Presence and distribution of alfalfa weevil (Coleoptera: Curculionidae) and broad-nosed weevils (Coleoptera: Curculionidae) in alfalfa fields grown for seed in southern Alberta

The alfalfa weevil ( Hypera postica (Gyllenhal), Curculionidae: Coleoptera) is a major defoliating pest of alfalfa ( Medicago sativa L., Fabaceae). High-density populations cause significant damage through leaf skeletonization. This study examines the location of adult alfalfa weevil in alfalfa fields grown for seed at various times throughout the growing season using different sampling techniques. Five fields with established alfalfa stands grown for seed were selected in 2021 and 2022, and eight fields in 2023. Alfalfa weevils captured in emergence cages, pitfall traps, sweep-net samples, and soil samples were collected in all study years. In 2023, the effect of alfalfa stand age on weevil abundance was tested. Sweep-net samples were more effective for capturing and monitoring adult alfalfa weevils than pitfall traps and soil samples, and also predicted subsequent larval densities. More adult alfalfa weevils were captured in emergence cages positioned outside alfalfa fields, revealing that some adult alfalfa weevils overwinter outside the field. More female than male alfalfa weevils were captured in the overwintered generation collected in the spring 2021. More overwintered alfalfa weevil adults were collected from mature than young alfalfa stands, suggesting the preference for or lack of dispersal from established alfalfa fields by adult alfalfa weevils. Weekly average temperature and relative humidity had variable effects on capture of alfalfa weevil adults in sweep-net samples. Two species of broad-nosed weevils, alfalfa curculio ( Sitona lineellus Bonsdorff) and pea leaf weevil ( Sitona lineatus L.) were identified from the by-catch of sweep-net samples.