Garden Variety: Use caution when attempting to trap Japanese beetles

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A Japanese beetle lands on a flower.

Since Japanese beetles have finally established a significant population in northeast Kansas and northwest Missouri, many gardeners are looking for options to control the voracious pests. Traps are an increasingly popular option, but research has shown that their scented lures can sometimes bring more beetles into an area than would otherwise be there. Gardeners wishing to control Japanese beetle should use traps with caution and consider a range of options.

Japanese beetle traps have a yellow plastic piece with a cone-shaped center. The piece attaches to a hanger and holds the lure. A bag or cylinder is attached to the bottom of the cone. Beetles fly to the lure, slide down the inside of the cone and are trapped inside the bag or cylinder. There are two types of lures that attract the beetles and most traps come with both lures in one package. One lure is a floral scent and the other is a sex pheromone specific to Japanese beetles.

The lures and traps are very effective at attracting and capturing Japanese beetles, which are strong flyers.

Traps are meant to be used to establish a perimeter — to be more attractive than the plants inside that perimeter, thus protecting the plants or crop inside. In a typical residential yard, four traps placed at the corners of the yard would be far more effective than one trap placed in the center of the yard. However, some beetles will still find their way in, and your neighbors will likely be unhappy with you either way.

In rural settings or large acreage yards, traps should still be set to establish a perimeter rather than luring beetles into the orchard, garden, landscape, etc. Lincoln University in central Missouri published research in 2017 on Japanese beetle trapping after six years of research and recommends seven traps per acre deployed outside of the area where the crop or desirable plants are grown.

The next consideration is emptying traps and disposing of captured beetles. Traps with small bags or cylinders can fill in a single day in peak weeks. Otherwise, traps likely need emptying weekly. Beetles are often still alive when traps are emptied, so dumping the live ones into a bucket of soapy water is recommended. They can also be fed to chickens or fish. Dead beetles (especially in full traps in the heat of summer) have a strong unpleasant odor.

Japanese beetles emerge over the summer, typically beginning in mid-June, peaking the second week of July and tapering off through August. Traps should be checked and emptied regularly during that time.

In the Lincoln University study, researchers report that more than 15.5 million Japanese beetles were trapped and killed over the six-year period between the two farms where research was conducted. In the study, traps were modified with lidded trash cans as the collection container to decrease the frequency of emptying.

Other options

Japanese beetles were accidentally shipped into the eastern U.S. more than a century ago. They have been slow to make their way across the U.S., but they are likely here to stay. As the adults feed through the summer months, females drop to the ground and lay eggs in moist soil. They especially like grasses and irrigated soil for egg-laying, so irrigated lawns, athletic fields and sod farms are prime targets. They will also lay eggs under whatever desirable plants they are feeding on, including roses, grapes, linden trees, and other favored species.

Diligent gardeners can pick or knock Japanese beetles from plants and drop them into a jar of soapy water. For small yards or locations where beetles favor certain species of small plants, hand-picking beetles is quite effective.

Eggs hatch into grubs that feed on plant material below the soil surface. They can do significant damage to turf as they feed on roots. Controlling the grubs instead of the adults is another consideration.

Organic options for Japanese beetle grub control are Milky Spore, parasitic nematodes and organic insecticides.

Milky Spore is a naturally occurring bacterium that grubs cannot digest, so they die after consuming it. They also release more spores into the soil when they die, increasing the amount of the bacterium in the soil. Milky Spore should be applied in early- to mid-August for best results and should be watered in or applied before rain is expected to help it soak into the soil. The manufacturer recommends making three applications per year two years in a row to see effective control.

Nematodes are microscopic worms. Certain species feed on grubs in the soil and can be purchased commercially for Japanese beetle control. Nematodes typically need to be applied twice a year, every year as they are not winter hardy in Kansas.

A few organic insecticides are labeled for use as a soil drench for Japanese beetle grub control. Price of the product, value of the crop and off-target effects should all be considered before opting to use an organic insecticide.

In conventional gardens, fields and farms, growers may opt to use a synthetic insecticide as a soil drench for grub control. Again, price of the product, value of the crop and off-target effects should be considered.

Japanese beetle grubs can also be partially controlled by cultivation.

For all Japanese beetle control methods, remember that more adults will always be flying into the area, so effectiveness is difficult to gauge in open-air spaces.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.


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