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Avid Gardener: The Japanese Beetle — A Summer Scourge

by and on July 29, 2020 3:38 PM

“In summer the empire

of insects spreads.” — Adam Zagajewski

This is the time of year when gardeners become very concerned about insect populations attacking their plants.The online sites are teeming with people bemoaning mites, hoppers, loopers, worms, gnats, ants, borers, miners, maggots and weevils destroying their hard work of planting and maintaining yards and gardens. Often they are desperate for answers.

It does feel like a war at times, with a desire to run for the spray can at the first sign of chewing, wilting, spots, eggs or webs.

However, the best way to vanquish any foe is to identify it and know how it operates. It’s also to take a deep breath and remember that the large majority of insects are beneficial and help keep each other in check when we do absolutely nothing.

Take the Japanese beetle (Popillia japonica Newman), which always makes it into the top five most abhorred insects in July. I think everyone is in agreement that it is destructive.

This year the infestation seems particularly bad with it feeding, as usual, on the foliage and fruit of several hundred species of fruit trees, ornamental trees, shrubs, vines and field and vegetable crops, leaving skeletonized leaves and large holes behind.

Was this pest always in the United States? Actually, the beetles were not discovered in this country until 1916 in a plant nursery near Riverton, New Jersey. It is thought that the larvae arrived in a Japanese shipment of iris bulbs before inspections of imports was the norm.

Without natural predators, such as it has in Japan, the pest spread to at least 22 states by 1972. By 2015 there were partial infestations in many of the midwestern states, too.

The adult beetles that are feeding now in our yards could be considered to be somewhat attractive. They are about ½-inch long and ¼-inch wide, with metallic green and coppery wing covers. If they are disturbed, they tuck their legs and drop off of plants, poking up their back legs to ward off threats. Odors, known as pheromones, that come from the other adults as well as any beetle-damaged leaves, attract more beetles to feed on the same plant. Aside from feeding, they also mate. At sunset, once the pheromone is no longer produced, the females fly to the grass to lay eggs several inches below the surface.

These eggs mature into larvae (grubs) which are C-shaped, plump and grayish white with light-brown heads. They feed on the roots of grass until cold weather eventually drives them further underground. There they overwinter below the frost line till spring.

Once the warmer weather arrives, they again feed on grass roots until they mature and pupate about one to three feet below the surface during May or early June. They later emerge as adult beetles to begin the cycle once again. In the meantime they have worked to destroy turf in lawns, parks, golf courses, and pastures. According to the United States Department of Agriculture, losses to turf grass from the larval stage alone have been estimated at $234 million a year.

What’s a lonely homeowner to do?

The United States Department of Agriculture advocates a program called Integrated Pest Management. This uses a combination of biological, cultural, mechanical and chemical controls to “keep pest populations below levels that cause economic damage.” Its steps are to survey the problem, figure out its severity and select control methods, apply them and evaluate success. The idea is to start at the least toxic solution and try it before moving on to something more potentially harmful.

To survey for populations is the first two-pronged step.

First, it means to use chemical lures to assess adult beetle numbers in an area. Second, it is to survey the lawn for their grubs. This is done by calculating the number of Japanese beetle grubs per square foot in the lawn at specific times during the year.

Once the severity of the infestation has been decided, control methods can include mechanical traps set at far boundaries of properties away from garden plants (be aware that they will attract more beetles to an area), choosing resistant plants, keeping trees and plants healthy, cleaning up around plants to keep from attracting the insects, maintaining turf grass, employing biological methods such as parasites, nematodes or fungi, or — as more of a final resort — using chemical controls. Pesticides are toxic materials and can pose serious hazards to people, wildlife, and the water supply. They are not to be chosen lightly and need to be applied following directions exactly.

For more specific information about the USDA’s Integrated Pest Management and how to implement it, the following link will help. It is their booklet called “Managing the Japanese Beetle: A Homeowner’s Handbook” at I especially like the “Best and Worst Plants to Have in Your Yard” section.

To be honest, I have found that the least harmful way to curtail Japanese beetle populations in my own yard is to walk it a few times a day with a jar of soapy water, picking the insects off and plopping them in. It’s oddly satisfying. (Note that they are more sluggish in early morning or evening.)

The good news in this is that though the Japanese beetle damage may look very unsightly, it rarely seriously harms the health of the attacked plant. For that reason, it may be wise to confine control to the most infected areas of the yard.

For all its bad press, the Japanese beetle still does one good deed. It feeds on poison oak, sumac and ivy.


This story was produced by the staff at the Centre County Gazette. It was re-published with permission. The Centre County Gazette is a weekly publication, available at many locations around Centre County every Thursday morning.

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