Overwintering ability of the emerald ash borer (Agrilus planipennis): host effects, microhabitat, and other factors

The emerald ash borer (EAB), Agrilus planipennis, is an invasive insect that was first found in North America in 2002. It is now found in 17 states and two Canadian provinces and killed tens of millions of trees in forests and in urban settings. In Minnesota EAB is currently known to be in four counties and has not yet been found northern Minnesota, home to more than 600 million black ash. Will EAB be able to overwinter in northern Minnesota? Here we consider some important factors when determining EAB's overwintering ability.

Minnesota has more than 900 million ash trees in the state, the majority of which are black and green ash.  Both tree species have been infested in Minnesota, but previous studies have shown varying levels of preference and utilization of these species. Does host species affect EAB's cold hardiness? We determine cold hardiness using thermocouple thermometry to measure supercooling points, the point at which the insect freezes; and lower lethal temperature, the lowest temperature an insect can be instantaneously exposed to before it dies.

To determine if the larvae could survive different levels of freezing, we also looked at removing supercooled insects from the freezer at different points along the exotherm: as close to the supercooling point as possible, at the peak of the exotherm, and at the completion of the exotherm.

EAB overwinter as larvae under the bark in the xylem and phloem of an ash tree where they are not directly exposed to the air temperature. How does the under-bark temperature differ from the air temperature, and are there differences in this microclimate between ash species? Under bark temperatures were recorded in both green and black ash trees at Great River Bluff State Park near Winona, MN, and in north central Minnesota near Grand Rapids, MN.