seasons that its destructiveness is most apparent. It is then 

 that the groves and forests are stripped of their leaves, and whole rows of trees in 

 orchards and along highways appear to have been stripped in a single night. 



The conditions as described seem to be comparable to those pre- 

 vailing at the present time, and at the same time to be inadequately 

 explained. Repeatedly personal observations have been made which 

 indicate beyond the shadow of a doubt that under certain circum- 

 stances the. gipsy moth has increased at a rate very far in excess of 

 sixfold annually at the present time. Counts of old egg masses as 

 compared with those newly laid, in several localities, in the spring 

 of 1908 and each spring subsequently, have shown positively that 

 an increase of at least twentyfold was not uncommon. In fact, 

 unless an unduly large number of old egg masses was concealed, it 

 could be said with equal certainty that increase sometimes amounted 

 to fiftyfold in the course of a single generation. The arguments pre- 

 sented by Forbush and Fernald, who evidently observed something 

 very similar, and who were inclined to credit it to seasonal or climatic 

 conditions (in part at least), do not stand, in view of the fact that the 

 rate of increase differs extraordinarily in localities nearly adjacent to 

 where the conditions are practically identical, saving only the varying 

 abundance of the moth; this latter, it may be noted, has in each 

 instance corresponded roughly and in direct ratio to the rate of in- 

 crease. The fact was not considered to be of more than coincidental 

 interest at first, but later, when an attempt was made to classify 

 according to their manner of operation, the various factors which 

 were already responsible for the partial control of the gipsy moth in 

 New England, the correlation between relative abundance and rate 

 of increase recurred and seemed to afford excellent support to the 

 contention which has been made as to the part which birds and most 

 other predators play in bringing this about. 

 95677°— Bull. 91—11 8 



