The role of chemical ecology and agroecological importance of smooth brome in biological control of the wheat stem sawfly by native parasitoids

Abstract

The wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae), poses a serious threat to cereal crops across the Northern Great Plains of North America. Effective management of WSS relies on native parasitoids Bracon cephi (Gahan) and B. lissogaster Muesebeck (Hymenoptera: Braconidae), which play a crucial role in suppressing WSS populations. Smooth brome grass (Bromus inermis Leyss.), when grown adjacent to wheat (Triticum aestivum L.) fields, serves as a potential trap crop that can both reduce WSS larvae and provide refuge for parasitoids. However, the agroecological benefits of smooth brome in providing second-generation hosts to B. cephi and B. lissogaster are not well understood. Therefore, our study explores the ecological dynamics involving WSS, smooth brome, winter wheat, emitted volatile compounds, and the associated parasitoids. We analyzed volatile organic compounds (VOCs) emitted by WSS-infested smooth brome and winter wheat using EAD-GC- FID techniques. Smooth brome under WSS infestation produced higher concentrations of (Z)-3- hexenyl acetate, 6-methyl-5-hepten-2-one, and (E)-2-hexenal. Both B. cephi and B. lissogaster were more attracted to WSS-induced volatile blends from smooth brome when given the choice against winter wheat. Field observations were conducted in central and northern Montana to assess the efficacy of smooth brome as a host refuge. We recorded WSS infestation rates and parasitoid prevalence in smooth brome and adjacent wheat fields. Infestation rates were notably high in both crop and adjacent brome fields (e.g., 64.5% in smooth brome and 65.7% in adjacent wheat in Big Sandy, Montana). Smooth brome had a maximum WSS larval survival of 5.7% and a 43.6% greater larval mortality rate compared to adjacent wheat. With similar parasitoid numbers in infested stems, smooth brome effectively reduced WSS survival and provided a sustainable habitat for crucial second-generation parasitoids. Based on our findings, we suggest the strategic conservation and maintenance of smooth brome in areas located adjacent to cultivated wheat fields to better manage populations of WSS.