Azadirachtin affected the intestinal structure and microbiota of adult Harmonia axyridis (Coleoptera: Coccinellidae) while controlling Spodoptera frugiperda (Lepidoptera: Noctuidae)

Azadirachtin impairs the growth and molting process of Lepidoptera and other insects and provokes strong developmental delays that affect insect survival. Predators of these insects will also be affected to a certain extent by the process of pesticide application. Therefore, how azadirachtin affects the growth and intestinal microbiota of adult Harmonia axyridis after feeding on azadirachtin-treated 2nd instar larvae of Spodoptera frugiperda was investigated. The results showed that the columnar cells in the midgut of the untreated ladybug adults were normal and that microvilli were abundant. Electron micrographs revealed that azadirachtin (2 and 10 mg/kg) destroyed the intestinal structure, which caused a decrease in the number of microvilli and sparsity of H. axyridis midgut columnar cell matrices, with many columnar cells bursting and releasing cell debris. In turn, azadirachtin affected the insect intestinal environment, with glutathione S-transferase (GST) and carboxylase (CarE) significantly decreasing, although mixed-functional oxidase (MFO) was not affected. Azadirachtin treatment significantly increased the abundance of Enterococcus, Serratia and Enterobacter and decreased the abundance of Hafnia-Obesumbacterium. It affected the absorption and transmission of material energy and resulted in an inhibition of the growth and development of H. axyridis. This study enriches the knowledge of the H. axyridis intestinal microbial community after treatment with azadirachtin and provides guidance for improving the control effect of S. frugiperda by using a combined application of biological control and pesticide.

Automated summary

Azadirachtin impacts the growth and molting process of insects, causing developmental delays and affecting their survival. This study investigated how azadirachtin affects the growth and intestinal microbiota of adult Harmonia axyridis after feeding on azadirachtin-treated larvae of Spodoptera frugiperda. The results showed that azadirachtin destroyed the intestinal structure, affected the insect intestinal environment, and inhibited the growth and development of H. axyridis. This study enriches the knowledge of the H. axyridis intestinal microbial community and provides guidance for improving the control effect of S. frugiperda.