Displacement of Tetropium cinnamopterum (Coleoptera: Cerambycidae) by Its Invasive Congener Tetropium fuscum.

Abstract

We examined the native community of insects interacting with an invasive species, Tetropium fuscum (F.) (Coleoptera: Cerambycidae), in its new range to explore reasons for the invader's relatively slow spread. Tetropium fuscum is a European spruce borer established in Nova Scotia since at least 1990, but it has spread only about 125 km from its site of introduction. We compared the densities of Tetropium spp., their known parasitoids, and the community of wood-boring insects at sites located within the invasion zone in Nova Scotia versus well outside this zone, in New Brunswick, Canada. Using red spruce trees stressed by girdling or felling, we tested whether: 1) T. fuscum had altered the native wood-boring community; 2) T. fuscum displaced a native congener, Tetropium cinnamopterum (Kirby); and 3) parasitism rates of Tetropium spp. differed between the invaded and noninvaded zones. Both Tetropium spp. and their parasitoid wasps emerged exclusively from felled trees as opposed to girdled trees. We found no difference in community diversity inside versus outside the invasion zone. The combined densities of both Tetropium spp. and their overall parasitism rates also did not differ between zones, but T. cinnamopterum density was significantly greater outside the invasion zone, suggesting T. fuscum may displace the native congener where they are sympatric. Our results suggest that the native and invasive Tetropium spp. act as a single functional species in the invasion zone. We speculate that natural control agents (predators, parasitoids, and competitors) might be limiting the rate of spread of T. fuscum.