The effect of Ricinus communis on larval behaviour and midgut microbe communities of Spodoptera frugiperda (Lepidoptera : Noctuidae)

Abstract

The Fall Armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is an invasive pest species that spread throughout sub-Sahara Africa after its introduction into west Africa during 2016. It is a destructive pest of many cultivated plant species. The greatest damage by the larvae is however done to its main hosts, maize and sorghum. This study aimed to determine the midgut microbiota community complex of fourth instar FAW larvae that fed on maize (Zea mays) and castor oil plants (Ricinus Communis). To identify the midgut microbial community the isolated bacteria were sequenced through the 16S rRNA gene. Molecular phylogenetic analyses revealed that the bacteria are affiliated to Proteobacteria, Actinobacteria and Firmicutes for both maize and castor oil reared larvae. The microbial midgut community structure and composition differed between larvae that fed on the two respective host plants. Cannibalism was also evaluated when larvae were kept at different densities on maize and castor oil plants to determine if host plants affect their cannibalism behaviour. Y-tube bioassays were conducted to determine if the larvae emit possible chemical compounds that either could attract or repel conspecific larvae and which could in turn enhance or suppress cannibalistic behaviour. The study showed that the castor oil plant affects cannibalism behaviour and midgut microbial community structure. Cannibalism occurs less when the larvae feed on castor oil plants, but the larvae are still cannibalistic when stressed in terms of higher numbers and food source. This study generated information regarding the gut microbe complex of FAW larvae as well as its cannibalism behaviour.