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 
