Japanese Beetle

These imported beetles originated in Japan (hence the name), where they were only minor pests in that environment. Unfortunately, in the US, they are one of the most destructive pests of lawns, gardens, and ornamental plants that gardeners deal with. These beetles have a one-year life cycle, with the adults hatching in May and laying eggs in July. Adults are less than ½” long, with a metallic green head and a bronze-colored body. The larvae or grub form is white and shaped like a “C.” The grubs feed on the roots of lawn grasses underground during the fall before burrowing deeper in the soil to overwinter. They eventually pupate in late spring before hatching into adults. The adults are equal opportunity feeders but prefer roses, maple trees, grapes, elms, and crabapples.
Most healthy plants can withstand a small number of Japanese beetles, so general watering and fertilizing will help reduce the impact. However, younger plants or vegetable crops can be quickly desiccated if numbers become too high (Like the grapevine in the picture below, Photo courtesy of Nancy Gordon). If you have a small number of beetles on your plants, you can pick them off and throw them into a bucket of soapy water. Larger populations of beetles may require some chemical treatment. Neem Oil is an organic option; however, it only discourages the beetles from eating the plant and needs to reapply frequently. Spinosad is another organic option that kills the adults but again needs to be reapplied frequently. Products containing permethrin, pyrethrin, or carbaryl will all provide a couple of weeks of control but must be reapplied. Any spraying should be done early in the morning or late in the evening to prevent harm to pollinators. Traps are not recommended as they attract more insects than enter the traps and can worsen feeding in the area rather than help control the issue.

Grubs are a significant issue in some lawns. Most grasses can tolerate up to 10 grubs per square foot. However, any number above that will cause dieback and browning of the grass. In heavily infested lawns, the grass can be peeled up like a carpet due to grub feeding or torn up by wildlife searching for the grubs. Control of the grubs should begin soon after they start to hatch in the late summer. Preventative herbicides can be applied up to four weeks before the eggs hatch, and various chemical options are available. A biological option called Milky Spore Disease has shown limited effectiveness against high populations and can take a while to establish. Still, it only impacts Japanese Beetles and is an organic option. For more information, check out this publication on Japanese Beetles.

 

Have questions? Contact our office where our Horticulture Extension Agent will assist you with questions.

Phone: (316) 321-9660

Email: callae@ksu.edu