SPRUCE BUDWORM IMPACT, ABUNDANCE AND PARASITISM RATE IN A PATCHY LANDSCAPE

The hypothesis that vegetational diversity may lessen the impact of fo rest insect pests by favoring natural enemies is appealing to those wh o seek ecologically sound solutions to pest problems, We investigated the effect of forest diversity on the impact of the spruce budworm Cho ristoneura fumiferana following the last outbreak, as well as the budw orm's current abundance and parasitism rate, in the boreal forest of n orthwestern. Quebec. Mortality of balsam fir caused by the budworm was greater ill extensive conifer stands than either in ''habitat islands '' of fir surrounded by deciduous forest or on true islands in the mid dle of a lake. Adult spruce budworm abundance, assessed by pheromone t raps, did not differ significantly between the three types of sites. L arval and pupal parasitism rates were examined by transferring cohorts of laboratory-reared larvae and pupae to trees in the three site type s and later collecting and rearing them. The tachinid Actia interrupta , a parasitoid of fifth and sixth instar larvae, as well as the ichneu monid pupal parasitoids Itoplectes conquisitor, Ephialtes ontario and Phaeogenes maculicornis, caused higher mortality in the habitat island s than on true islands or in extensive stands, Exochus nigripalpis tec tulum, an ichneumonid that attacks the larvae and emerges from the pup ae, caused greater mortality in the extensive stands of conifers.