Japanese Beetles - What Do I Do Now That My Plants Looks Like Swiss Cheese?

Japanese Beetles - What Do I Do Now That My Plants Looks Like Swiss Cheese?

Japanese Beetles, Acreage Insights July 2017, http://acreage.unl.edu
Japanese beetles feeding on rose flowers at Lauritzen Gardens in Omaha. Photo by Jody Green, Nebraska Extension Entomologist.
Japanese Beetles, Acreage Insights July 2017, http://acreage.unl.eduSevere damage to an aspen tree from Japanese beetle feeding. Photo by Keith Glewen, Nebraska Extension.

Many gardeners are seeing severe damage from feeding by Japanese beetle adults in their landscapes. Hardest hit include lindens, fruit trees - especially apple and pear - and roses.  Many homeowner didn't realize these insects were present until their linden tree started turning brown or their rose plants develop holes in the leaves. But after looking closely, many found clusters of beetles congregating on their plants. 

Large numbers congregate on favored plants, due to a pheromone the adult beetles release when they find a particularly yummy plant, such as rose, linden, grape and many other ornamentals. They also feed on soybeans and sweet corn silk, so can be a problem in landscapes or vegetable gardens near farm fields.

Where Did They Come From?
Japanese beetles have been in the United States for about 100 years. They are native to Japan and were first reported in New Jersey in 1916. Since they have no natural predators or diseases in North America, they have gradually spread and now have an established range from the east coast to Nebraska, north to Canada and south to Alabama.

Japanese beetles were first reported on Nebraska field crops in July 2011. In 2013, a statewide Japanese beetle survey by the Nebraska Department of Agriculture declared 19 counties “infested”, including Saunders, Lancaster, Cass, Seward and Saline counties. They have continued to moved westward; 29 counties now have confirmed infestations. 2016 Distribution Map

Japanese beetle adults congregating on a rose flower. Image by Jody Green, Nebraska Extension Entomologist. 

What Do They Look Like?
Both adult and immature Japanese beetles are damaging to landscapes. The adult beetles are shaped similar to our common June beetles, but a little smaller - about ½ inch long. Their head and thorax is metallic green, their wing covers are copper brown. But you have to look closely sometimes to see these colors. At first glance they often just appear dark brown. They also have a series of five white tufts of hairs on both sides of their abdomen, distinguishing them from a native insect called the false Japanese beetle or spring rose beetle.

Immature Japanese beetles are similar to white grubs. They are creamy white colored with a brown head and three pairs of legs on the front part of their body. As immature insects, the white grubs feed on plant roots and are a frequent lawn pest. Root feeding causes browning and death of the grass.

Adults feed on over 300 common plants in our landscapes, including Japanese and Norway maple, hollyhock, Rose-of-Sharon, birch, cherry, plum, peach, rose, American elm, American linden, as well as marigolds, grape, Virginia creeper and Boston ivy. Adults feed during the day, preferring hot weather and plants in full sun. They have chewing mouthparts and consume tissue between the leaf veins, leaving behind a lacy or skeletonized leaf that quickly turns brown.

Lifecycle

There is one generation of Japanese beetles each year. Adults lay eggs in the soil for about a 4 to 6 week period beginning in late June. These eggs hatch out in the summer and begin feeding, so most turf damage happens in late summer. Turf products for the control of our common white grub also provides control of Japanese beetle grubs and should be applied from mid to late June. If extensive damage occurs and grub control products were not applied earlier in the summer, then Dylox works well in late summer as a rescue treatment.

Japanese beetle white grubs go through three instars during the summer and overwinter in the last stage as larvae in the soil. They pupate the following spring and emerge as adults in late June the following year.

How Can I Protect My Plants?
Adult feeding right now is high and causing significant damage. Do not use the Japanese beetle traps found at some hardware stores. They tend to attract more insects to your landscape than they trap, resulting in heavier damage on your plants!

If only one or two plants in your landscape are affected, then handpicking beetles and dropping them into a can of soapy water is an option.

The organic pesticides Neem and Pyola will give some control of the adult beetles, but only for about 3-7 days. Other insecticides, including carbaryl, bifenthrin, cyfluthrin, permethrin and lambda-cyfluthrin provide about 2 weeks of control following a thorough application.

As always when using pesticides, read and follow all label directions including the use of personal protective equipment.

Keep in mind that trees and shrubs tolerate damage with little significant impact on the plant's health, even severe defoliation, if it only occurs a few times. Repeated defoliation several years in a row is a greater concern for long term health of the plant. 

Long Term Management
If Japanese beetle damage becomes an annual event in your landscape, consider swapping out the plants they feast on for less preferred ornamentals. Take a look at the publication Japanese Beetles in the Urban Landscape from Purdue University Extension for a list of resistant plants.

Sarah Browning
Sarah Browning
Extension Educator, Horticulture & Urban Agriculture
Sarah Browning has been an Extension Educator with the University of Nebraska-Lincoln for seventeen years. Sarah's programming has focused on environmental horticulture, fruit & vegetable production and food safety. Working with the general public and commercial green industry professionals, her major program goals include conserving water, protecting water quality, promoting local food production and protecting human health. Sarah has her Bachelor of Science in Horticulture, and Master of Science in Plant Breeding from the University of Nebraska, Lincoln.

Lancaster County Extension
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