Is overwintering biology of invasive insects affected by climate change? Plasticity of cold tolerance in the emerald ash borer (Agrilus planipennis)

Background/Question/Methods The emerald ash borer (EAB, Agrilus planipennis, Buprestidae) is a saproxylic beetle introduced to North America from Asia. EAB larvae feed on the cambium of living ash trees (Fraxinus spp.), which quickly kills the trees. EAB infestations were first detected in Michigan in 2002, and the beetles have since spread as far northwards as Northern Ontario, Canada. The potential geographic range of invasive species like the EAB could be limited by temperature constraints on survival, but the climatic conditions in the native geographic range of the EAB in Asia are similar to the northern US and Canada. However, determining the potential geographic range from meteorological records is difficult, because the prepupae are only indirectly exposed to the ambient macroclimate. In the sapwood of ash trees, prepupae and larvae experience microclimatic conditions that likely vary with factors like ambient temperature, light exposure, and snow cover. In particular, local winter conditions in Ontario are characterised by periods of winter thaw and unexpected cold snaps in spring. Here we investigate the plasticity of cold tolerance in overwintering EAB prepupae in respect to winter climate variability. Results/Conclusions EAB prepupae were freeze avoidant with a mean supercooling point (SCP) of -30.6 °C. The SCP appeared to be equivalent to the lower lethal temperature. Survival after prolonged chilling at -10 and -20 °C for up to 12 h at various cooling rates did not differ from survival in unexposed controls. Simulated mid-winter warm spells at 15 °C, only slightly above the developmental threshold of 13.4 °C, resulted in deacclimation to cold, and the SCP increased significantly within 7 days to -22 °C. No significant response was observed after exposure to warm temperatures below the developmental threshold (4 °C, 10 °C). Eventually, our data will be combined with measurements of bark microclimate in various locations in Canada, and used to model the potential spread of the EAB and other invasive wood-boring species in North America.