Japanese Beetles Life Cycle

Amazingly this year, we've not had many Japanese Beetles. We've had a much longer Spring here in Virginia and a much cooler summer with nighttime temps being in the high 50's at times and almost always in the 60's. Until the end of July we didn't really have any high temps in the afternoons.

Japanese Beetles love hot weather. They're most active in the full sun of afternoons. That's why you can effectively control them by picking them off the plants in the morning hours before it warms up. Not a fun thing to do, tossing them into a bucket of water. I don't like taking any life away, so my method of dealing with them is to use a natural spray from Gardens Alive that repels them.

Regardless, this year there have been few of them and our Rose of Sharon blooms have been gorgeous. As you can see from the photo, the bees are very happy.

The University of Wisconsin offers this information on Japanese Beetle life cycle which helped to explain why we had so few:

Japanese beetles have only one generation per year. In mid-June, as soon as they emerge, adult females mate and begin laying eggs. The adults are most active in the afternoon in full-sun.

Females leave ornamental plants where they feed and mate, and burrow two to four inches into the soil (under the turf) in a suitable area to lay their eggs. Eggs hatch in about two weeks, after which grubs begin feeding on the roots of turfgrass.

The grubs grow quickly and by late-August are nearly full-grown (about one inch long). Mid-summer rainfall and adequate soil moisture are needed to prevent eggs and newly-hatched grubs from drying-out. Adult females instinctively select areas with higher soil moisture content to lay their eggs to ensure survival of their offspring.

Older grubs are more drought tolerant and will move deeper into the soil if conditions become dry. Grubs can also withstand high levels of soil moisture, so excessive rainfall or irrigation will not effect them. As soil temperatures cool in the fall, and the first meaningful frost occurs, grubs begin to move deeper into the soil. Grubs overwinter in the soil about two to six inches below the surface, although some may be a deep as 20 inches.

They become inactive when soil temperatures fall below 50°F. In the spring, when soil temperatures reach 50°F, the grubs begin to move up into the root-zone to resume feeding for about three to five weeks. Thereafter, the grubs stop feeding and begin creating an earthen cell whereby they transform (i.e., pupate) into adults.