Canadian Forest Service Publications
Modeling landscape-level spatial variation in sex ratio skew in the mountain pine beetle (Coleoptera: Curculionidae). 2016. James, P.M.A.; Janes, J.K.; Roe, A.D.; Cooke, B.J. Environmental Entomology 45(4):790-801.
Issued by: Northern Forestry Centre
Catalog ID: 37119
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Through their influence on effective population sizes, sex ratio skew affects population dynamics. We examined spatial variation in female-biased sex ratios in the mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in western Canada to better understand how environmental context affects sex ratio skew. Our specific objectives were to: 1) characterize spatial variation in mountain pine beetle sex ratio; 2) test previously asserted hypotheses that beetle sex ratio varies with tree diameter and year in outbreak; and 3) develop predictive models of sex ratio skew for larval and adult populations. Using logistic regression, we modeled the probability that an individual beetle (n = 2,369) was female as a function of multiple environmental variables across 34 stands in British Columbia and Alberta, Canada. We identified a consistent female-biased sex ratio with significantly greater skew in adults (2:1, n = 713) than in larvae (1.76:1, n = 1,643). We found that the proportion of larval females increased with decreasing tree size and with outbreak age. However, adults did not respond to tree size and larvae did not respond to outbreak age. Predictive models differed between larvae and adults. All identified models perform well and included predictors related to weather, tree diameter, and year in outbreak. Female-biased sex ratios appear to originate from differential male mortality during development rather than from sex-biased oviposition, suggesting sex ratio skew is not the cause of outbreaks, but rather a consequence.
Plain Language Summary
An unprecedented outbreak of the mountain pine beetle has affected millions of hectares of forest in western North America. The sex ratio (i.e., the ratio of males to females) of this beetle varies considerably in time and space. Information on the factors driving variations in the sex ratio is needed to effectively monitor and manage beetle populations. This study documents an unusual skew in the sex ratio toward a strong female bias in populations at the eastern front of the beetle invasion and examines environmental factors responsible for the skew. In larval populations the degree of skew is most affected by tree size, but in adult populations it is more affected by the age of the beetle outbreak. The sex ratio of larval beetles is influenced by factors linked to individual tree diameter whereas the sex ratio of adult beetles is influenced by broader scale geographic variation. The skew is ultimately associated with differential mortality of the two sexes, with males dying in larger numbers, particularly in small trees in older outbreaks. The implications for population dynamics are discussed.