Japanese beetles are one of the prettiest garden pests ever. At almost half an inch long, with a metallic green body, coppery wings and flashy white tufts along the abdomen, they’re easily identified.
But if you have a heavy infestation, you don’t need to see them to know that something bad is happening in your garden.
In severe years, Japanese beetles will defoliate trees, eat the flowers off roses and skeletonize grape and linden leaves, creating havoc for gardeners.
Earlier this month, “they’re baaack” reports from around the state lit up the Minnesota Master Gardener e-mail list. While gardeners from Edina to Dakota County to Chanhassen reported large numbers of Japanese beetles, other gardeners had seen just one or two.
The U.S. Department of Agriculture calls Japanese beetles the most destructive pest of ornamental plants and turf in the eastern U.S., with more than $450 million spent each year to combat the bug and replace damaged plants. With no real natural enemies in this country, the beetles have steadily marched west across the nation.
In Minnesota we’ve been dealing with the beetle for about 20 years, but people on the East Coast have known this pest for almost a century. While anxious gardeners will find plenty of heavy-duty chemical options and anti-beetle gadgets at garden stores, over the years, research on controlling Japanese beetles has yielded control techniques that are kinder to the environment and to bees, which can be killed by the same chemicals that knock off beetles.
One of the most satisfying organic ways for gardeners to seek revenge on their colorful enemy is to walk around the garden early in the morning when the beetles are drowsy and inactive. Holding a bucket of soapy water, shake branches so beetles fall, or flick individual beetles into the bucket, where they will drown.
Grapevines are a favorite of Japanese beetles. Shaking the vines first thing in the morning and stomping on the fallen beetles is an invigorating way to start the day.
While removing beetles by hand sounds hopeless when an infestation is big, Japanese beetles like to feed in groups; removing them from plants actually makes that plant less attractive to other beetles.
Annual geraniums are another favorite of the beetles, and some gardeners have planted pots of geraniums as bait plants to pull beetles away from more valued plants. Research indicates that once beetles feast on geraniums, they keel over and are paralyzed for several hours — another opportunity to give your shoes a workout. Birds seem to find the bugs tasty and will often prey on downed beetles.
Though many garden stores offer Japanese beetle traps, the University of Minnesota Extension Service is no longer recommending them. The traps use pheronomes and the scent of geraniums and roses to attract the beetles. But according to the U, they also lure beetles from several thousand feet around your yard, and those beetles may feed on your plants.
Some gardeners position the traps above basins of soapy water, so the beetles enter the trap, drop into the basin and die.
Plants are surprisingly resilient, and even trees that suffer serious defoliation will bounce back if they are not attacked by large numbers of Japanese beetles year after year. But if serious infestations occur season after season, trees and plants may need to be treated lest they become so weakened that they die.
When to go with a pro
It’s worth hiring a professional to treat trees that have been repeatedly attacked. Foliar applications (treating the leaves) will kill beetles for two to three weeks. Systemic chemicals can be injected into trees for longer beetle control, but when used on flowering trees like lindens, which are favorites of both Japanese beetles and pollinators, bees could be killed, too.
The beetles overwinter as grubs under lawns. While insecticides can control grubs, treatments are not very effective unless an entire neighborhood is treated, because beetles can fly into your yard from surrounding properties.
Information about Japanese beetles and control options is available at the U of M Extension website: www.extension.umn.edu/garden/insects/find/japanese-beetles.
While Minnesota gardeners were in a bit of a frenzy about Japanese beetles when the insect was new to the state in the 1990s, beetle populations are cyclical. Some years are bad, others not so much. In most years, gardeners can take a live-and-let-live approach to the bug.
Here’s a link to an interesting page about possible natural enemies for the beetles from the University of Maryland Extension Service: extension.umd.edu/ipm/japanese-beetles-and-natural-enemies.
While Japanese beetles are invaders that aren’t a natural target of predators in this country, all of nature is intertwined. It’s another reminder to be careful about chemical use in your yard, to avoid hurting beneficial bugs.
Adult Japanese beetles live about 30 to 45 days, so in a few weeks we’ll be rid of them for the summer. For me, a walk around the yard with a bucket of soapy water each day is all the control I need.
But if they’re a real and continuing pest in your yard, consider selecting landscape plants that aren’t targeted by beetles. Arborvitae, lilac, burning bush, oaks and pines are on the “seldom damaged” list on that U of M beetle fact sheet. Check out that website for a complete list of favored and non-favored plants.
Mary Jane Smetanka is a Master Gardener and Minneapolis-based freelance writer.