The wood tiger moth is home to communities of bacteria in both its gut and defensive fluids. Experiments on the frass produced by larvae have revealed a correlation between the homozygous white allele genotype of the moth and the bacterial taxa present in the gut community. We also observe faster development in larvae when gut microbiota from the white morph is transplanted to homozygous yellow genotypes (Juottonen, Nasiri et al., in prep).

By contrast, the microbiome in the defence fluids is more conserved. Across populations in Finland and Estonia, we have found very little divergence in bacterial community content. Similarly, within a single population, a robust community of bacteria remain present over multiple years and generations (Murphy et al., in prep).

The bacteria present in the defensive secretions appears to play a role in the efficacy of the fluids against avian predators. Secretions from moths whose bacterial communities were depleted by antibiotics during development elicited less hesitation in experiments with blue tits than secretions from untreated moths (Galarza, Murphy et al., in prep.). This points towards the bacteria being involved in the moth’s anti-predator defences and wider ecological interactions.

Following the depletion of the microbiomes bacterial community, a number of changes have been recorded in laboratory-based experiments. The larvae reach pupation significantly faster following multiple applications of antibiotics. Gene expression analysis of these larvae revealed that genes related to immune functions were down-regulated relative to control group larvae, while genes related to growth and cuticle development were up-regulated following antibiotic injections (Galarza, Murphy, et al., under review). This reveals to us that the immune system is required to maintain homeostasis in the moth’s microbiome and this comes with its own costs for the host.