The emerald ash borer, Agrilus planipennis Fairmaire, is an invasive insect in North America that has caused extensive damage to ash trees throughout its invaded range. The range of Fraxinus spp., specifically black ash (Fraxinus nigra) extends farther north than the known A. planipennis distribution. The ability of an insect to withstand cold, or its cold hardiness, is important in limiting its northern distribution. The cold hardiness of A. planipennis from green ash and black ash was assessed using two laboratory measures, supercooling points and lower lethal temperature, in laboratory- and naturally-infested A. planipennis larvae. Supercooling points of A. planipennis larvae from black and green ash, with medians ranging from 25.5 to 32.6°C and 24.0 to 34.2°C respectively, were not significantly different during the winters of 2012-13 and 2013-14, although the laboratory infested larvae tested in November 2011 had significantly warmer supercooling points. Kaplan-Meier estimates of the likelihood of freezing of larvae from black ash and green ash were not significantly different between tree species or between years. A high proportion of larvae died after being frozen, but freezing did not kill all larvae. Lower lethal temperatures for 50% of the population (LT50) were estimated after accounting for the proportion of larvae that would survive freezing. LT50s of larvae from green ash were 35.2°C (95% CI 35.9°C, 34.7°C) in the winter of 2012-13 and 33.4°C ( 34.1°C, 32.1°C) in the winter of 2013-14. Larvae from black ash had estimated lower lethal temperatures of 33.4°C ( 34.4°C, 32.3°C) in 2012-13 and 33.1°C (≤ 37.5°C, 35.5°C) in 2013-14. Agrilus planipennis larvae overwinter under the bark of ash trees, so larvae may not be experiencing winter air temperatures. To determine temperatures under the bark of ash trees, temperature probes were placed under the bark of both green and black ash at 1.4m high and at the base of the tree on the north and south faces. Weather stations recorded air temperature. Daily minimum temperatures under the bark of green ash ranged from 0.5 to 4.7°C warmer than daily minimum air temperatures, and temperatures were 1.2 to 5 degrees warmer under the bark of black ash. Temperatures at the base of the trees provided the most insulation, possibly due to any snow cover adding to the insulative effect of the bark. During the time in which we took measurements, the air temperature fell below 30°C for an average of 16 hours during the winter of 2012-13. Temperatures under the bark of green ash did not reach -30°C, and temperatures under the bark of black ash fell below -30°C, on average, for 2.6 hours. In 2013-14, air temperature was 30°C or colder for an average of 146.1 hours, 78.1 hours under the bark of green ash, and 118.7 hours under the bark of black ash. Because of the differences in air temperatures and temperatures under the bark of trees, we cannot use air temperature to directly predict A. planipennis mortality. Because under-bark temperatures do not reach the temperature required to kill 50% of the population of A. planipennis every year, further research should consider how the larvae survive sublethal temperatures for longer periods of time.
University of Minnesota M.S. thesis. December 2014. Major: Entomology. Advisor: Robert Venette. 1 computer file (PDF); vii, 80 pages.
Host influence on the cold hardiness of the emerald ash borer, *Agrilus planipennis* Fairmaire (Coleoptera: Buprestidae).
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