Can herbivore-induced volatiles protect plants by increasing the herbivores’ susceptibility to natural pathogens?

In response to insect herbivory, plants mobilize various defenses. Defense responses include the release of herbivore-induced plant volatiles (HIPVs) that can serve as signals to alert undamaged tissues and to attract natural enemies of the herbivores. It has also been shown that some HIPVs can have a direct negative impact on herbivore survival, but it is not yet understood by what mechanism. Here we tested the hypothesis that exposure to HIPVs renders insects more susceptible to natural pathogens. Exposing caterpillars of the noctuid Spodoptera exigua to indole and linalool, but not exposure to (Z)-3-hexenyl acetate increased the susceptibility to its nucleopolyhedrovirus (SeMNPV). We also found that exposure to indole, but not exposure to linalool or (Z)-3-hexenyl acetate, increased the pathogenicity of Bacillus thuringiensis. Additional experiments revealed significant changes on gut microbiota composition after forty-eight hours of larval exposure to indole. Overall, these results provide evidences that certain HIPVs can strongly enhance the susceptibility of caterpillars to pathogens, possibly through effects on the insects’ gut microbiota. These findings suggest a novel mechanism by which HIPVs can protect plants from herbivorous insects.