Canadian Forest Service Publications

Do natural enemies explain fluctuations in low-density spruce budworm populations? 2018. Bouchard, M.; Martel, V.; Régnière, J.; Therrien, P.; Correia, D. Ecology 99(9): 2047-2057.

Year: 2018

Issued by: Laurentian Forestry Centre

Catalog ID: 39209

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1002/ecy.2417

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Understanding the causal pathways through which forest insect outbreaks are triggered is important for resource managers. However, detailed population dynamics studies are hard to conduct in low-density, pre-outbreak populations because the insects are difficult to sample in sufficient numbers. Using laboratory-raised larvae installed in the field across a 1000 km east-west gradient in Québec (Canada) over an 11-year period, we examined if parasitism and predation were likely to explain fluctuations in low-density spruce budworm (Choristoneura fumiferana; SBW) populations. Parasitism rates by the two main larval parasitoid species, Elachertus cacoeciae and Tranosema rostrale, peaked during different years. This suggests that temporal fluctuations in overall parasitism were partly buffered by compensatory dynamics among parasitoid species. Still, spatial covariance analyses indicate that the residual interannual variation in parasitism rates was substantial and correlated over large distances (up to 700 km). On the other hand, interannual variation in predation rates was not spatially correlated. Piecewise structural equation models indicate that temporal variation in parasitism and predation does not influence temporal variation in wild SBW abundance. Spatially, however, SBWs installed in warmer locations tend to show higher parasitism rates, and these higher rates correlate with lower wild SBW population levels. Overall, the results indicate that large-scale drops in parasitism occur and could potentially contribute to SBW population increases. However, during the period covered by this study, other factors, such as direct effects of weather on SBW larval development or indirect effects through host tree physiology or phenology, are more likely to have explained large-scale variation in wild SBW populations.

Plain Language Summary

In this paper, the researchers studied predation and parasitism among spruce budworm (SBW) larvae under pre-epidemic conditions. SBW larvae were reared in the laboratory and placed in various locations throughout the province of Quebec over a period of 11 years. Key findings include that the only rates that were similar over long distances (over 600 km) were those related to parasitism and that the fluctuations in time of predation and parasitism did not affect spruce budworm populations.

Large-scale decreases in parasitism may occur and could potentially explain epidemic outbreaks since parasitism keeps population in check. However, the study results show that other factors, such as the influence of temperature on SBW larvae development, also come into play.