What Are These Black Beetles Taking Over my Bathroom? Back »

Written collaboratively by Amanda Bachman, Patrick Wagner, Adam Varenhorst, and John Ball.

A common call this time of year is about small, black insects usually crawling up bathroom walls, hanging out in basements, or showing up in other parts of the house. These insects usually turn out to be either root weevils (Figure 1) or the imported longhorned weevil (Figure 2).

Weevils (Family Curculionidae) are a kind of beetle (Order Coleoptera) that is characterized by a distinct snout on the front of their head with antennae originating halfway down the snout. Root weevils are members of the genus Otiorhynchus, and there are at least four species in South Dakota; the strawberry root weevil (Otiorhynchus ovatus), the rough strawberry root weevil (O. rugostriatus), the lilac root weevil (O. meridionalis) and the black vine weevil (O. sulcatus). The imported longhorned weevil (Calomycterus setarius) is similar in size to root weevils, but is a mottled gray color instead of black. Adult root weevils and the imported longhorned weevil are approximately 1/3 of an inch long. The adults of these weevils do not fly, and will sometimes play dead if disturbed (e.g., roll on their backs, fold up their legs, and not move for several seconds). Identification to species is not necessary for management since they are all similar in behavior.

Black beetle on a white background.
Figure 1. Adult root weevil. Photo: Amanda Bachmann

Speckled grey beetle on a green leaf.
Figure 2. Adult imported longhorned weevil. Photo: Adam Varenhorst

Adult root weevils feed on many plants in the landscape, but appear to prefer lilac leaves and yews. Both of these plants are abundant in residential landscapes. Their larvae feed on the roots of a variety of plants. The mature larvae overwinter, pupate in the spring, and new adults emerge around June. Imported longhorned weevil adults also feed on a variety of plants, but are noted for their presence in soybean.

These insects are considered nuisance pests indoors, and their presence could be an indication of a larger outdoor population. To determine if plants around the home are being fed on, examine them for “saw-toothed” leaf edges, which is characteristic feeding injury by these weevils.

Management for root weevils or imported longhorned weevils that find their way indoors is the same regardless of species. Since they do not feed or reproduce inside structures, the best course of action is to sweep or vacuum them up. To prevent future invasions by the root weevils (or other insects seeking shelter inside) check around doors and windows for any possible entry points. Perimeter insecticide sprays will reduce the occurrence of these invaders as fall approaches. If using an insecticide, always read and follow the label directions.

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