Desmodium volatiles in 'push-pull' cropping systems and protection against the fall armyworm, Spodoptera frugiperda

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. While initial research indicated that Desmodium volatiles can dampen the attraction of FAW to maize, a recent study recovered very low volatile emission from the commonly used D. intortum and found that the D. intortum headspace did not reduce FAW oviposition on maize (Erdei et al., 2024). Here, we detect volatiles from the headspace of two Desmodium species sampled within the activity window of FAW: D. intortum and the more recently adopted D. incanum; and we present 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 headspace. In flight tunnel assays, moths approached the headspace of maize more than shared headspaces 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 suggest that direct effects of Desmodium volatiles on FAW behavior are insufficient to explain reduced FAW infestation of maize under push-pull cultivation.