Desmodium Volatiles in “Push-Pull” Agriculture and Protection Against the Fall Armyworm, Spodoptera frugiperda

Abstract

Push-pull systems for sustainable pest management of crop plants employ repellent stimuli from intercrops (“push”) to repel herbivores and attract their predators and parasitoids, and attracting stimuli from border plants (“pull”) to lead herbivorous insects out of the crop. The most widespread implementation, intercropping with the legume Desmodium, reduces herbivory damage from the invasive fall armyworm (FAW) Spodoptera frugiperda. However, the three publications to date investigating underlying mechanisms disagree whether the Desmodium intercrop emits bioactive volatiles that repel FAW. We aimed to resolve this controversy by measuring volatile emission from Desmodium intortum (greenleaf Desmodium) and D. incanum intercrops in push-pull fields, and assaying their effects on the behavior of the FAW in oviposition and wind tunnel choice bioassays. We detected many volatile substances in the headspaces of both Desmodium species, which exhibited similar profiles, including substances previously reported to repel lepidopteran herbivores. FAW moths preferred to oviposit on maize over Desmodium, but not on maize further from versus closer to Desmodium plants that were inaccessible to the moths, but sharing the air. In a wind tunnel bioassay, the moths tended to prefer maize volatiles over a combination of volatiles from maize and D. intortum, but not D. incanum. In conclusion, we found D. intortum and D. incanum to emit volatiles that have been shown to be bioactive, and FAW moths to prefer maize over either Desmodium species. Moreover, additional mechanisms are likely important for reducing FAW damage to maize under push-pull cultivation.