Plant volatile organic compounds (VOCs) play many important ecological roles, including the mediation of plant-insect interactions. Phytophagous insects can exploit constitutive VOCs to find suitable hosts, while herbivore-induced VOCs may provide plants indirect defense against herbivores through recruitment of their natural enemies. Plant VOCs can also vary in response to changing conditions in the soil environment, including shifts in the soil microbiome. The soil-mediated plasticity of plant VOCs is of particular interest in agricultural settings where crop rotations and management practices cause significant shifts in the soil microbiome. While studies have shown that soil microbiomes can alter plant growth and foliar chemistry, their impact on volatile emissions is unclear. To determine the effect of farming management system on VOC emissions of crops through microbe-mediated soil effects, we measured constitutive and herbivore-induced VOC emissions from wheat (Triticum aestivum) in a greenhouse experiment. Plants were grown in sterilized soil inoculated with soil from a wheat-fallow rotation or a wheat-cover crop rotation in which the cover crops were terminated by grazing cows. Herbivore-induced VOCs were collected from plants infested with larvae of the wheat stem sawfly (Cephus cinctus), a major pest of wheat in the Northern Great Plains.
Results/Conclusions
Wheat grown in fallow soils showed similar quantity and quality of VOC emissions whether infested with sawfly larvae or not. However, in response to sawfly feeding, wheat grown in cover crop soils emitted greater total VOCs, including higher concentrations of nonanal, a compound important in the recruitment of natural enemies, and 2-pentadecanone, a known insect repellent. These results indicate that agricultural practices can indirectly influence crop VOC emissions through microbe-mediated soil legacy effects, suggesting that agricultural practices may be used to steer soil microbiomes that improve crop resistance to insect pests. Further, our results suggest that crops grown in diversified rotations may be more successful at recruiting natural enemies of pests through herbivore-induced VOC signaling.