Desmodium Volatiles in "Push-Pull" Cropping Systems and Protection Against the Fall Armyworm, Spodoptera frugiperda

Abstract

Push-pull systems for sustainable pest management combine repellent stimuli from intercrops ("push") and attractive stimuli from border plants ("pull") to repel herbivorous insects from a main crop and attract the herbivores natural enemies. The most widespread implementation, intercropping the legume Desmodium with maize surrounded by border grass, reduces damage from the invasive fall armyworm (FAW) Spodoptera frugiperda. However, the three publications to date investigating underlying mechanisms disagree about the role of the Desmodium intercrop and whether it emits volatiles that repel FAW. Here, we synthesize knowledge on the effects of maize-Desmodium push-pull on FAW, report volatiles from two commonly used Desmodium species: Desmodium intortum and D. incanum, and test whether these affect the behavior of gravid FAW moths in bioassays. We detected 25 volatiles from field-grown Desmodium, many in the headspaces of both species, including volatiles previously reported to repel lepidopteran herbivores. In cage oviposition assays, 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 flight tunnel assays, moths approached the headspace of maize more than the shared headspace of maize and Desmodium, but pairwise differences were often insignificant. Thus, headspaces of Desmodium species include volatiles that could repel FAW moths, and gravid moths were generally more attracted to maize and its headspace than to either Desmodium species or mixed maize-Desmodium headspaces. However, our results indicate that volatiles from Desmodium are not sufficient to explain reduced FAW infestation of maize under push-pull cultivation.