Effects of host tree chemical defenses on the overwintering physiology and success of mountain pine beetle larvae

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Two major factors influencing mountain pine beetle population growth are their ability to withstand host tree chemical defenses and the overwintering success of their larvae. After emerging from a brood tree, adult beetles search for a suitable host. Upon finding the host, the adults and their symbiotic fungi overcome the tree's potent chemical defenses via a pheromone-mediated mass attack. While much past research has focused on the easily accessible adult life stage, very little research has been done on the cryptic larvae as they develop and overwinter in the inner bark (phloem). Research in my lab has shown that larvae face at least as much of a challenge from the host's residual defensive chemicals that saturate the phloem as did their parents during initial host colonization. There is good reason to believe that early larval success in this toxic environment plays an important role in winter survival. In a recently published study of overwintering beetles we have shown that autumn larvae must detoxify host defensive chemicals; that they accumulate stress physiology proteins; and that they devote energy to the production of glycerol and other cryoprotectants in order to survive winter temperatures. Thus, in the short time between hatching and the onset of winter, larvae are faced with obtaining sufficient energy from toxin-saturated phloem tissue while also maintaining energy-intensive physiological processes for survival during months of extreme cold. We propose to study mountain pine beetle overwintering physiology in the context of the substantial residual host tree defenses that the larvae encounter upon hatching. We hypothesize that the ability of autumn larvae to obtain vital nutrients and energy from defensive-resin soaked phloem has a major impact on their cold tolerance physiology and their ultimate overwintering success. We will use traditional field-based research along with a variety of genomics-based methods to investigate the connection between host toxins and insect overwintering physiology. Results of this work will help to improve predictive models for mountain pine beetle population dynamics and will provide potential targets for direct control of this ecologically and economically important insect.