Microbial Associates of the Elm Leaf Beetle: Uncovering the Absence of Resident Bacteria and the Influence of Fungi on Insect Performance

Abstract

AbstractMicrobial symbionts play crucial roles in the biology of many insects. While bacteria have been the primary focus of research on insect-microbe symbiosis, recent studies suggest that fungal symbionts may be just as important. The elm leaf beetle (ELB,Xanthogaleruca luteola) is a serious pest species of elm (Ulmus minor). Using culture-dependent and independent methods, we investigated the abundance and species richness of bacteria and fungi throughout various ELB life stages and generations, while concurrently analysing microbial communities on elm leaves. No persistent bacterial community was found to be associated with the ELB or elm leaves. By contrast, fungi were persistently present in the beetle’s feeding life stages and on elm leaves. Fungal community sequencing revealed a predominance of the generaPenicilliumandAspergillusin insects and on leaves. Culture-dependent surveys showed a high prevalence of two fungal colony morphotypes closely related toPenicillium lanosocoeruleumandAspergillus flavus. Among these, thePenicilliummorphotype was significantly more abundant on feeding-damaged compared to intact leaves, suggesting that the fungus thrives in the presence of the ELB. We assessed whether the detected prevalent fungal morphotypes influenced ELB’s performance by rearing insects on i) surface-sterilised leaves, ii) leaves inoculated withPenicilliumspores, and iii) leaves inoculated withAspergillusspores. Insects feeding onPenicillium-inoculated leaves gained more biomass and tended to lay larger egg clutches than those consuming surface-sterilised leaves orAspergillus-inoculated leaves. Our results demonstrate that the ELB does not harbour resident bacteria and that it might benefit from associating withPenicilliumfungi.ImportanceOur study provides insights into the still understudied role of microbial symbionts in the biology of the ELB, a major pest of elms. Contrary to expectations, we found no persistent bacterial symbionts associated with the ELB or elm leaves. Our research thus contributes to the growing body of knowledge that not all insects rely on bacterial symbionts. While no persistent bacterial symbionts were detectable in the ELB and elm leaf samples, our analyses revealed the persistent presence of fungi, particularlyPenicilliumandAspergilluson both elm leaves and in the feeding ELB stages. Moreover, when ELB were fed with fungus-treated elm leaves, we detected a potentially beneficial effect ofPenicilliumon the ELB’s development and fecundity. Our results highlight the significance of fungal symbionts in the biology of this insect.