The light at the end of the tunnel: photosensitivity in larvae of the mountain pine beetle (Coleoptera: Curculionidae: Scolytinae)

Investigations of light sensitivity and its physiological effects on insects developing within subcortical tree tissues are limited, presumably due to the assumption that cryptic microhabitats are completely devoid of light. In this study, we documented light-mediated behaviour and opsin gene expression in larvae of the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), an extremely important forest insect that is well adapted for development beneath the bark of pine (Pinus Linnaeus; Pinaceae) trees and is eyeless in the larval stage. Larvae were negatively phototactic, as they selected dark over light microhabitats in phototaxis assays. We recovered long-wavelength opsin transcripts from all life stages, including eggs and larvae, suggesting that D. ponderosae is photosensitive throughout its entire life cycle. Our results imply that photosensitivity contributes to immature D. ponderosae survival and that extraocular photoreception could be common among bark beetle larvae.