Japanese Beetle Activity on the Rise

Adult Japanese beetles burrowing into a golf course putting green.

The number of adult Japanese beetles has exploded the last couple of weeks. Once confined to the northeastern region of the country, this destructive insect has become a permanent part of the Midwestern landscape. This pest has continued its trek west across the country since being introduced into the U.S. from the Orient. Japanese beeltes were first reported in Iowa in 1994 and have gone on to inhabit approximately half of the counties in the state.

The Japanese beetle is one of the white grubs that includes the May and June beetle, masked chafer, green June beetle, European chafer, Asiatic garden beetle, Oriental beetle, and black turfgrass ataenius. The adult beetles can be identified by their green and bronze metallic head and shell and by the white tufts of hair that run along their abdomen. The larval stage must be identified by their raster pattern.

As with all white grubs, the Japanese Beetle larvae feed on the roots of grass plants just below the soil surface. Injury first often appears as drought that fails to respond favorably to irrigation. Each year the adult beetles emerge from the soil and begin mating and laying their eggs. This is the period we are currently experiencing. The eggs hatch in 2-3 weeks and the larvae begin feeding. Feeding can continue through the fall up until the first frost. Injury can also occur in the spring but is usually less severe due to the vigorous growth of cool-season grasses.

This particular grub species is somewhat unique in that the adult beetles also are significant pests of a wide range of ornamental plants. Japanese beetles feeding on leaf tissue leave a skeleton framework of veins following damage. Damage typically occurs at the top of the tree and works downward. Below is a picture of a Linden tree that is under attack from Japanese beetles.

Adult Japanese beetles feeding on a Linden tree.  The beetles usually start feeding at the top of the tree and work their way down.

Monitoring for white grubs can give you an indication of the severity of damage you may be able to expect. Sites with heavy beetle infestation in the summer months are likely incur grub damage during the fall months. Also, be sure to watch areas that have been damaged in the past as grubs often reinfest the same areas.

There are a number of insecticides on the market that are effective at controlling white grub species. The key to effective control is proper timing and placement of the products. Products applied preventatively or curatively are more effective against the grubs when they are small.

Regardless of the timing of the application, it is essential that the product be effectively watered in. Using nozzles that produce larger droplet sizes will help place the product further down in the canopy. Irrigation is normally recommended to help move the product down below the thatch layer and into the soil where the product will be most effective.

Based on the number of adults we are seeing we could be set for significant damage this fall. Monitoring for adult beetle activity is a great tool for those of who haven’t treated and are trying to decide what to do. The margin for error appears as if it will be quite slim as the turf will already be experience summer decline as we move through a week with high environmental stress.

Let us know what you’re seeing out there.

Marcus Jones
Assistant Scientist