Indirect interactions between the invasive brown spruce longhorn beetle, Tetropium fuscum, and a native defoliator, Choristoneura fumiferana

Abstract

Two important pests in Atlantic Canada are an invasive beetle, the brown spruce longhorn beetle (BSLB), and a native defoliator, the spruce budworm. Both species are currently attacking spruce in separate regions; however, there is a high likelihood that these attack ranges will overlap. It remains unknown what impact concurrent attacks of these herbivores might have on trees, or on population trends for either species. We investigated the effects of BSLB on interactions between spruce budworm and red spruce, using field experiments near Halifax, Nova Scotia. We established four stress treatments involving natural and simulated BSLB attack, with five branches on each tree assigned densities of spruce budworm. We measured the impact of natural and simulated BSLB attack on red spruce, and spruce budworm defoliation and examined its dependence on budworm density. We also examined performance of adult moths through fecundity measurements. Natural and simulated BSLB attack did not change density defoliation relationships, relative to no attack. Survival of spruce budworm on red spruce appeared to be reduced as spruce budworm density, presumably due to reductions in the availability of the preferred developing foliage. There are indications that, in general, simulated BSLB stressed trees allow higher larval survival than trees with natural BSLB attack or trees that are unattacked.