Increasing prevalence of Ovavesicula popilliae (Microsporidia), a pathogen of Japanese beetle (Coleoptera: Scarabaeidae), in the Midwest and Northcentral United States from 1999 to 2025.

Distribution of Ovavesicula popilliae Andreadis & Hanula (Microsporidia) in the United States was updated by sampling 232 new sites, with a focus on prevalence in the Northcentral and Midwest regions. Results were added to 189 previously sampled sites to detect infections of Japanese beetle (Popillia japonica Newman, Scarabaeidae) by O. popilliae. Following site inoculations, prevalence of O. popilliae at irrigated golf courses and parks in regions with a dry summer climate increased from 0% to 4.0% over 7 yrs, but establishment was poor at nonirrigated airports and parks. The pattern of O. popilliae establishment in Minnesota, Michigan, Colorado, Iowa, and Wisconsin were similar: O. popilliae was first found in densely populated or frequently visited areas, dispersing from there to some but not all sites within 20 k over the next 10 to 20 yrs. Dispersal of O. popilliae beyond 20 k was sporadic, and most likely due to the movement of infected beetles in sod or nursery stock. The prevalence of positive sites in 4 Northcentral and Midwest states increased from 11.9% in the first survey conducted in each state, to 70.9% in a second survey, 4 to 13 yrs later, although infection rates were mostly below 10% and still increasing at the time of the second survey. In conclusion, we recommend that golf courses, vineyards, small fruit farms and hops farms introduce O. popilliae as soon as possible after Japanese beetles are found to minimize the delay in biological control of Japanese beetle by O. popilliae in concert with other natural enemies.