Fungal symbionts of the polyphagous and Kuroshio shot hole borers (Coleoptera: Scolytinae, Euwallacea spp.) in California can support both ambrosia beetle systems on artificial media

The specificity of ambrosia beetle-fungus relationships is unknown for the vast majority of ambrosia symbioses, yet has important implications for the success of these beetles and fungi in their surrounding environments. The polyphagous shot hole borer and the Kuroshio shot hole borer (Euwallacea spp.) are invasive ambrosia beetles that exist sympatrically in California, and together with their fungi cause Fusarium Dieback in a variety of hosts. The potential for the two beetle species to exchange symbionts is unknown, but has important ecological consequences. Here we perform symbiont switching experiments, to determine if the polyphagous and Kuroshio shot hole borers can survive and reproduce on each other’s symbiotic fungi in their invasive range in California. Aposymbiotic foundresses were assessed on their ability to reproduce on each of the beetles’ primary (Fusarium spp.) and auxiliary (Graphium spp., Paracremonium pembeum) fungal symbionts, grown on sawdust-based artificial media. Offspring were collected to confirm which fungi were present in the different sexes and life stages. Additionally, the ability of aposymbiotic and non-aposymbiotic foundresses to survive and reproduce on non-ambrosial Fusarium species was tested. We found that reproduction is maximized on the beetles’ primary Fusarium symbionts, regardless of whether a beetle was paired with its own Fusarium symbiont or its congener’s, indicating that there is potential for symbiont switching in natural populations of these beetles. The addition of auxiliary fungi to treatments containing Fusarium spp. had no effect on overall offspring production, and females that were fed solely on auxiliary fungi produced significantly fewer offspring. Females that were fed non-ambrosial Fusarium species produced significantly fewer offspring than when reared on symbiotic Fusarium, and this was especially pronounced in aposymbiotic females. From our experiments, the putative roles of the primary versus auxiliary fungal symbionts, as well as the implications of these roles for beetle reproduction and development in their surrounding environment, are discussed.