Japanese beetles swarm Wisconsin yards as feeding peaks

Japanese beetles have entered their peak feeding phase across north‑central Wisconsin, damaging a wide range of garden and yard plants. Populations have risen over the past five‑to‑seven years, especially in wet soils.

Krista Hamilton, an entomologist with the Wisconsin Department of Agriculture, Trade and Consumer Protection, said the state is focusing on a biological control agent known as the “winsome fly.” The fly, similar to a house fly, lays eggs inside beetles; the hatching larvae kill the host within five to ten days. The flies, first introduced in Minnesota two decades ago, are now being recorded in Marathon County and are expected to spread further.

Beetle numbers have already fallen in southern Wisconsin, and a comparable decline is anticipated northward. Homeowners can manually remove beetles, use soapy water to kill them, and install barrier netting on vulnerable plants from late June through September. While no quick fix exists, natural enemies like the winsome fly should gradually reduce beetle populations over several years.