112 PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 
hatching and on every day thereafter during this period a decrease 
in numbers of 10 per cent should be brought about through natural 
causes, there would still be enough survivors to permit of a substan- 
tial increase in the abundance of the insect. 
Twelve gipsy moths (6 pairs) from each egg mass would be suffi- 
cient to provide for a sixfold annual increase. If reference be made 
to the preceding table, it will be seen that if all destruction ceased 
after the caterpillars had reached the fourth stage, the survivors 
would permit of slightly in excess of sixfold increase; that is, the 
mortality during the first to the fourth stage, inclusive, with a part 
of that which resulted during the fifth stage, would be sufficient to 
account for all of the control at present exerted by natural agencies 
in New England, and this gives, at the same time, an idea as to the 
amount of additional control which the parasites must accomplish if 
they are to become effective. 
The conditions under which the gipsy moth was studied at the 
time when the material for the report just quoted was accumulated 
were, for the most part, abnormal. In only relatively few localities 
was it allowed to increase undeterred, and there were relatively very 
few examples of unrestricted increase to the point when defoliation 
resulted. This, in part, explains what seems to be an element of 
indecision concerning the character of conditions which favored more 
rapid increase of the moth, as quoted below, from the same source. 
CONDITIONS FAVORING RAPID INCREASE. 
When any colony under average normal conditions has grown to a considerable 
size and then received an added impetus from exceptionally favorable conditions, its 
power of multiplication and its expansive energy are greatly augmented, and its 
annual increase arises above all calculations. 1 Under such influences hundreds of 
egg clusters will appear in the fall where few were to be seen in the spring, and thou- 
sands are found where scores only were known before. It is probable that the season 
of 1889 was particularly favorable for the moth's increase. The season of 1894 and 
that of 1895 appear also to have furnished conditions especially favorable for an 
abnormal multiplication of the insect. 
The operation of the causes of these sudden outbreaks is not understood. It is 
evident, however, that the warm, pleasant spring weather of the past two years (1894 
and 1895) hastened the development of the caterpillars, thereby shortening their term 
of life. The length of life of the caterpillars varies from six to twelve weeks. During 
cold, rainy weather the caterpillars eat little and grow slowly. During warm, dry 
weather they consume much more food and grow with great rapidity. In the unusu- 
ally warm spring and early summer of 1895 many of the caterpillars molted a less 
number of times than usual, and their length of life did not exceed six or seven weeks- 
Under these conditions they proved more quickly injurious to foliage than in a more 
normal season, and were more completely destructive within any given area in which 
their numbers were great. And they were not so long exposed to the attacks of their 
i The increase of these large colonies seems to he limited only by the supply of food. Whenever food 
becomes scarce many of the moths are less prolific. The larv;v which do not find sufficient food either die 
or develop early, and the female moths lay fewer eggs than those which transform from well-nourished 
caterpillars. 
