Proce^ings, 1920. 



FURTHER NOTES ON THE TENT-CATERPILLAR AND 

 ITS NATURAL CONTROL, 1919. 



Bv A. B. Baird. 



The tent-caterpillar outbreak was again very severe in and around the 

 Cities of Vancouver and \'ictoria, causing much damage to shade and fruit 

 trees and making the streets and parks very unsightly. The web-building 

 species (Malacosoma pluvialis) was predominant and spread over a much 

 larger area than the forest tent ( M. disstria var. erosa). The latter was 

 confined chiefly to a part of the Uplands District in Victoria, and in Van- 

 couver to a section between Seventh and Thirtieth Avenues and extending 

 for some fifteen to twenty-five blocks east and west of Main Street. 

 Occasional caterpillars and small colonies were found scattered all over the 

 infested area, but only in the above-mentioned areas was it in the majority. 

 The species are equally destructive and their life-histories, habits, and control 

 are practically the same, the chief difference being that T. pluvialis hatches 

 about a week earlier in spring. Owing to the very cold, backward spring 

 the trees had nearly all leaved out before the larvae issued, which is a rather 

 unusual condition. The cold weather also reduced the work of insect para- 

 sites to a large extent. 



As conditions in \'ancouver and \'ictoria are quite dififerent, it is neces- 

 sary to discuss them separately. 



In \^ancouver overcrowding and diseases were the chief control factors 

 noted, and together they were responsible for the death of large numbers of 

 the larvae. It is rather ditificult to estimate the percentage thus destroyed, 

 but judging from the abundance of egg-masses it was not suflicient to pre- 

 vent another outbreak next year. Insect parasites were almost entirely 

 absent. Apart from a few egg parasites which destroyed about 2 per cent, 

 of the eggs and a species of tachinid fly which destroyed less than i/io of t 

 per cent, of the pupje, none were found, although several careful searches 

 were made in the field covering most of the infested territory, and each week 

 collections of 100 larvae and pupae were made from different sections, which 

 were either dissected or reared tJirough in trays in the insectary. If para- 

 sites were present they must have been exceedingly scarce and localized. 

 Birds of various kinds devoured a small percentage and insect predators, 

 such as ants and carabid beetles, no doubt also took a small percentage. 

 None of these factors, however, have had any appreciable eft'ect in reducing 

 the outbreak, and there will be another outbreak to face this coming season 

 if the eggs now on the trees hatch and yield the usual number of caterpillars. 



In \'ictoria conditions were quite different. Insect parasites were very 

 al)undant in early June — sufficient under ordinary conditions to control the 

 outbreak. The cold, windy weather, however, reduced their effectiveness, 

 so they were almost a negligible quantity in the control. IMillions of 

 parasite-eggs failed to hatch in time to do their deadly work, and millions 

 more were not deposited at all, or were deposited out of season. One 

 tachinid fly deposited eggs on fully 50 per cent, of the caterpillars, but, so 



