Microbiota changes in the Olive Fruit Fly Bactrocera oleae driven by antimicrobial treatments of egg-infested olives

The olive fruit fly, Bactrocera oleae (Diptera: Tephritidae), is one of the most damaging pests of olive cultivation, and its management still largely depends on chemical insecticides. Increasing regulatory constraints and environmental concerns have shifted attention toward alternative strategies that target key biological processes of the pest. A central biological dependency is the obligate association with Candidatus Erwinia dacicola, which enables larval development within olives through its nutritional and detoxification functions and contributes to adult fitness. In a first study, the symbiotic association was challenged using five antimicrobial compounds (copper oxychloride, dodine, tannins, flavonoids, and a Zn-Cu-citric acid biocomplex) applied to egg-infested olives. Treatments resulted in a marked reduction of E. dacicola abundance in larvae, with dodine and the Zn-Cu-citric acid biocomplex producing the strongest effects. Symbiont depletion was consistently associated with impaired pest performance, as reflected by reduced pupation success. Based on these results, a second study focused on the effect of the antimicrobial treatment on the microbiota across pest development. Using 16S rRNA gene metabarcoding combined with qPCR, bacterial communities were characterized in larvae, pupae, and adults emerging from treated infested olives. The antimicrobial exposure caused a pronounced reduction in bacterial richness at the larval stage and induced shifts in community composition that persisted through pupation into adulthood. Alongside E. dacicola, four recurrent bacterial taxa involved in nutrient metabolism and detoxification (Acinetobacter, Bacillus, Flavobacterium, and Corynebacterium) were consistently depleted, with effects varying across developmental stages. Together, these studies show that antimicrobial treatments targeting egg-infested olives can disrupt a key symbiotic relationship and reshape the associated microbiota, imposing an early microbial bottleneck that affects both the primary symbiont and the broader bacterial community. The resulting microbiota alterations persist across development and are translated into reduced host performance, supporting microbial symbiosis as a viable target within integrated pest management programs.