Bioactive Volatiles From Push-Pull Companion Crops Repel Fall Armyworm and Attract Its Parasitoids

Fall armyworm,Spodoptera frugiperda, is a serious invasive pest in Africa but “Push-Pull” companion cropping can substantially reduce infestation. Here, we elucidate the underpinning chemical ecology mechanisms. We hypothesized that companion crop volatiles repel herbivores (push) while attracting natural enemies (pull). Headspace volatiles collected from companion plants (Desmodium intortum, Desmodium uncinatum,BrachiariaMulato II) were used in bioassays and electrophysiological recordings withS. frugiperdaand parasitoid wasps. Insect populations, plant damage and herbivore parasitism were assessed in field plots. Coupled GC-electroantennogram (GC-EAG) recordings showed robust responses to certain aromatic and terpenoid volatile compounds. In wind tunnel bioassays, maize volatiles mixed withDesmodiumvolatiles were less attractive to moths than maize alone. In oviposition bioassays,S. frugiperdalaid significantly fewer eggs on maize whenDesmodiumvolatiles were present. Conversely, in an olfactometer bioassay, parasitoid wasps were attracted to the scent of bothDesmodiumspp. (intercrop) and theBrachiariaborder crop. Our data provide evidence of the mechanisms underpinning reducedS. frugiperdainfestation in the Push-Pull companion cropping system, i.e., volatiles from companion crops repelS. frugiperdawhile attracting its parasitoid natural enemies. These findings explain why Push-Pull field plots had fewerS. frugiperdalarvae and lower crop damage than monocropped maize.