Canadian Forest Service Publications
Do visual cues associated with larger diameter trees influence host selection by Tetropium fuscum (Coleoptera: Cerambycidae)? Nelson, T.D.; Sweeney, J.D.; Hillier, N.K. 2017. The Canadian Entomologist 149: 487-490.
Issued by: Atlantic Forestry Centre
Catalog ID: 39130
CFS Availability: PDF (download)
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Tetropium fuscum (Fabricius) (Coleoptera: Cerambycidae) is an invasive phloeophagous beetle established in Atlantic Canada that infests stressed and moribund Picea Dietrich (Pinaceae) species. Successfully colonised trees tend to be large in diameter (>10 cm diameter at breast height), but whether diameter influences host selection, larval performance, or both, is unknown. We tested the hypothesis that T. fuscum host selection is influenced by visual cues associated with tree diameter by counting the number of adults landing on 29 Picea rubens Sargent ranging in diameter at breast height from 12.2 to 37.5 cm. All trees were wrapped with sticky bands and baited with aggregation pheromone and host volatiles to make them equally attractive with regard to olfactory cues. We found significant positive relationships between the mean number of T. fuscum per sticky band and tree diameter, and also between phloem thickness and tree diameter. We conclude that the positive association between host diameter and T. fuscum infestation is at least partially due to the positive influence of diameter on landing rate, and that this may benefit the beetle because larger diameter trees have more food for developing larvae. However, there was no effect of tree diameter on the mean number of adults per m2 of sticky band and thus no evidence that T. fuscum actively selects larger diameter hosts based on visual cues. The positive relationship between landing rate and host diameter may simply be due to greater chances of airborne beetles being passively intercepted on larger versus smaller trees.
Plain Language Summary
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 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 number 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 regard 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.