Density‐dependent variability in an eruptive bark beetle and its value in predicting outbreaks

Abstract

AbstractSeveral species of aggressive bark beetle in the genus Dendroctonus (Coleoptera: Curculionidae) undergo large fluctuations in population density with distinct outbreak and non‐outbreak phases. We investigated attributes we hypothesized as subject to density‐dependent variation (in particular, those likely to express phenotypic plasticity) in the southern pine beetle (SPB), Dendroctonus frontalis, as possible indicators of population fluctuations. These traits were morphology (body size and hindwing shape) and the sex ratio of trap‐captured, dispersing SPB populations. We compared attributes of beetles from locations that ranged from having zero to high numbers (>1500) of SPB infestations (spots) at the county level for two summers. Southern pine beetle were obtained from six states in the southeastern USA and had been collected during a springtime, region‐wide trapping survey used for forecasting outbreaks annually. Although we detected an expected but weak sexual dimorphism in both size and shape‐related traits, no morphological differences were found between SPB collected from traps in counties with low or high densities of spots (≤10 or >10 per county, respectively). We found no relationship between numbers of SPB spots per county and trapped sex ratios in 2016, but we observed a strong trend in 2017, with about three times higher proportions of females trapped in counties with low compared with high numbers of spots. This implies that one or more known or possible factors influencing trapped sex ratios (e.g., disparities between the sexes in their responses to semiochemicals or in their propensity to disperse) can be density‐dependent. Including trap‐captured sex ratios in prediction models may improve current forecasting of SPB outbreaks in the southeastern USA, informing more timely and effective management of one of the most economically and ecologically important beetle species of this region.