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. l 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 be limited only by the supply of food. Whenever food 

 becomes scarce many of the moths are less prolific. The larvae 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. 



