Life History 1. The life cycle requires 1 year. Japanese beetles overwinter as grubs about 3 to 6 inches below the soil surface. Grubs change to the pupal stage in May or June, and adults emerge from June to August. The adult female deposits eggs in the soil during the summer.
Grubs soon hatch and feed on roots of plants until cold weather approaches.
Then they move deeper into the soil. Next Page Rutgers Cooperative Extension is an Equal Opportunity Employer. If Plants Could Talk'' 2000 William T. Hlubik , Rutgers, The State University of New Jersey.