<i>Desmodium</i> volatiles in 'push-pull' cropping systems and protection against the fall armyworm, <i>Spodoptera frugiperda</i>.

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 <i>Desmodium</i> with maize surrounded by border grass, reduces damage from the invasive fall armyworm (FAW) <i>Spodoptera frugiperda</i>. While initial research indicated that <i>Desmodium</i> volatiles can dampen the attraction of FAW to maize, a recent study recovered very low volatile emission from the commonly used <i>D. intortum</i> and found that the <i>D. intortum</i> headspace did not reduce FAW oviposition on maize (Erdei et al., 2024). Here, we detect volatiles from the headspace of two <i>Desmodium</i> species sampled within the activity window of FAW: <i>D. intortum</i> and the more recently adopted <i>D. incanum</i>; and we present the behavior of gravid FAW moths in bioassays. We detected 25 volatiles from field-grown <i>Desmodium</i>, 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 <i>Desmodium</i>, but not on maize further from, versus closer to <i>Desmodium</i> 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 <i>Desmodium</i>, but pairwise differences were often insignificant. Thus, headspaces of <i>Desmodium</i> species include volatiles that could repel FAW moths, and gravid moths were generally more attracted to maize and its headspace than to either <i>Desmodium</i> species or mixed maize-<i>Desmodium</i> headspaces. However, our results suggest that direct effects of <i>Desmodium</i> volatiles on FAW behavior are insufficient to explain reduced FAW infestation of maize under push-pull cultivation.