Forecasting the beetle storm: Abiotic drivers of Japanese beetle phenology and abundance in Midwestern vineyards

The invasive Japanese beetle (Popillia japonica, Coleoptera: Scarabaeidae) is an expanding pest in the US, causing millions of dollars in annual damage to crops, turf, and ornamentals. In Wisconsin, adult beetles are the primary defoliator of grapes. Larvae develop in the soil feeding on turf grass roots, and their distribution, development timing, and influence on adult populations in vineyards remain difficult to predict. Adult beetles are highly mobile and fly several miles in search of suitable food and abiotic conditions, further complicating population forecasts. Accurate predictions of both larval and adult populations are essential for improving pest managements strategies of this important pest. Several abiotic factors – including temperature, moisture, and surrounding landscape characteristics – may influence beetle phenology and abundance. While previous models have successfully predicted Japanese beetle phenology with air temperature, they often overlook soil characteristics which more directly reflect larval conditions and may better predict phenology than air characteristics alone. This study tracks soil temperature, soil moisture, and air temperature in vineyards across Wisconsin and one site in Minnesota to predict adult emergence timing in the spring. Additionally, although research has shown that landscape composition influences adult beetle distribution, it did not distinguish between cropping types. Crops such as soybean are known to attract adults and cause population spillover into nearby areas. This study incorporates fine-scale landscape distinctions to better predict adult population abundance and distribution throughout the summer. These findings enhance our understanding of Japanese beetle ecology and support more targeted pest management in vineyard systems.