38 B. C. ESTTOMOLOGICAL SOCIETY 



coast to coast distribution. Certain findings in particular seemed to 

 indicate that such was the case. For instance, a few years after the work 

 was started it was found that useful parasites were occasionally killed 

 out over an area as large as New Brunswick. A Tachinid parasite that 

 for three years in succession had killed off more than sixty per cent, 

 of the fall webworm crop (Hyphantria) was suddenly itself killed off 

 by a very simple but quite natural set of conditions. The crux of the 

 situation was that the fly had to compete with another parasite that 

 could oviposit in wet weather. While the Tachinid flies were sheltering 

 under leaves, their food supply was parasitized by their competitor. 

 In another instance a light frost in June indirectly killed off the principal 

 parasites of the forest tent (Malacosoma disstria). Such cases indicated 

 that parasites present on one coast or in one locality might be absent 

 in other places. 



With these things in view the same set of host insects were studied 

 last year in the West as had been studied for several years previously 

 either in Massachusetts or New Brunswick. 



The forest tent caterpillar (M. disstria) was studied at Red Deer, 

 Alberta, and in the lower Fraser Valley. At the first of these places 

 there has been a serious outbreak of the insect for three years, and a 

 great deal of leaf stripping has taken place each year. In an outbreak 

 of such long standing one usually finds that parasites of various kinds 

 are becoming very abundant. At Red Deer, however, such was not the 

 case, and I was unable to find a single specimen of any of the parasites 

 ordinarily preying on this host in both the East and far West. Parasites 

 of the right kind could be secured in great quantities at the present time 

 from the Western Tent insect (M. pluvialis) on the lower end of Van- 

 couver Island, and it seems well worth an effort to collect these for 

 liberation at Red Deer. The only reason for the absence of parasites at 

 Red Deer that I am able to suggest is that as the present outbreak seems 

 to be quite isolated, it probably arose from a few moths being blown in 

 from such a great distance that the parasites have been unable to follow. 

 As none of the parasites are on the wing at the flying time of the moths 

 they could reach such an isolated outbreak only by being accidently 

 blown to it. In their case, of course, the barrier would consist of a great 

 area of country supporting no food material. 



In the lower Fraser Valley the environment for the forest tent is 

 exceedingly unfavourable. None of the native trees it feeds upon are 

 abundant enough to support an outbreak, and the insect seems to be 

 eking out a rather unhappy existence. Most of the larvae from twenty- 

 four egg masses that I had under observation there were eaten by spiders. 

 Though the usual insect parasites were present and even abundant, this 

 was undoubtedly due to the great numbers of M. pluvialis which were 

 suffering a great mortalit)' from the same parasites. 



