Impact of the exotic fall armyworm on larval parasitoids associated with the lepidopteran maize stemborers in Kenya

Exotic invasive insect herbivores have the potential to interfere with existing herbivore-natural enemy interactions in new environments. Fall armyworm (FAW), Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae), is a new invasive pest in maize fields in Africa. Understanding the acceptability and suitability of FAW to existing maize stemborer-parasitoid interactions is the first step in elucidating the impact that this exotic insect pest can have on the existing natural enemies used in biological control of maize stemborers in Kenya. The most commonly used larval parasitoids for biological control programs against maize stemborer communities in East Africa are Cotesia flavipes Cameron (Hymenoptera: Braconidae) and two populations of the native Cotesia sesamiae (Cs-Inland and Cs-Coast) Cameron (Hymenoptera: Braconidae). All these parasitoid species attacked FAW larvae but none yielded offspring, although they induced high non-reproductive host mortality when compared to natural mortality. Furthermore, the parasitoids that inserted their ovipositor into FAW larvae exhibited no significant preference between FAW larvae and their respective stemborer hosts under dual-choice bioassays. In olfactometer bioassays, the parasitoids were more attracted to plants infested by FAW than uninfested plants and even showed a marked preference for the odours of plants infested by FAW over those of plants infested by their natural host counterparts. This study illustrates that exotic pests, such as FAW, can impact existing stemborer-parasitoid interactions associated with maize, even if they cannot be used as hosts by parasitoids associated with these stemborers. Although additional studies are needed, FAW might therefore have a negative impact on stemborer biological control existing before its invasion.