Identification of odorant-binding proteins in the antennal transcriptome of Cotesia ruficrus and the response of CrufOBP3 and CrufOBP17 to maize volatiles

Cotesia ruficrus (Hymenoptera: Braconidae) is an endoparasitic natural enemy of various Lepidopteran pests, and it plays a significant role in integrated pest management. In the field survey of maize fields, we found for the first time that the indigenous parasitoid C. ruficrus parasitized the major invasive pest Spodoptera frugiperda (Lepidoptera: Noctuidae) in China. We identified 18 odorant-binding proteins (CrufOBPs) of C. ruficrus through the RNA-Seq method, and PCR analysis showed that 13 out of the CrufOBPs were highly expressed in antennae. Two CrufOBPs, CrufOBP3 with the highest antennal expression and CrufOBP17 with antenna-specific expression, were selected for protein biosynthesis in the prokaryotic protein expression system and subjected to protein purification. Through fluorescence binding assay, seven maize volatile compounds were selected for the antennal electroantennography (EAG) study, and the results indicated that female C. ruficrus exhibited distinct responses to these volatiles at different concentrations. Octanal and 2-heptanone of 10 mg mL⁻¹ were chosen for olfactory behavioral experiments because of their high EAG responses, and the results showed that C. ruficrus adults were significantly attracted at certain concentrations. Furthermore, in the cage experiments, 10 mg mL⁻¹ of 2-heptanone significantly increased the parasitism rate of C. ruficrus on S. frugiperda. These findings not only provide a theoretical basis for studying the odorant-binding protein family of C. ruficrus but also contribute to the development of novel pest control strategies for this invasive pest management.