Garden Tips: Watch for destructive Japanese beetles in Northwest

It was never a matter of if the Japanese beetle would reach the Northwest, it was a matter of when.

That could be now. Numerous adult beetles have been trapped and found eating on roses and other plants in Portland, according to the Oregon Department of Agriculture (ODA). ODA says that this suggests “a breeding population of the non-native insect has been established.”

This is terrible news for gardeners and growers. A University of Kentucky publication says that “the Japanese beetle is probably the most devastating pest of urban landscape plants in the eastern United States.”

I am willing to bet that if you came to this area from the eastern part of the U.S., you already know too well why a Japanese beetle infestation is scary. As a pest, this pretty beetle packs a double whammy. Its grubs feed on grass roots and can be damaging to lawns. The extremely voracious adults are omnivorous, feeding on a variety of plant hosts, including roses, ornamentals, trees, shrubs, fruit and vegetables, often devouring the upper sides of plant foliage, leaving only the skeleton of veins and midribs behind.

The Japanese beetle is one of the scarab beetles and could be considered attractive if you like beetles. The adult beetle is almost a half-inch in length, with metallic copper wing covers and clubbed antennae. The head and thorax in front of the abdomen are metallic green. Along the sides of the abdomen are hairy patches that look like white spots.

Like so many insect pests, the Japanese beetle is an alien, coming here from Japan. It was first identified in the eastern U.S. in 1916, and it became established in all the states east of the Mississippi, except for Florida, by 1998.

Besides its rapacious appetite, this beetle’s propensity for aggregating on plants can lead to rapid and complete defoliation of a plant. Japanese beetle-damaged plants emit volatile chemicals that bring more beetles to the party. In addition, the unmated females emit a pheromone to attract more hungry beetles. Once done with one plant, they move onto another.

Thankfully, there is only one generation of Japanese beetles a year, but with each adult female laying about 40 to 60 eggs, populations can build quickly. Control with pesticide applications is aimed at the adults on plants and their grubs in the soil. You may see Japanese beetle traps advertised for their control, but university research shows that the traps are effective in monitoring for the beetle’s presence, but not for control. In fact, the traps can result in more of the beetles finding your yard and causing damage.

Throughout the years, ODA has been working to contain and eradicate any Japanese beetle infestations. They believe that the origin of these infestations are air cargo carriers flying in through the Portland International Airport. ODA estimates that the infestation has been present for more than a year without detection. Right now, they are trying to pinpoint where the breeding population is so they can treat the infestation next year.

For us in Washington, it is good to know that the Washington State Department of Agriculture has been on the watch for the Japanese beetle and has been conducting trapping each year since the mid-1980s. So far, WSDA has not had to conduct eradication measures yet. I hope it stays that way.

This story was originally published September 4, 2016 at 5:24 AM with the headline "Garden Tips: Watch for destructive Japanese beetles in Northwest."