Can conservation biocontrol of wheat stem sawfly be improved? Contributions of supplemental nutrition to longevity, egg load, and egg volume of Bracon cephi and B. lissogaster

Abstract

Wheat stem sawfly (WSS), Cephus cinctus, is a major pest of wheat, causing losses that exceed $350 million annually across the Northern Great Plains. Two native parasitoids, Bracon cephi and B. lissogaster, suppress C. cinctus populations in wheat fields, where the immatures feed on and kill C. cinctus larvae. The success of natural enemies is linked to access to supplemental food in the field, which increases their life span and benefits their reproductive parameters. To assess the benefits that supplemental nutrition might have on B. cephi and B. lissogaster, we reared adult females on carbohydrate-rich diets combined with and without amino acids, with water and water plus amino acids as controls. We also conducted greenhouse experiments with the non-native, warm-season cover crop cowpea, Vigna unguiculata, as an extrafloral nectar source for parasitoids. Cages containing individual females were placed on living plants enclosing either cowpea inflorescence stalk extrafloral nectar (IS-EFN) or the leaf stipel extrafloral nectar (LS-EFN), with a supply of water as the negative control and buckwheat nectar as the positive control. In both lab and greenhouse experiments, females were observed daily to assess longevity, while egg load and volume were assessed 2, 5, and 10 days after placement. Results show that both species 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 benefit these parameters. Results show that IS-EFN increases longevity in both parasitoids. B. lissogaster kept constant egg load and volume, while B. cephi increased reproductive parameters with IS-EFN. These native braconid parasitoids are unfamiliar with non- native cowpea plants. Hence, we tested their attraction to cowpea volatiles, where positive responses were observed. Our results show that supplemental nutrition greatly benefits B. cephi and B. lissogaster females, with cowpea IS-EFN improving their longevity and reproductive parameters. Therefore, we suggest that cowpea has potential to be considered as a food supplement crop in conservation biocontrol programs to reduce WSS populations.