Suboptimal host tree benefits the overwintering of a destructive forest insect pest

Abstract

Winter represents a stressful period for many organisms, and terrestrial insects are particularly susceptible to adverse thermal conditions. However, exposure to thermal stress and its effects on insect survival and performance can be modulated by various external factors. In forest systems, the overwintering microsite, stand structure, and regional climatic conditions may play a role in overwintering survival and performance at the individual microsite, local stand, and landscape level, respectively. To better understand the effects of these potential influences, rarely examined simultaneously, we deployed wild and lab-reared diapausing spruce budworm in a series of field experiments established across a range of spatial scales.Overwintering survival was higher on black spruce than balsam fir, likely owing to differences in substrate structure (and hence microsite quality/availability) between these two hosts. At the stand level, neither tree species nor age had an impact on insect survival; however, we observed a positive carryover effect of overwintering in a black spruce stand, enhancing performance in post-diapause development, possibly owing to buffering against warm spells during winter. Lastly, despite exposure to a broad range of winter temperatures across an extensive latitudinal gradient, overwintering survival was high (>80%) and did not vary at the landscape level.Our results indicate that insect overwintering was influenced by environmental factors at multiple spatial scales. Interestingly, despite it being considered a poor host for larval feeding, overwintering on black spruce clearly benefited spruce budworm survival and post-diapause performance. Secondly, microsite and stand-level factors had a greater impact than did regional climate variation. Collectively, these findings highlight the importance of finer-scale environmental factors on larval overwintering at both the lethal and sub-lethal level, with implications for basic and applied ecology of this iconic forest insect pest.