Japanese beetles: Keeping these bejeweled bugs from devouring landscapes

Photo: Katja Schulz/FlickrPhoto: Katja Schulz/Flickr

Japanese beetles are an invasive pest that causes damage to the landscape throughout its life cycle as both a grub and an adult.

These voracious insects feed on over 300 plant species as adults, while the grubs plague lawns, parks and golf courses.

According to the United States Department of Agriculture (USDA), Japanese beetles have spread through most states east of the Mississippi River. Meanwhile, states including Arkansas, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota and Oklahoma are currently experiencing partial infestations.

Where does it come from?

As the name would suggest, the Japanese beetle is originally from Japan and the pest was first spotted in 1916 at a New Jersey nursery. Entomologists believe they entered the country as grubs in the soil on Japanese iris roots.

A lack of natural predators and an abundance of food has allowed this insect to thrive and breed prolifically.

Over time, the beetles have spread westward and a quarantine is currently being conducted by the USDA to prevent the bug from spreading further. The quarantine has not stopped the spread, but it has slowed the process. Plant material shipped with soil from Japanese beetle infested regions are required to be inspected before they can be sent to unaffected areas.

What does it look like/what are the signs?

Adult Japanese beetles are about ½ inch in length and have a scarab appearance with a shiny green head and bronze body. They can be distinguished from other beetles with similar coloration by the white tufts on their sides and a pair at the tip of the abdomen.

The adults emerge from the ground anywhere from June to early August, where they begin to feed. While they are willing to feed on numerous plant varieties, they still have a preference for certain plants, and damage will be more severe on these. Some of the favored plants include roses, birch, elms, Japanese maple, crape myrtle, linden, raspberries, and grapes.

The beetles tend to feed in groups and will work their way down from the top of a plant. They skeletonize foliage by eating the tissue between the veins, leaving a lace-like appearance. They are most active on warm, sunny days and prefer plants in sunlight.

During their adult phase of 30 to 60 days, the beetles will also mate on the host plants. Mated females will burrow into the nearby soil to deposit 40 to 60 eggs during her lifespan.

By late August, the larvae are almost full-grown and will feed on the roots of turfgrass and vegetable seedlings. They will overwinter in the soil, burying deeper when the temperatures fall. In the spring as the temperature climbs above 50 F, they will return to feed until they turn into pupae and emerge as adult beetles about two weeks later.

The larvae are C-shaped white grubs with a brown head and a grayish-black rear. Signs of their feeding on turf include a general wilting and a gradual thinning. Heavily damaged turf can be rolled back like a rug due to the connecting roots being destroyed.

Healthy turf will not appear damaged when there are fewer than 10 grubs per square foot, according to the University of Tennessee extension, but poorly maintained turf will show injury with just four or five grubs per square foot.

How can I control them?

Eradication of the beetles is not possible, but there are multiple control methods. It is important to remember that Japanese beetles can fly considerable distances, so control of one life stage will not necessarily prevent problems from the other.

When beetle numbers are low, simply picking or shaking the insects off of plants and putting them into a bucket of soapy water will work. Covering high-valued plants with a fine netting during peak activity can also help.

There are two common ways to fight the Japanese beetles if they have targeted your client’s property. One route is insecticides, while the other is cultural controls. There are numerous insecticides available to treat adult Japanese beetles including cyfluthrin, bifenthrin and carbaryl.

Choosing the correct insecticide depends on features such as what plants they can be used on, how long they persist and their threat to pollinators. It is crucial to follow the label to avoid harming bees as well.

Imidacloprid is a chemical option for controlling the larvae, but treating the larvae in the yard will have little effect on the number of Japanese beetles feeding on ornamental plants due to their ability to travel. There are biological controls that also target the grubs, but these take considerable time before any effects are seen.

Japanese beetle traps are commonly sold and touted as a solution, but multiple sources say they attract more beetles than they catch. If they are used, they should be placed 20 to 30 feet away from the plants they are protecting.

The cultural controls include habitat manipulation and the planting of resistant flora. Diseased and poorly nourished plants are more susceptible to attacks, so it’s important to keep the landscape healthy. Watering can also affect the survival chances of the beetle eggs and young larvae. If the lawn can tolerate being dry during the egg period – July and early August – many can be killed.

While the Japanese beetles will feed on many types of plants, designing a landscape with a mixture of non-preferred species will reduce the level of damage they will wreak. Some of the plants that are resistant include dogwood, red maple, magnolias, boxwood, ashes, yew, forsythia and lilac.

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