RELATIONS BETWEEN INVERTEBRATE PREDATORS AND PREY ASSOCIATED WITH LARCH SAWFLY EGGS AND LARVAE ON TAMARACK

Abstract

Suspected predatory invertebrates associated with the larch sawfly, Pristiphora erichsonii (Hartig), on tamarack were confined in cages with sawfly eggs or larvae to determine which species were predaceous. Most of the insects were found to feed on larch sawfly eggs or larvae under these conditions, but very few spiders ate larch sawfly. The amount of food eaten by various predators is presented.Seasonal population trends of the more common predators and prey available to them were determined from samples of tamarack branches taken at weekly intervals. Associations between the numbers of various predators and prey per 20-branch sample were investigated, and a number of significant predator–prey correlations were obtained. These included correlations between Deraeocoris and sawfly eggs, Plagiognathus and sawfly eggs, and Tetraphleps and sawfly eggs. However, estimates of larch sawfly egg mortality under field conditions showed that egg mortality was largely independent of egg density in the range of populations studied, and showed little relationship to predator density per 20-branch sample. This is because the distribution of larch sawfly egg predators per individual branch was random and appeared to be independent of egg density. Single factor analysis of annual egg populations and egg survival indicated that survival in the egg stage is of no value in predicting population trends of the larch sawfly.