28 BULLETIN 1363, U. S. DEPARTMENT OF AGRICULTURE 



establishment of Compsilura, by its subsequent parasitism upon 

 native species, and by its present relation to the native parasites, is 

 more obvious. 



As regards the majority of native parasites there is little to indi- 

 cate anything detrimental to their welfare in the introduction of 

 Compsilura, except an occasional scarcity of host material brought 

 about by the successful attack of this insect. That this absence of 

 host material is in itself of importance, inasmuch as it might materi- 

 ally change or upset the natural balance already existing, does not 

 seem to be borne out by the records of the writers. It is true that 

 there were no systematic collections of native larvae prior to 1915 

 against which to check the larval collections of the last few years; 

 there is. however, first-hand knowledge concerning serious outbreaks 

 of native insects in this locality since 1907. The abundance of any 

 insects for which the introduction of Compsilura might be held re- 

 sponsible, because of its interference with the native parasites of 

 the species in question, has not been recorded. On the other hand, 

 infestations of insects which are due for their periodic outbreaks 

 are, so far as known, not on the increase. That there is a consider- 

 able amount of duplicate parasitism, there can be no doubt ; and that 

 this would be detrimental to one species or another, must also be 

 true. Such an occurrence would take place in any case, however; 

 and, although the presence' of Compsilura probably adds to it, the 

 results are not necessarily serious. It is possible that, because of 

 the rapid larval development of Compsilura, it would crowd out 

 forms which develop more slowly. This probably happens where 

 Compsilura and a native tachinid attack the host simultaneously. 

 If, however, the native tachinid were in a stage of development more 

 advanced than that of Compsilura, the latter would in all probability 

 be the loser. There are few positive data on the rapidity of develop- 

 ment of many of the competing species, but it is believed that in 

 some of them the development is much slower than in Compsilura. 



What appears best to illustrate an occasion where there is a likeli- 

 hood of Compsilura usurping the position of a native tachinid is 

 found in its relation to Tachina mella Walk. Here is a species far 

 less specialized than Compsilura, its reproductive habit being host- 

 oviposition of a flat macrot}^pe egg. Not only have the two species 

 apparently similar hosts, but in nearly every instance where there 

 is competition the native tachinid is the one that suffers most. That 

 there has been a marked decrease in the number of T. mella in the 

 gipsy-moth area since the establishment of Compsilura is probable. 

 The parasite records of the writers show this, and there is corrobo- 

 rative evidence as well. Forbush and Fernald, in 1896 (#, p. 388), 

 cited several instances of rearing mella from the gipsy moth. It is 

 rare that any mella are reared to-day, however, and in their aggre- 

 gate the larval collections of the gipsy moth are far greater than in 

 the past. Forbush and Fernald (4, p. 385) call attention to the great 

 number of tachinid eggs deposited upon gipsy-moth larvae. It is 

 possible, of course, that these eggs were not all laid by one species ; but 

 it is probable that a large percentage of them belonged to T. mella, 

 as the records of rearing will show. During the past few years 

 thousands of gipsy-moth larvae have been sent to the laboratory and 

 less than 1 per cent have tachinid eggs on them. Evidently one of 



