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

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 legumeDesmodium, reduces herbivory damage from the invasive fall armyworm (FAW)Spodoptera frugiperda. However, the three publications to date investigating underlying mechanisms disagree whether theDesmodiumintercrop emits bioactive volatiles that repel FAW. We aimed to resolve this controversy by measuring volatile emission fromDesmodium intortum(greenleafDesmodium) andD. incanumintercrops 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 bothDesmodiumspecies, which exhibited similar profiles, including substances previously reported to repel lepidopteran herbivores. FAW moths preferred to oviposit on maize overDesmodium, but not on maize further from versus closer toDesmodiumplants 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 andD. intortum, but notD. incanum. In conclusion, we foundD. intortumandD. incanumto emit volatiles that have been shown to be bioactive, and FAW moths to prefer maize over eitherDesmodiumspecies. Moreover, additional mechanisms are likely important for reducing FAW damage to maize under push-pull cultivation.