KC Gardens

Japanese beetles are wrecking havoc: What you need to know

Japanese beetles are particularly fond of rose petals, though they will eat over 300 speciess of plants.
Japanese beetles are particularly fond of rose petals, though they will eat over 300 speciess of plants.

Japanese beetles have been wreaking havoc on our landscape for weeks. Thie year and last have seen the worst outbreak. Japanese beetles were first found in the United States over a 100 years ago and have slowly moved across the country.

The adult stage devours the soft tissue of over 300 species of plants. They love rose petals, grapes and linden trees but will eat everything from green beans to crepe myrtle. The Johnson County Extension office has fielded hundreds of calls. Below are the most common questions.

How long will they last?

There is one generation of beetles each year. Adults live for about 30 to 45 days. Unfortunately, they don’t emerge from the ground at one time but in waves. The highest number of adults lasts from late June through early August, with a peak number somewhere in the middle.

Where did they come from?

The species was first introduced into the country from Japan. The adult beetles come from larvae or grubs that can feed on your lawn. Luckily we have not seen damage to lawns, as they can be controlled with readily available treatments for other grubs.

If we are controlling the grubs in our lawns, why are there still so many? The majority of the beetles are not coming from managed lawns, but from roadsides, pastures and other open areas that are not treated with chemicals. The high number of beetles reflect their wide distribution. Since this is a flying insect, treating your lawn for grubs will have little effect on the adult feeding.

Will they return next year?

While insect populations are difficult to predict, there is no doubt this pest is here to stay. The numbers have been building in the KC area over 20 years: It has taken this long for the problem to be widespread.

How can I control them?

Control is not easy. Since they are a hard-shelled beetle, insecticides are not as effective but can provide some relief. However, since they come in waves, one treatment does not last. Here are your options:

▪ Do nothing and let them run their course. Most plants will recover. The greatest damage is done to annuals and food crops. Ornamental trees will releaf, maybe not till next year, but they will recover.

Japanese Beetles in Bucket Soapy Water
One way to control the beetles: Knock them into a bucket of soapy water.

▪ Hand remove. This requires almost daily monitoring. Mix up a bucket of soapy water, get out in the landscape early in the morning and simply knock them into the bucket. This works only for small plants not trees.

▪ Chemical sprays will reduce the populations but must be reapplied every couple of weeks. These sprays will also have an adverse effect on beneficial insects.

There is no simple answer to this pest. The good news is they usually do not kill a plant, they just leave it looking ragged.

Dennis Patton is a horticulture agent with Kansas State University Research and Extension. Got a question for him or other university extension experts? Email them to garden.help@jocogov.org.