PARASITISM OF NATIVE INSECTS. 103 



extent as to have made a study in parasitism under strictly rural con- 

 ditions very difficult, except when eggs or caterpillars have been arti- 

 ficially colonized for the purpose. 



The outbreak of the Heterocampa in New Hampshire and Maine is 

 another exceptional instance. In many respects the results of the 

 relatively limited study given to this insect were the most remarkable 

 of any, since there was offered what, to the writers, was the unique 

 spectacle of unrestricted increase being checked through starvation 

 without the intervention of disease. Notwithstanding the fact that 

 the abundance of this insect was so great as to bring about complete 

 defoliation of its favored trees over a very wide area, not a sign of dis- 

 ease was observed in the fall of 1909 in forests where millions of cater- 

 pillars were literally starving to death. The final, thoroughly effect- 

 ive, and miraculously complete subjugation of the outbreak, which 

 resulted in the insect dropping from the abundance above mentioned 

 to what is perhaps less than its normal numbers in the course of a 

 single year, has already been described in a paper which appeared in a 

 recent number of the Journal of Economic Entomology. There is 

 every reason to believe that it was entirely the result of insect enemies, 

 including both parasites and a predaceous beetle, which latter, 

 through its ability to increase abnormally at the direct expense of 

 that particular insect, played a role exactly comparable to that of the 

 true facultative parasites. Such another outbreak of Heterocampa 

 has never been known, and it is probable that it will be very many 

 years before a combination of conditions makes its repetition possible. 

 It is altogether probable that during this period the parasites will 

 remain in full control. 



A third exceptional instance is the present outbreak of the "pine 

 tussock moth" in Wisconsin. This interesting and, as it has proved 

 itself, potentially destructive insect is decidedly rare in Massachusetts, 

 but notwithstanding its scarcity a sufficient number was collected 

 in 1908 and 1909 to make possible a study of its parasites. Para- 

 sitism to an extent rarely exceeded amongst leaf-feeding Lepidoptera 

 was found to be existent, and it is safe to say that had it not been for 

 its parasites the host would have increased at least fivefold or six- 

 fold in 1909 over the numbers which were present in 1908. Such a 

 rate of increase, if continued, would have placed it among - the ranks 

 of destructive insects in a very few years, and it appears that some- 

 thing of this sort actually occurred in northern Wisconsin. There, 

 some years ago, it reached a stage of abundance which resulted in 

 partial or complete defoliation of pine throughout a considerable 

 territory and, as was expected, a relatively small percentage of the 

 caterpillars and pupae were found to be parasitized. Existing para- 

 sitism in 1910 was not sufficiently effective to prevent its increase 

 to a point which would have made complete defoliation of its food 



