High temperature tolerance and thermal plasticity in emerald ash borer Agrilus planipennis

1 The emerald ash borer Agrilus planipennis (Coleoptera: Buprestidae) (EAB), an invasive wood-boring beetle, has recently caused significant losses of native ash (Fraxinus spp.) trees in North America. Movement of wood products has facilitated EAB spread, and heat sanitation of wooden materials according to International Standards for Phytosanitary Measures No. 15 (ISPM 15) is used to prevent this. 2 In the present study, we assessed the thermal conditions experienced during a typical heat-treatment at a facility using protocols for pallet wood treatment under policy PI-07, as implemented in Canada. The basal high temperature tolerance of EAB larvae and pupae was determined, and the observed heating rates were used to investigate whether the heat shock response and expression of heat shock proteins occurred in fourth-instar larvae. 3 The temperature regime during heat treatment greatly exceeded the ISPM 15 requirements of 56 degrees C for 30 min. Emerald ash borer larvae were highly tolerant of elevated temperatures, with some instars surviving exposure to 53 degrees C without any heat pre-treatments. High temperature survival was increased by either slow warming or pre-exposure to elevated temperatures and a recovery regime that was accompanied by up-regulated hsp70 expression under some of these conditions. 4 Because EAB is highly heat tolerant and exhibits a fully functional heat shock response, we conclude that greater survival than measured in vitro is possible under industry treatment conditions (with the larvae still embedded in the wood). We propose that the phenotypic plasticity of EAB may lead to high temperature tolerance very close to conditions experienced in an ISPM 15 standard treatment.