Canadian Forest Service Publications
Evaluating seasonal variation in bottom-up and top-down forces and their impact on an exotic wood borer, Tetropium fuscum (Coleoptera: Cerambycidae). 2013. Flaherty, L.; Quiring, D.T.; Pureswaran, D.; Sweeney, J.D. Environmental Entomology 42(5): 831-1122.
Issued by: Atlantic Forestry Centre
Catalog ID: 35120
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It is well understood that forces from multiple trophic levels simultaneously influence herbivore performance, but how the relative strength of these forces vary over space and time is less clear. We evaluated seasonal variation in the impact of bottom-up forces (host condition), top-down forces (natural enemies), and competition on the performance of an exotic wood borer. Tetropium fuscum (F.) (Coleoptera: Cerambycidae) eggs that were either protected or exposed were placed on healthy and stressed red spruce, Picea rubens Sargent, trees at three different times during the natural T. fuscum flight period. We also measured the length of necrotic lesions (i.e., an induced hypersensitive response) that developed in response to attacking T. fuscum. As predicted, T. fuscum performance was usually greater, and induced host defenses lower, on stressed than on healthy trees, but the impact of host condition on T. fuscum performance varied seasonally. Timing of attack was critical, influencing the strength of bottomÐup forces and consequently all measures of T. fuscum performance. Survival was reduced when T. fuscum attacked too early (late-May) or too late (late-June), which may result in stabilizing selection for attack time in this species. Parasitism and competition were generally negligible during this study. Our results suggest that timing of attack is critical for this wood borer and that temporal variation in the impact of top-down and bottom-up forces should be considered in other systems.
Plain Language Summary
The brown spruce longhorn beetle, Tetropium fuscum, (BSLB) is an invasive species from Europe that has established in Nova Scotia and is slowly spreading westward. In Europe it infests severely weakened Norway spruce but in Canada it has infested and killed apparently healthy spruce. In previous studies in which we placed eggs or adult beetles on trees (i.e., the beetles had no choice) we found that BSLB survival was much greater on stressed trees than on healthy trees. The “preference-performance hypothesis” predicts that when given a choice, females will prefer to lay eggs on trees on which their offspring have greater survival. We tested this prediction with the BSLB by comparing healthy versus stressed spruce for the numbers of BSLB that landed on the trees and the number of eggs laid per tree. We found that ten times as many BSLB landed on stressed spruce than on healthy spruce, and once they had landed, laid three times as many eggs on stressed spruce than on healthy spruce. These results support the preference-performance hypothesis, i.e., female BSLB made smart choices when looking for trees in which to lay their eggs. Our results also showed that these choices were made largely before landing on the trees, probably using olfactory cues. Finally, the results also suggest that healthy spruce trees are at much lower risk of attack by the BSLB than are stressed spruce.