Hardwood-softwood composition influences early-instar larval dispersal mortality during a spruce budworm outbreak

Abstract

Spruce budworm (Choristoneura fumiferana Clem.) defoliation of balsam fir (Abies balsamea (L.) Mill.) has been shown to be less severe in stands with higher hardwood (broadleaved trees) content during both building and declining phases of budworm outbreaks. Reduced defoliation associated with forest composition could be explained by habitat fragmentation and/or natural enemy hypotheses, which posit higher early-instar larval dispersal loss or more intense parasitism in hardwood-softwood mixed stands than in pure balsam fir stands, respectively. We carried out field studies in 27 hardwood, mixedwood, and softwood plots with varied balsam fir-hardwood composition to assess effects of forest composition on: 1) first- and second-instar larval dispersal loss, and 2) stage-specific spruce budworm density and parasitism rates. Results indicated that increasing hardwood content increased second-instar (L2) dispersal losses. Stand type significantly affected dispersal loss of L2 larvae, both measured directly for three years and sampled on balsam fir seedlings. Post-hoc pair-wise comparisons indicated that L2 dispersal loss was less in softwood than in mixedwood or hardwood stand types based on measurements on ground traps and less in softwood and mixedwood than in hardwood stands based on sampling budworm larvae on regeneration. This suggested that high early-instar larval dispersal loss is a plausible explanation for the decreased spruce budworm density and balsam fir defoliation associated with increasing hardwood content. Stand type did not significantly affect parasitism rates.