Plant Palette

Plant Palette

Japanese Beetle-- Popillia japonica

Photo of Jennifer Schultz Nelson

Jennifer Schultz Nelson
Extension Educator, Horticulture

My least favorite pest of the season has made its presence known in a big way in the last few weeks. I was out in my yard one evening in late June, and I spotted him on my river birch tree–one lone Japanese beetle in all his awful metallic oily-colored glory. With visions of defoliated roses and a stream of colorful language escaping my mouth, I scurried to apply the systemic imidacloprid insecticide to my roses that I was supposed to apply the month before, but somehow it never got done. (I think perhaps my wedding got in the way of proper pest management!) I also dusted the roses with carbaryl, in hopes of stopping the beetles dead in their tracks while waiting for the imidacloprid to be taken up by the plant and become effective at killing the hungry little rose-munching machines.

The Japanese beetle is native to the main island of Japan. As with many of our most obnoxious pests, the Japanese beetle was introduced to the U.S. by accident in a shipment of nursery stock. The beetles were first noticed during a routine inspection of a nursery in New Jersey in August 1916. Attempts to identify it as an American species failed. It was well into the winter of 1917 before the beetle was identified as Popilla japonica, a Japanese species.

The scientists initially involved were alarmed when they read the literature available surrounding this beetle, and were concerned about potential damage to U.S. agriculture. Attempts were made to confine the pest to the area in which it was found through extensive use of pesticides and removal of plants the beetles favored. Unfortunately, due to shortages of both money and labor, it was impossible to keep up with the control measures. At first it was thought that the Japanese beetle preferred to feed on a short list of plants. This was a very wrong conclusion. To date, adult beetles have spread through most of the eastern U.S. and will feed on a list of over 300 different plants, many of which are favorite landscape plants, like roses and linden trees. The larvae consume roots of turf grass and vegetables, and can do considerable damage if the population in an area is large enough.

The beetle spread to nearby areas, and into Pennsylvania. At one desperate point in time, federal officials even offered children cash rewards for each quart jar of dead Japanese beetles they turned in. Nothing seemed to even put a dent in the beetle population. Did Japan have the same problem?

In Japan, the Japanese beetle is not really a pest. This is surprising until considering a few facts about the environment of Japan versus the U.S. For one, not only is Japan the home of the Japanese beetle, it is also home to several natural enemies and parasites that have co-evolved with their Japanese beetle prey or host. The other major factor is turf grass. Female Japanese beetles prefer to lay their eggs in large expanses of turf grass that is in full sun. They especially love turf grass that is watered regularly.

Japan is a pretty crowded country. Large expanses of lawn are a rare sight. Compare that to the U.S., where a stroll through nearly any neighborhood will reveal large stretches of turf grass. Plus it is not uncommon to find lawns that are serious competitors for the "green grass award", being watered and fertilized heavily in the quest to have the greenest lawn on the block. To the Japanese beetle, this is heaven!

Everyone plagued by Japanese beetles in their landscape wants to know how to control them. Not surprisingly, people are not in a rush to eliminate their lawns. Thankfully, there are much less drastic measures that can reduce populations of all life stages of the Japanese Beetle when used appropriately.

There are chemical and physical control measures for Japanese beetles. Remember though, that these methods are intended to manage beetle populations, not eliminate them altogether, which would be nearly impossible. Following the philosophy of integrated pest management (IPM) practices, it is possible to have an insect pest present, but in such low numbers it does not do significant damage. Aiming to significantly lower the numbers of Japanese Beetles on your property is the most realistic scenario.

Chemically, there are two major chemicals the University of Illinois recommends for controlling Japanese beetles. The first is carbaryl, commonly sold under the brand name Sevin, which is usually a spray or fine dust used to coat the leaves and/or flowers on affected plants. Control can last for up to two weeks, but the beetles must actually contact the carbaryl, and it must be reapplied if washed off by rain.

The second is imidacloprid, commonly sold as Merit, which is a systemic treatment for non-food crops such as shrubs, trees and flowers. Formulations differ, but imidacloprid is generally applied early in the growing season, in mid to late spring, to allow sufficient time for the plant to absorb the chemical and develop some protection from the beetles. Control can last for months to years depending on the formulation. The beetles must ingest the chemical to be affected, so some plant damage will occur. Also, imidacloprid does not accumulate well in petals, so the beetles may still devour your roses. But the foliage should be protected, making their bounce-back that much quicker once the adult feeding ends.

Remember that with either chemical method, potentially beneficial insects will be affected, including valuable pollinators such as bees. Remember too that the crop or plant you wish to treat must be listed on the label of the insecticide. Some formulations are not appropriate for some plants. Read the label!

There are some natural bacterial controls available such as milky spore, sold under the name Doom or Grub Attack intended to target only the Japanese beetle grub, or larval stage of the beetle. Other grub treatments contain synthetic insecticides like Merit that targets grubs in general, not just Japanese beetle grubs. Considering that adults can travel ten to fifteen miles from where they were larvae, controlling Japanese beetle grubs is not likely to have any effect on numbers of adults feasting on your garden. But if you have significant damage from grubs, you may want to consider their use.

A device mistakenly used by many for Japanese beetle control that will not work, and may in fact make your problem worse is traps. These exploit the beetles clumsy flying ability and amorous intentions with a cardboard or plastic wall they slam into while seeking the source of the irresistible pheromone in the bottom of the trap. The only real use for them is for monitoring beetle populations, not controlling them. If using them for monitoring, they should be placed at the edge of your property, away from valued landscape plants, as they will attract beetles from miles around. If the trap fills in a short time, treatment is probably in order. A few beetles over the course of the day would not indicate any need to use controls.

Practically speaking, we can just as easily observe Japanese beetles feeding in our landscape and note the numbers. Also, considering the proximity of other homes in most neighborhoods, your neighbors will likely not appreciate your use of a trap. One person I know never had a huge Japanese beetle problem until this year, when her neighbor installed a trap near their property line. Now the beetles are devouring everything in sight in her yard!

The best physical control method for adult beetles continues to be hand picking or shaking them from plants and dropping them into soapy water. They are very clumsy, so if you place a pan of soapy water below an affected plant and shake the branches, the beetles should easily drop to their death using this soapy weapon. This does get tedious, and sometimes the beetle population will be too large for hand picking. The two basic options at that point are: try the chemical sprays, or just let nature take its course. Most woody plants will survive near to total defoliation by the beetles, despite their ragged appearance post-feeding.

For those that feel the need to inflict bodily harm on future generations of Japanese beetles, the solution comes from those "aerator sandals" sold in many garden catalogs that are not effective in aerating your lawn, but may effectively break your ankle as you attempt to do so. Researcher and Extension specialist from Colorado State University, Dr. Whitney Cranshaw, has dubbed these sandals the "Spikes O' Death" when it comes to grub control in lawns. He and his graduate students did an actual, legitimate experiment on a lawn heavily infested with grubs, comparing effectiveness of chemical grub control with the "Spikes O' Death" sandals. The sandals were just as effective as chemical controls. The "Spikes O' Death" would not prevent adult beetles from coming into the yard the next year, but using the sandals would potentially save the environment from application of some amount of pesticides, and probably would be beneficial for the homeowner's mental health, just knowing he or she has rid the world of some future Japanese beetles!

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