Economic thresholds for <i>Hypera nigrirostris</i> (Coleoptera: Curculionidae) in red clover seed production from on-farm trials

Red clover, Trifolium pratense L., is a perennial forage legume grown for seed production in Western Canada, contributing significantly to regional agricultural economies. The lesser clover leaf weevil, Hypera nigrirostris Fab. (Coleoptera: Curculionidae), is a key pest of red clover seed crops, with larval feeding capable of reducing seed yields by up to 50%. Despite its impact, no economic thresholds (ET) currently exist to guide insecticide application for this pest. As a result, prophylactic treatments are used that increase production costs, increase the potential for insecticide resistance, and pose risks to pollinators, which are essential for red clover seed production. This study established a quantitative relationship between larval density and red clover seed yield under field conditions and used these data to develop economic injury levels (EIL) and practical ETs across a range of seed price scenarios. Yield losses were strongly associated with increasing larval densities, and thresholds varied with changing economic conditions. These resulting thresholds provide the first data-driven guidance for managing H. nigrirostris in North American red clover seed systems, offering a foundation for cost-effective, pollinator-conscious pest management strategies.