Investigating thermal buffering from ambrosia beetle fungal symbionts during heat treatments: Implications for the wood packaging pathway

There are roughly 100 species of ambrosia beetles (Curculionidae: Scolytinae and Platypodinae) within the United States, 60% of which are non-native. Many of these non-native species are believed to have arrived inside solid wood packaging materials via international trade. Even with International Standards for Phytosanitary Measures No. 15 (ISPM 15), which requires wood packaging materials to be treated with heat or methyl bromide, some ambrosia beetle species appear to bypass biosecurity measures and invade. Within the scolytinae, ambrosia beetles are unique in that they obligately feed on symbiotic fungi that they vector in specialized structures called mycangia. Fungi are characteristically hypothermic due to evaporative cooling, which stems from having a high moisture content, and could potentially provide thermal buffering. To determine if ambrosia beetle fungal symbionts help protect beetles during heat treatments, we investigated the hypothermic properties of two fungal symbionts, Ambrosiella cleistominuta and Ambrosiella roeperi, each associated with a species of non-native ambrosia beetle. We grew the fungi on agar and used thermal imaging to quantify temperature differences between fungal-colonized and un-colonized sections of agar. We also conducted heat treatments on American beech bolts with and without colonies of fungal symbionts and tracked temperatures inside bolts to quantify in situ buffering effects. Our results indicate the extent to which Ambrosiella spp. are hypothermic and provide insights into the potential role of fungal hypothermia in helping ambrosia beetles survive ISPM 15 heat treatments.