The Bane of June: Japanese Beetles

By Patricia Lunn Adsit, UGA Master Gardener Extension Volunteer with Walton County

CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=487963

Usually June is a month that is associated with jubilation such as the first day of summer, a bounty of blooms, and a ripe tomato from the home garden. However, Georgia gardeners dread the pests that arrive every year around this time: Japanese beetles. In the past week, I have found them on several of my roses and on two pepper plants in our vegetable garden. If you don’t already have them on your property, you soon shall. 

Japanese beetles are an invasive species from Asia that were first found in the USA in a shipment of irises to New Jersey in 1916. With so few natural enemies to keep them in check, they spread rapidly to the rest of the Eastern states and to Southern Canada and are well-established across the Mississippi River into Missouri. A century later they are considered a major pest of agricultural crops, home gardens, lawns, and ornamental plants. 

Each year they emerge mid-June from their larval state of greyish-white grubs in the ground and begin their feeding frenzy on the flowers, fruit, and leaves of the such plants as grapes, peach, rose, cherry, soybeans, hibiscus, crape myrtles, dahlia, and zinnia. Unfortunately that list is not exclusive, as they will reportedly feed on 300-400 varieties of plants. Damage will look like holes chewed in the leaves, where the tissue has been eaten between the leaf veins in a process referred to as “skeletonization.”

Extension offices and Master Gardeners in the area of infestation always field questions from June through August about Japanese beetle damage and how to control these bad bugs. And the question heard most often is usually: “what about Japanese beetle traps?” Let’s take those three things in turn.

We typically respond to questions about damage with reassurance that most affected plants that are generally healthy and well-established will eventually recover once adult Japanese beetles have left the area. With that knowledge, many gardeners can raise their threshold of response.

Control of Japanese beetles is rarely easy, but the best time to begin is with the grubs. There are species of parasitic wasps that attack Japanese beetles in the grub and adult stages, but they are not a consistent response. There are also nematodes and soil pathogens that attack the grubs in the soil. One such soil bacterium known as Milky Spore, Paenibacillus popilliae, causes a fatal disease for the grubs, but can take several years to build to lethal levels in your soil. For both nematode and Milky Spore applications, you must manage your expectations. Remember that reduced numbers of adult Japanese beetles emerging in ‘your’ yard does not prevent them from flying in from neighboring areas that are untreated.

If you have a low threshold to pest damage, there are insecticides available. Because Japanese beetles have hard exoskeletons and chewing mouthparts and must ingest the poison for any result, contact sprays have little effect; you must treat the leaves of the plant that will be eaten. Choose insecticides labeled for use against Japanese beetles and for application on the host plant, referring to the most current version of the UGA Extension Special Bulletin 48 • Georgia Pest Management Handbook. Please remember that insecticides kill insects, both pests and pollinators alike. While there are several products labeled for control of Japanese beetle, you might wish to use organic solutions with active ingredients such as neem or spinosad. As always, follow all directions, particularly safety precautions on the insecticide label.

Personally, I carry a repurposed coffee canister filled with soapy water. When I scout my gardens and see a Japanese beetle (or three, since they tend to congregate) on a plant, I shake the plant to dislodge the beetles into that container of soapy water. I hate to admit how satisfying it feels.

And those Japanese beetle traps? As the Georgia Pest Management Handbook states: “(t)here is no clear evidence that using Japanese beetle traps will reduce Japanese beetle damage in your landscape.” Japanese beetle traps are likely much more effective in attracting beetles than trapping them, meaning you may have more beetles in your yard than before you used them. 

Want to learn more about this pest? The UGA Cooperative Extension Circular 1167Japanese Beetles in the Nursery and Landscape is available online.

Have home gardening questions? Need a soil test kit? Your Walton County Extension office can help! The Master Gardener Help Desk is staffed on Tuesdays and Thursdays (1:00-4:00) through August. You can also email us at waltonmg@uga.edu or call 770-267-1925.

Check out the newAsk a Master Gardener” pages on the Walton County Extension website: 

https://extension.uga.edu/county-offices/walton/master-gardeners/Ask_MGEV.html 

Patricia Lunn Adsit is a UGA Master Gardener Extension Volunteer in Walton County, as well as a member of the Garden Writers Association (GardenComm). She lives and gardens in Loganville, GA

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