Oak gall wasps lack conserved microbial symbionts

Shayan Naeem , Diana Rojas-Guerrero, Piotr Łukasik

Herbivorous insects frequently rely on microbial partners to exploit their host plants, yet for gall-inducing insect taxa, there were few attempts to comprehensively resolve their microbial associations. To address this gap, we characterised microbial abundance, composition and specificity in three Polish populations of cherry oak gall wasps (Cynips quercusfolii), a highly specialized phytophagous lineage. Using quantitative sequencing of bacterial and fungal marker region amplicons, we tested whether these insects harbour conserved microbiota likely associated with gall induction or host specialisation.
Cynips quercusfolii and their associated parasitoid Torymus auratus harboured notably low bacterial loads, with a median of ca. 2.28 × 10⁴ 16S rRNA gene copies per insect. Their bacterial communities, primarily comprising genera with broad environmental distributions, were highly inconsistent among individuals and populations, and no bacterial lineage was consistently abundant or broadly conserved across the sampled gall wasps. In turn, fungal communities showed a stronger population-specific pattern.
These results indicate oak gall wasps do not maintain abundant, conserved microbial symbionts despite their specialised herbivorous lifestyle. Instead, their bacterial communities appear sparse, variable, and dominated by environmental or transient taxa. This contrasts with symbiont-dependent lifestyles in many herbivores and suggests gall induction is unlikely to rely on stable bacterial mutualisms.