The causes of wheat stem sawfly (Hymenoptera: Cephidae) larval mortality in the Canadian prairies

Abstract

Cephus cinctus Norton (Hymenoptera: Cephidae), the wheat stem sawfly, is a well-established and important pest of wheat, Triticum aestivum Linnaeus (Poaceae), and its relatives in North America. Crop losses are caused directly by C. cinctus feeding inside wheat stems during larval development and indirectly when weakened plants lodge before being harvested. Understanding the factors that affect population dynamics of C. cinctus can help farmers to better manage it. Our study therefore explored how C. cinctus and natural enemy densities vary in space (southern Alberta, Canada) and over time. Five fields were sampled using an established protocol in fall 2019 and resampled in spring 2020; six additional fields were sampled in fall 2020 and resampled in spring 2021. Wheat stubs were dissected to record numbers of cut stems, C. cinctus larvae, and sources of larval mortality (i.e., parasitism, fungal infection). Densities of wheat stem sawfly and the impact of natural enemies varied between the sampled fields. No C. cinctus mortality was observed during the winter, indicating that C. cinctus population dynamics are not susceptible to mortality (abiotic or biotic) between years. Results of our study will be incorporated into new models to predict wheat stem sawfly phenology and risk to crop production.