D3391: Do host plant resistance and parasitism regulate wheat stem sawfly damage in winter wheat?

The wheat stem sawfly (WSS), Cephus cinctus Norton, is a key pest of wheat on the High Plains, affecting yield through larval stem boring that interrupts plant vascular systems and induces tiller lodging via pupal chamber formation. However, the role of Bracon spp. (a WSS parasitoid) in regulating WSS impacts on wheat yield is poorly understood. In 2024, we assessed the impact of WSS infestation on wheat head mass in winter wheat varieties and measured the effectiveness of Bracon spp. in protecting head mass. We examined twelve hard red winter wheat varieties according to their stem solidness trait attributes encompassing hollow, semi-solid, and solid-stem varieties using a randomized complete block design with four replications for three years. At harvest, kernel count and head mass per wheat tiller were evaluated. Thirty-five stem samples per plot were split to examine WSS larval presence and morbidity. WSS infestation levels varied significantly among different wheat varieties and the Bracon spp. population demonstrated a consistent increase from previous years. We measured no significant difference in injury caused by WSS based on stem solidness traits of varieties. WSS negatively impacted head weight; however, the presence of Bracon spp. mitigate this loss by effectively reducing the WSS injury by paralyzing the larvae and thus preventing head weight loss. By mitigating yield loss, the parasitoid preserved 95% of head weight, resulting in a financial loss of only $50.00 per hectare, compared to a loss of $479.00 per hectare when parasitoids were absent.