Survey and dose-response of Ovavesicula popilliae (Microsporidia: Ovavesiculidae) as a biological control agent for Popillia japonica (Coleoptera: Scarabaeidae) in Wisconsin vineyards

Japanese beetle, Popillia japonica (Coleoptera: Scarabaeidae), is an invasive generalist insect in the United States that is an economically significant pest of many crops and is the primary defoliator of grape. Current vineyard management for P. japonica relies on broad-spectrum insecticides, while alternative methods provide insufficient population reduction. Ovavesicula popilliae infects P. japonica via ingestion and is linked to reduced overwintering survival, fecundity, and population declines in highly infected sites. The refinement of soil inoculation methods targeting P. japonica larvae is necessary, particularly regarding optimal spore density in soil inoculations and adult infection potential.

The objectives of this 2024–2025 study were to 1) survey the natural incidence of O. popilliae in 22 Wisconsin vineyards; 2) refine soil inoculation procedures through a dose-response assay in 1.25 m² field plots at five sites with high pest pressure; and 3) evaluate the potential for adult infection via a lab-based dose-response feeding assay using spore-coated grape leaves, testing infection after feeding on clean leaves five days post-treatment. Both dose-response experiments assess spore infection at three concentrations based on current research, with 20 replicates per treatment, and determine infection via qPCR analysis. Results from the 2024 survey show two Wisconsin sites with high infection rates and one with trace presence. Determining the distribution of O. popilliae in Wisconsin and optimizing inoculation procedures will advance the development of this biological control method for P. japonica and provide a foundation for future inoculation efforts in the state.