The Emerald Ash Borer (EAB) is a wood-boring invasive beetle feeding on all species of native ash (Fraxinus spp.) in North America, thereby causing tremendous ecological and economic damage. EAB is rapidly extending its range following the distribution of ash, but could potentially be limited by unsuitable climate, such as low winter temperatures. However, modeling species range extensions solely based on macroclimate is questionable, since it neglects microclimatic effects and phenotypic plasticity of the organism of interest. Here we investigate if EAB will be able to survive cold temperatures in Canada, and present a first synthesis of overwintering physiology, habitat microclimate, and ambient macroclimate for predicting its future distribution, rather than simply relying on climatic envelope models alone.
We recorded temperature in the overwintering habitat (underneath the bark of ash) over a period of two years in eight different locations that could potentially be invaded by EAB. We determined whether EAB is freeze tolerant or freeze avoidant (dies upon freezing) by measuring supercooling points (SCPs) and monitored survival. Carbohydrates, proteins and lipids were assayed, and we conducted a series of experiments examining phenotypic plasticity and response to climate change induced extreme temperature events.
Results/Conclusions
EAB is freeze-avoidant with mean SCPs being lowest in January (-30.5°C) and increased SCPs in spring. Accordingly, hemolymph cryoprotectants were upregulated at the onset of winter and depressed in spring. Simulated extreme weather events led to deacclimation to cold when warm-spell temperatures were within range of the developmental threshold of EAB. Prolonged exposures to sublethal cold for up to one week had no effect on survival or on any of the physiological parameters measured. The same was found for fluctuating temperature regimes. Repeated exposures in spring led to increased survival in the cold exposed group. In the field, we observed a delay between changes in air temperature and changes in under-bark temperature. In one location we recorded a minimum under-bark temperature that was below the mean winter SCP of EAB, and the microclimatic conditions at this site were generally more extreme than the local macroclimate. We conclude that even though phenotypic plasticity is low, off-season extreme weather events, as well as site-specific microclimatic conditions, might impact the distribution of EAB on a local scale.