32 THE EEPOET OF THE No. 36 



In New Brunswick our last outbreak of the Forest Tent insect subsided 

 suddenly in 1915. The outbreak seeois to have been due to an over abundance 

 of the poplar supply, as a direct result of civilization and forest fires. 



In the case of the Spruce Budworm a study of the New Brunswick outbreak 

 has shown the fundamental cause to be a relaxing of the normal food pressure 

 in the form of an increased supply of balsam fir, which is the favored food plant. 



This relaxing of the food pressure has been brought about by the hand of 

 man and has been an inevitable result of existing lumbering practices. 



Without going into details it can be said that the increase of balsam fir has 

 not only meant an increased food supply, but has also meant a decreased bird 

 supply. For the birds that under conditions of the primeval mixed type of growth 

 keep this insect properly subdued, seldom nest or feed in pure stands of balsam fir. 



In New Brunswick we now have an incipient outbreak of the Fall We])worm, 

 and as our studies on this insect have been carried through the best part of a decade, 

 it may be of interest to examine this case a little more closely than the others. 

 In order to show the causes of the present outbreak let us glance for a moment: 

 at the situation obtaining toward the end of the last outbreak, and then let us 

 follow the situation through a short tenn of years until the insect became almost 

 extinct in the Province, and finally let us glance at the conditions of the present 

 incipient outbreak. 



In 1912 the Fall Webwojm was abundant in New Brunswick and from the 

 fact that the environmental pressures were then in a very nice state of equilibrium 

 I infer that the insect had been a fairly conspicuous member of the fauna for at 

 least a decade. 



The food pressure was not very great because the staple diet is Alder and 

 there is an abundance of this shrub along our streams and waterways ; the fooil 

 supply is not great enough to produce menacing millions of the insects but it is 

 sufficient for their maintenance in a condition of mild outbreak. 



On the basis of an average egg mass of 260 eggs, there were about 26 that 

 for some reason or other failed to hatch. Of the 234 that did hatch about 42 

 were attacked in the young caterpillar stage by a four-winged parasite called 

 Apanteles. Of the 192 left to tell the tale about 6 were attacked by another little 

 four-winged fly Meteorus. Then as the larva? grew in stature about 22 of those 

 surviving fell prey to a fair-sized Ichneumon that is now known as a Campoplex. 

 In spite of these attacks by insect parasites there were still left about 164 half- 

 grown caterpillars. Of these about 85 were parasitized and so removed from the 

 contest by another species of Campoplex. The 79 remaining larvae became about 

 three parts grown when a two-winged fly Varicliaeta began an attack upon 

 them. This fly victimised about 45 and this attack together with that of another 

 species of minor importance reduced the inmates of our average nest to about 32. 

 About this time the young red-eyed vireos were getting very hungry and the 

 webworm caterpillars fell a prey to them. Of the 32 remaining these birds devoured 

 over 90 per cent, leaving only about two in each nest. The few not att-acked by 

 birds were able to pupate, but some of them fell victims to pupal parasites of 

 which an Exochilum was the most effective. 



As a result of the combined environmental pressures the average number 

 of moths yielded by each egg mass was less than two, so that in the following 

 year there was a measurable decrease in the numbers of the webworm. 



This decrease continued very regularly year after year until 1916 when the 

 insect became almost extinct in the Province. 



