Ask the Expert: What is the best way to stop Japanese beetles from eating plants?

Editor’s note: Throughout the growing season, Mike Hogan, OSU Extension educator for Agriculture and Natural Resources in Franklin County, will answer gardening and home landscape questions submitted by Dispatch readers. Send your questions to hogan.1@osu.edu.

Q: I have a hedge of burning bush which grows profusely every spring, so in early summer, I have to trim them back pretty hard so they do not outgrow the space in my landscape. Soon after trimming these shrubs, the foliage gets brown and dead spots are on leaves. While the shrubs typically recover and push out some new growth, this year, the problem seems to be worse. What causes this condition?

A: Burning bush (Euonymus alatus) typically has a significant growth spurt in late summer and especially in early spring, and routine trimming each year is needed to maintain the shape of these woody ornamentals, and to prevent them from getting too large for the space which they occupy in the landscape.

If these shrubs are trimmed pretty aggressively in early summer, removing a significant amount of foliage, you expose some foliage in the interior of the canopy to direct sunlight. This can cause the tender foliage, which had previously been protected from the sun by layers of other leaves, to become sunburned, with symptoms exactly like you describe.

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Portions of leaves will turn white and then brown, and sometimes holes develop in the leaves. This problem is worse when the shrubs are in full-sun locations.

This may be worse this year because of the hotter-than-normal temperatures we have experienced in Greater Columbus so far this summer, as well as the recent lack of rainfall. While this condition may appear somewhat unsightly, it should not affect the overall health of the plants.

If regular precipitation does not return, I would provide some supplemental water to these shrubs, even if they are mature size. This will help restore some vigor to the plants and will likely stimulate some new growth.

The best way to water these plants is to punch a few holes in a plastic 5-gallon bucket, place the bucket over the root zone of the shrub, fill it with water and let it drain slowly. This will provide a deep watering to the root zone of the plants.

Q: My tomato plants have brown leaves on the bottom of the plant, even though I have been watering frequently. Would too much water cause leaves to die?

A: Excess soil moisture would typically present as a wilting of foliage of the entire plant, not usually browning foliage at the base of the plant. My guess is that you have some early blight on the foliage of your tomato plants.

Early blight is a common disease which we see every year. It is more prevalent in seasons when the weather is abnormally wet and humid. Look closely on the leaves toward the bottom of the plant, and if blight is present, you should see lesions with concentric circles on the leaves. The disease starts at the bottom of the plant and works its way up.

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The disease spreads mostly by water splashing spores of the disease organism up onto foliage not yet infected. For this reason, be sure to water the soil and not the foliage. Don’t spray water on the foliage; simply direct the water to the soil at the base of the plants. Also remove infected leaves and dispose of them in household trash.

Q: What is the best way to control Japanese beetles? They seem to be eating the leaves of many different plants around my yard this year.

A: For most plants in the landscape, control of adult Japanese beetles (Popilla japonica) is not necessary. In most cases, the damage from these insects is cosmetic and healthy mature plants can tolerate a lot of feeding without significant long-term injury to the health of the plant.

Young or unthrifty plants may be harmed by extensive feeding, so an application of a pesticide may be warranted on these plants. Consider a low-risk pesticide such as neem oil or B.t. in order to protect bees and other pollinators. Physically removing the beetles by knocking them into a cup or bucket is also effective.

The one thing you don’t want to do is use a Japanese beetle trap in your landscape! Research has shown that instead of providing protection from the beetles, these traps can actually increase localized damage from the insects simply because they attract an abundance of the insects to the vicinity of the trap. These traps only work if they are located in your neighbors' yard, a few houses down the street!