Host influence on overwintering ability of the emerald ash borer (Agrilus planipennis)

Emerald ash borer (EAB), Agrilus planipennis is an invasive insect native to Asia that appeared in Minnesota in 2009. Most of Minnesota’s 900 million ash trees are black ash (Fraxinus nigra) in northern forests, where EAB has not yet been found. Cold hardiness is an important factor in determining where a species can establish, and this study looks at host effects on two measures of cold hardiness: supercooling points (SCP), the point at which an insect freezes, and lower lethal temperature (LL-Temp) of overwintering larvae. Larvae from green (F. pennsylvanica) and black ash were cooled at 1°C/min until the insect froze. For LL-Temp, larvae were exposed to one of five treatments: room temperature, -20°C, -25°C, -30°C, or -35°C. Larvae were observed until pupation to determine survivorship.  SCPs between hosts were not significantly different (Mean_{black ash} = -31.16°C, Mean_{green ash} = -30.48°C,  Kruskal-Wallis χ² = 0.8907, df = 1, P = 0.3453). When larvae were removed at the peak of the exotherm, 75% from green ash and 26.7% from black ash survived. For LL-Temp, host type and presence of an exotherm affected mortality (P_host = 0.0318, P_exotherm < 0.001). The treatment temperature had a significant effect on whether or not there was an exotherm (P_treatment < 0.001), but did not directly affect mortality. Although host does not appear to affect the temperatures at which EAB larvae freeze, overwintering survival appears lower in larvae from black ash than from green ash.