Direct and indirect effects of host tree condition on the preference and performance of an exotic wood-borer, Tetropium fuscum (coleoptera: cerambycidae)
University of New Brunswick
Tetropiumfuscum (F.) (Coleoptera: Cerambycidae) primarily colonizes weakened Norway spruce (Picea abies Karst.) in its native Europe, but has been reported to attack and kill apparently healthy spruce (Picea sp.) in Atlantic Canada. Recent research has improved our ability to monitor and control this species, but little is known about its population ecology. This research evaluates the effect of host tree (red spruce, Picea rubens Sarg.) condition and exposure to natural enemies and competitors on T. fuscum performance in Canada, examines whether the impact of these factors varies seasonally, and evaluates T. fuscum preference for host trees of varying condition. Results suggest that T. fuscum performs better when larvae develop on stressed (i.e., girdled) compared to apparently healthy spruce. However, T. fuscum developing in stressed trees sometimes experienced higher rates of parasitism and competition than did T. fuscum developing in healthy trees, and therefore host condition may mediate the impact of factors from higher trophic levels. Timing of attack by larvae was a critical factor, influencing all measures of T. fuscum performance, as well as the size of lesions that developed on trees attacked by T. fuscum (i.e., induced defences). As selective pressures from natural enemies did not vary temporally, the effect of timing of attack is likely due to bottom-up factors (i.e., host defences and herbivore nutrition). Tetropium fuscum survival was highest when larvae attacked trees mid-season, which may lead to stabilizing selection on attack time and attack synchrony for this species. As predicted by the preference-performance hypothesis, T. fuscum adults landed more frequently and females laid more eggs on girdled compared to apparently healthy trees. This suggests that T. fuscum makes adaptive host selection decisions both before and after landing on a potential host. Taken together, these studies rigorously examine novel tri-trophic interactions between an exotic wood-borer and its host tree, competitors, and natural enemies in an introduced habitat and provide basic morphometric information necessary for future research and management of this species.