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Japanese beetle season begins: We have been behind about 150 degree days this spring. Degree days are accumulated heat units that coincide with insect and plant developmental stages. For example, lindens bloomed this year the last week of June instead of mid-June. Usually around June 23-25, light blue chicory flowers start to bloom along the roadsides. When that occurs, we can expect to see Japanese beetles appearing in our gardens and yards. This year that timetable has been delayed, but we getting there now.

Japanese beetles overwinter as grubs in the soil, feeding on roots of turfgrass and other plants as they develop in spring. When they hatch into adult beetles, they have a wide range of plants they feed on. Some of their favorites are linden trees, birch and crabapple foliage, raspberries, grapes, roses, witchhazel, hazelnuts and Boston ivy. Since the beetles are around for an extended period from late June to September, they are very difficult to control, especially if you are trying to minimize the use of insecticides.

Adults also tend to fly in from other locations to feed on your plants, so just controlling the grub population in your lawn is not likely to provide sufficient control. Some tips are to NOT use the beetle traps that are sold at many retail outlets unless you have a larger property. Research had shown that the traps actually ATTRACT about four times as many beetles as would have visited your property than if you had not used the traps, so they are not helpful in most urban settings.

However, if you can set the traps a few hundred feet away from your garden and you know the prevailing direction from which the beetles are coming, they might be useful.

Milky spore is also not recommended, as it has to have a fairly large grub population already in the soil to be effective, and it has not been found to be as potent as it once was. Also, it only controls grubs in the lawn — not adult beetles that have flown and are feeding on your plants.

An organic product you may want to try on certain plants is 70% neem oil — but realize you will need to apply it every five days or so — including after heavy rains. You can always hunt for the beetles when they are less active in the early morning or early evening and flick the leaves they are perched on, tumbling them into a bucket of soapy water.

The good news is that although when they first invade a new area their population levels are quite high and damaging, after 5-7 years the numbers seem to level out and they aren’t quite so problematic.

For more information on Japanese beetles and controlling them, visit https://pddc.wisc.edu/ and look for “Japanese Beetles” under the Fact Sheet tab.

Lisa Johnson, Dane County UW-Extension horticulture educator

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