Ask a Master Gardener: Shiny, bright and annoying! Coping with Japanese beetles

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Japanese beetles damaged these rose bush leaves.

(Posted Aug. 5, 2021)

By Jane Kutzley, Madison County Master Gardener

Well, the cicadas have come and gone and other garden insects are now making their presence known. Some are friends; some are not so friendly. This one really doesn’t deserve much love.

Japanese beetles, lots of them, have found my new rose bushes. Roses are one of their favorite snacks. They skeletonize the leaves; only the ribs remain when they’ve finished. They seek out and devour hydrangeas and other shrubs, all sorts of fruit trees, beans, peas, even tomato leaves.

Adult Japanese beetles are easily recognized by their iridescent wings. They would be beautiful, except for the part about devouring leaves. Before you find them on your roses, during late spring and early summer, they’ve already lived another whole life beneath the ground as grubs, eating the roots of your grass. Occasionally, if the infestation is severe, you can actually lift away a chunk of the browned-out sod, no shovel required, and expose the white c-shaped grubs beneath. Or, before the grass even turns brown, you might find areas of your lawn with shallow pits dug all over. That’s the work of a night-time marauder (oftentimes a skunk) feasting on those larval forms.

If left alone, the grubs will pupate into the shiny adult form, emerge from the soil and seek out a nearby yummy plant. The adults will find each other, mate, and lay their eggs on the soil surface. In a couple weeks, those eggs hatch and the tiny larvae begin feeding, burrowing deeper into the soil as temperatures drop in autumn, hibernating through the winter, then coming up to grass-root level when the soil warms in the spring.

In Japan, the beetles have natural predators and are not much of a problem. Here, the only predator is that skunk that dug up the yard, and they are very inefficient. The best way to control Japanese beetles is to literally pick them off your plants and drop them into a bucket of water with a few drops of soap added. Don’t forget the soap as it reduces the surface tension of the water. Without it, the beetles can stay on top the water and just fly away. The beetles attract each other by secreting pheromones, so it is important to pick them off daily. Otherwise, they will leave their scent and attract others even after they themselves are gone.

Japanese beetle traps are sold as a way to reduce the problem. However, studies have shown that the beetles are more attracted to the plants near the trap than they are to the trap. Traps increase the number of beetles on nearby plants. The common joke is the best place to put a Japanese beetle trap is in your neighbor’s yard. Please don’t.

Controlling the larvae at the grub stage is another effective option. A natural product called Milky Spore, applied to the lawn, targets the grubs while not harming beneficial insects. It is also safe for pets. A quick internet search provides lots of information. Be sure to read and follow all directions carefully as timing is critical. And be patient. Natural processes like this need time to work.

he worst thing to do is use an insecticidal spray. The beetles appear during the time of year when flowers are in full bloom and bees and other pollinators are busy at work. The hard-shelled, winged, Japanese beetles will be far less impacted by the spray than will a bee or butterfly. Go for the option that does the least harm.

To learn more about the Madison County Master Gardener program, call the Madison County OSU Extension Office at (740) 852-0975.

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