Mountain pine beetle seasonal timing and constraints to bivoltinism.

Mountain pine beetle tree colonization typically occurs in July and August, with completion of a generation one (univoltinism) or two (semivoltinism) years later. In a 2012 publication, Mitton and Ferrenberg suggested that climate change resulted in an unprecedented generation between June and September (a summer generation), with a concomitant shift to two generations in one year (bivoltinism). Although summer generations are not uncommon in this species, completion of a second generation across winter, between September and June, would be required for bivoltinism, a phenomenon not previously observed. Mitton and Ferrenberg showed that a summer generation can occur, but they failed to adequately track cohorts and provided no compelling evidence for bivoltinism. We demonstrate that a winter generation-and hence bivoltinism-would have been physiologically impossible at the high-elevation site used in Mitton and Ferrenberg due to lower thermal developmental thresholds. The mountain pine beetle is indeed being influenced by climate change. To address the challenges of future population outbreaks of this significant tree mortality agent, however, it is imperative to consider evolved, thermally dependent traits that serve to maintain seasonality.