David Robson Extension Educator, Springfield Extension Center, University of Illinois
Yard & Garden
Prevention of Unpredictable Pests
How to stop an invasion of Japanese beetles
If there’s one thing you can count on with invasive pests, it’s the fact that they aren’t predictable. Some can move fast, spreading from coast to coast within years. Others migrate slowly. Think Dutch Elm Disease - it started out east in the late 1920s, hitting the Midwest about 30 to 40 years later.
Japanese beetles supposedly arrived about 10 years earlier. Many credit a shipment of iris bulbs as hiding the pests. Slowly but surely, it made its way to us some 60-plus years later.
(Interesting historical trivia - General George Marshall once stated during World War II that he would often forget about his campaign against the Japanese by his battling against the Japanese beetles at his estate in Virginia.)
Japanese beetles can be fascinating if you’re not battling them. Much like a kudzu or the Ebola virus.
Few insects do serious damage in their two active life stages. As an adult beetle, which is metallic green with copper colored wings, they will feed on just about any plant. They’ll feed on the upper surfaces, basking in the sun at the same time. They don’t like leaf veins, instead feeding on the tender tissue between the veins.
Of all the plants, linden trees seem to be the hardest hit. And as one of the trees the Illinois Department of Transportation has planted along highways throughout the state, there is a great feeding trail from Cairo to Rockford and Danville to Quincy.
Roses and all the rose relatives from apples to peach to cherries to hawthorns run a close second to the lindens. Follow that with grapes, hydrangeas, tomatoes, peppers, peas and birch.
And just about anything that is green, except grass, seems to be a food source if nothing else is available. Curiously, most shade plants are unaffected, probably because shaded areas seem to be the last place Japanese beetles visit.
As mentioned above, both the larval and adult stages cause problems. For Japanese beetles, while the adults don’t feed on grass, the larval stage does. Japanese beetle grubs now account for a vast amount of turf grass insect problems.
All the history and fun facts don’t mean squat when the pests arrive. All the gardener can think about is how to control them.
Japanese beetle traps get some bad raps, not because they work, but because they tend to work too well. Instead of just trapping the insects in your yard, they seem to attract just about all the Japanese beetles in the neighborhood, and sadly many don’t end up in the trap.
Jokingly, people say to give the Japanese beetle trap to your neighbor a couple doors down the street, or post the trap about a quarter mile away. Of course, this being a civilized society, no one would ever think about doing that or even condoning it. At least not in daylight.
You can control the grubs in your lawn with many of the insecticides on the market, or go the organic route with Milky Spore Disease, which only works on Japanese beetle grubs. However, remember that the beetles fly five to 25 miles per year. You can control all your grubs in the yard, and be invaded the next year. If you have beetles in your neighborhood, you’ll probably have them in your yard.
There are some insecticides that work, but nothing is surefire. Sprays may not penetrate the hard shells of the beetles. Chemicals that are absorbed by the tree seem to work better.
Some people will flick them into a bag or jar of soapy water. Others suggest gasoline or petroleum products, which work, but present a definite environmental problem when disposing of the insects. When they’re young, they tend to fly away; as they mature, they become more complacent and are easily picked off. But nature abhors a vacuum, and you’ll probably have to pick more off the following day.
Most years, the beetles are gone by Labor Day, though their damage stays around for the rest of the year.
Maintaining plant health is the best bet. Make sure you fertilize trees in the fall, and water them, if possible, during droughts.
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: 217-782-6515.
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