Supplemental nutrition may improve the effectiveness of wheat stem sawfly parasitoids: Observations on longevity, egg load and egg volume

The success of natural enemies is often linked to accessing supplemental food in the field, consequently contributing to reduced pest numbers. The wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae), a major pest of wheat, is suppressed by two native parasitoids, Bracon cephi (Gahan) and B. lissogaster Muesebeck (Hymenoptera: Braconidae), with the immatures feeding on and killing WSS larvae. To assess the benefits of supplemental nutrition on B. cephi and B. lissogaster, we reared adult females on carbohydrate-rich diets combined with amino acids or uncombined, with water and amino acid controls. We also conducted greenhouse experiments with cultivars of the cover crop cowpea, Vigna unguiculata L. Walpers, as extrafloral nectar sources that vary in amino acid and carbohydrate concentrations. Cages containing individual females were placed on living plants enclosing either cowpea inflorescence stalk extrafloral nectar (IS-EFN) or leaf stipel extrafloral nectar (LS-EFN), with water and buckwheat floral nectar as negative and positive controls. In both experiments, females were observed daily to assess longevity, while egg load and volume were assessed 2, 5, and 10 days after placement. Our results show that parasitoids increased longevity when provided carbohydrate-rich food. B. cephi enhanced egg load and volume with carbohydrate-rich diets, and the addition of amino acids tending to further benefit these parameters. IS-EFN increased longevity in both parasitoid species. B. lissogaster had constant egg load and volume, while B. cephi increased egg parameters when accessing IS-EFN. Therefore, supplemental nutrition benefits B. cephi and B. lissogaster female longevity and reproductive parameters.