Molecular profiling of the Australian plague locust pathobiome reveals a microbial driver of population suppression.

Despite recent advances in surveillance, forecasting, and management, locust plagues remain a chronic threat to global agriculture. Part of the intractability of this issue lies in the complex interplay of ecological factors driving locust population dynamics, of which the locust microbiome remains an understudied-but crucial-component. During the wet Australian summers of 2020-2022, forecasting models predicted that Australian plague locust (Chortoicetes terminifera) populations would reach plague proportions, while field-surveillance revealed the contrary. To investigate a putative microbiological cause for this population suppression, a diagnostic framework integrating metabarcoding, microbial isolation, genomic characterisation, and pathogenicity assays was applied. This approach identified a bacterial pathogen belonging to the Serratia marcescens species complex and demonstrated its entomopathogenicity through controlled feeding assays. Genome mining of the isolated S. marcescens strain revealed virulence factors associated with insecticidal activity, including urease-mediated cytotoxicity and metal-scavenging biosynthetic gene clusters. Finally, field population screening indicated that S. marcescens presence was not uniform across surveyed APL populations, suggesting that infections may be transient or epidemic in nature, potentially influenced by environmental conditions such as temperature and humidity. These findings stress the importance of microbial interactions in shaping locust population dynamics and establish a basis for future research into ecological strategies for locust control.