Microbial associates of the elm leaf beetle: Uncovering the absence of resident bacteria and the influence of fungi on insect performance

Microbial symbionts play a crucial role in the biology of many insects. While most research on insect-microbe symbioses has focused on bacteria, recent studies investigating insect-fungus interactions suggest that fungal symbionts may be just as important as bacteria in influencing insect biology. The elm leaf beetle, Xanthogaleruca luteola, is a significant pest species that causes extensive damage to elm (Ulmus minor) trees. Here, we investigated the diversity, abundance, and persistence of bacteria and fungi across different life stages and generations of the elm leaf beetle. In addition, we analysed the microbial community associated with elm leaves. We found no evidence of a persistent bacterial community within the beetle or on host plant leaves. By contrast, fungi were found to be persistently present in the beetle's feeding life stages and on beetle-fed elm leaves. Fungal community sequencing showed a predominance of members from the Penicillium and Aspergillus genera in insects and leaves. Culture-dependent surveys of beetles and leaves showed a high prevalence of two distinct fungal colony morphotypes closely related to Penicillium lanosocoeruleum and Aspergillus flavus. We assessed whether these morphotypes influenced the elm leaf beetle's performance by rearing insects on three types of leaves: surface-sterilised leaves, leaves inoculated with Penicillium spores, and leaves inoculated with Aspergillus spores. Insects feeding on Penicillium-inoculated leaves gained more biomass than those consuming surface-sterilised leaves or Aspergillus-inoculated leaves. Females reared on Penicillium-inoculated leaves tended to lay larger egg clutches than those reared on the other types of leaves. Our results suggest that the elm leaf beetle does not harbour resident bacteria and that it benefits from associating with Penicillium fungi.