30 BULLETIN 1363, U. S. DEPARTMENT OF AGRICULTURE 



the native tachinid — PelatacMna pellucida Coq. — is well able to hold 

 its own against Compsilura. A spring emergence at least two- 

 weeks in advance of Compsilura and an ability to hibernate in a 

 puparium sufficiently guarantee its survival. 



EFFECT UPON HOST SPECIES 



Data concerning the relation of the native tachinids to their hosts 

 prior to the introduction of Compsilura have been obtained through 

 miscellaneous host records and published accounts of the insect. 

 Data have also been obtained from two years of systematic collecting 

 of native larvae outside the Compsilura area, together with a few 

 collections of larva* made in a territory where Compsilura was 

 hardly established. 



From comparative data thus obtained and from what can be sur- 

 mised, it seems very clear that the addition of Compsilura to our 

 fauna has been of great benefit. Not only has it accomplished the 

 purpose for which it was introduced, namely, to act as a primary 

 parasite on the larvae of the gipsy and brown-tail moths, but the 

 rapidity of its dispersion has exceeded all expectations and it is now 

 found established in an area independent of its primary hosts. It 

 is in this area and especially along the outskirts of the moth infesta- 

 tion that the presence of Compsilura means so much. The reasons 

 are twofold: (1) Its attack upon a light infestation of gipsy-moth 

 larvse, such as would be found along the border of the infested area, 

 would be, or at least has always been, attended with maximum re- 

 sults, and the check exerted here is of the greatest importance; (2) 

 possible dispersion of the gipsy-moth larvae by the wind or otherwise 

 in an unscouted area, and subsequent infestation, would result per- 

 haps in complete annihilation by Compsilura. The establishment of 

 Compsilura in this area is, of course, due to the alternate hosts upon 

 which its existence is dependent. 



Of the many native hosts from which Compsilura has been reared, 

 at least one-half are decidedly favorable for its development. The 

 fact that Compsilura has, since its introduction into New England in 

 1906, made its presence felt in no less than 92 species of native in- 

 sects is in itself a manifestation of its efficiency. So thoroughly has 

 this tachinid established itself in this region that it is now able to act 

 as automatically as any native species whose mission is that of a con- 

 trolling agent. Surely no better example of its ability to cope with 

 the unexpected can be asked than its encounter with the satin moth 

 (Stilpnotia salicis). Here an injurious insect, recently imported 

 from Europe, in a single year appeared in astonishing numbers with- 

 out the least semblance of control by native tachinids. It is ex- 

 tremely doubtful if any of the native parasites would have proved 

 effective had they had the field to themselves. Fortunately, however, 

 Compsilura had become acclimated, and the control exercised by it 

 was remarkable. From certain last-stage larval collections, there 

 have been obtained as high as 78 per cent parasitism. In their ag- 

 gregate (last-stage larval collections) they will average at least 50 

 per cent parasitism. On the other hand, the sum total of the native 

 tachinids obtained from 20 collections is but 19 individuals, less than 

 2 per cent parasitism. Although this is perhaps one of the most spec- 

 tacular instances of Compsilura's potency, it is by no means excep- 



