Temperature-based model for predicting univoltine brood proportions in spruce beetle (Coleoptera: Scolytidae)

The spruce beetle, Dendroctonus rufipennis (Kirby), has possible life cycles of 1 or 2 years. Empirical and experimental evidence suggest that temperature is the primary regulator of these life-history pathways. These different life cycles potentially result in substantial differences in population dynamics and subsequent spruce mortality. A multiyear field study was conducted in Utah, Colorado, and Alaska, to monitor spruce beetle development under a variety of field conditions with concurrent air temperature measurements. This information was used to model the tree- or stand-level proportion Of univoltine beetles as a function of air temperature. Temperatures were summarized as averages, cumulative time, and accumulated heat units above Specified thresholds over various seasonal intervals. Sampled proportions of univoltine insects were regressed against the summarized temperature values in logistic models. The best predictive variable, as evaluated by Akaike's Information Criterion, was found to be cumulative hours above a threshold of 17 degreesC elapsed from 40 to 90 days following peak adult funnel-trap captures. Because the model can be used to forecast trends in spruce beetle populations and associated spruce mortality, it is a tool for forest planning.