How a cover crop can benefit parasitoid natural enemies of wheat stem sawfly

Wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a major pest of wheat, causing losses that exceed $350 million annually across the Northern Great Plains. However, two congeneric parasitoids, Bracon cephi (Gahan) and B. lissogaster Muesebeck (Hymenoptera: Braconidae), have the potential to be effective biological control agents of wheat stem sawfly. The success of natural enemies is linked to their access to supplemental food in the field, which increases their longevity and benefits their reproduction. Therefore, we are testing in the greenhouse the cover crop cowpea, Vigna unguiculata, as an extrafloral nectar resource for parasitoids. Females of B. cephi and B. lissogaster were placed in cages over cowpea plants. Females were assessed daily for longevity, and egg load and volume were measured at 0, 5, and 10 days after placement. Our results show that extrafloral nectar increases parasitoid longevity and maintains constant egg volume, but egg load varies. Based on these results, we investigated different types of cowpea extrafloral nectar (from the stipels of trifoliolate leaves and from the inflorescence stalk). Females lived longer and produced more eggs when feeding on inflorescence stalk extrafloral nectar compared to the stipel leaf extrafloral nectar. Our results demonstrate that cowpea may be a good nutrient source for adult parasitoids, increasing their ability to reduce wheat stem sawfly populations. Cowpea extrafloral nectar is easily accessed by parasitoids and other beneficial insects, contributing to greater ecosystem services in agroecosystems.