The brown spruce longhorned beetle lands more frequently on larger diameter trees

What is it about?

The brown spruce longhorned beetle, Tetropium fuscum, (or BSLB) is an invasive wood boring beetle from Europe that infests stressed spruce trees in Nova Scotia. Previous research has demonstrated that the BSLB uses chemical cues emitted from spruce trees to locate its hosts and that it prefers to land on and lay eggs on stressed- rather than healthy spruce trees. It makes sense for the beetles to target stressed trees because survival from egg to adult is much greater in stressed- than in healthy trees. BSLB infestation also tends to be more prevalent in larger diameter trees than in smaller trees but why this happens is unknown. It could be that the beetles preferentially select larger diameter trees when seeking hosts, or alternatively, it could be that brood survival and successful colonization of a host tree is greater on large diameter trees than on small ones. We tested the hypothesis that BSLB host selection is influenced by tree diameter by counting the numbers of adults landing on red spruce trees of varying diameters. All trees were wrapped with sticky bands and baited with aggregation pheromone and host volatiles to make them equally attractive with regards to olfactory host stress cues. We found significant positive relationships between the mean number of BSLB per sticky band and tree diameter, and also between phloem thickness and tree diameter. We concluded that the positive association between host diameter and BSLB infestation is due in part to greater attack rate on larger diameter trees. This may be adaptive because larger diameter trees have more food (thicker phloem) for developing larvae than do smaller diameter trees. However, there was no evidence for active diameter-based discrimination by the BSLB. When numbers of BSLB landing on trees were expressed per m2 surface area, there was no increase in landing rate with increasing tree diameter. Therefore, the positive relationship between the number of BSLB landing per tree and host diameter may simply be due to greater chances of intercepting airborne beetles (that are drawn to volatiles emitted from stressed spruce) on larger vs. smaller diameter host trees.

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