Japanese Beetles: Small Pest, Big Threat
By Nanette Morgan, Anoka County Extension Master Gardener
It’s the time of the year when peonies are blooming, rose bushes are leafing out, and Japanese beetles start visiting our gardens, eating their way through our prize flowers and foliage.
Japanese beetles can be identified by their metallic blue-green heads and thoraxes, their tan wings, and the small white half-inch hairs that line each side of their abdomen. They are considered an invasive species and are monitored by the Minnesota Department of Agriculture. They are a two-pronged threat because they are pests in both adult and grub form.
The adult stage of a Japanese beetle is only around 40 days, but their damage can be devastating to the home garden. They appreciate a good buffet and especially like roses, hollyhocks, Virginia creeper, grapes, raspberries, beans, apples and fruit trees. However, as most Anoka County gardeners know, they will eat almost anything and will travel to do so. The females eat and lay eggs constantly between July and August, and the eggs hatch into grubs after only two weeks.
The Japanese beetle spends about 10 months of its life as a grub, where it feasts on the roots of lawn grasses for the energy to pupate into an adult beetle. Grubs can cause problems in turf that isn’t irrigated regularly, resulting in brown patches of grass that pull up easily, like carpet fibers. As summer turns to fall and the soil temperature falls below 50 degrees, the grubs tunnel deep in the soil to overwinter, and the cycle will start over again next spring.
It’s important to note that adult beetles inflict mostly cosmetic damage to mature, healthy plants and shrubs. However, a severe infestation can damage or kill young or diseased plants. You can minimize damage to food crops by harvesting continuously during July and August. Here are some other ways to control them:
Choose less preferred plants or trees. Boxwood, clematis, chrysanthemums, daylilies, geraniums, lilacs, and conifers apparently don’t taste as good as tea roses.
Pick beetles off your plants in the early morning or evening when they are sluggish, and dispose of them in a bucket of soapy water.
Use row covers, although this will interfere with pollination.
Neem oil is a deterrent, but it needs to be reapplied every few weeks.
Bacillus thuringiensis galleriae (BT) is moderately effective and lower risk to bees; however, it is toxic to Monarch caterpillars.
Products containing pyrethrins can be effective when sprayed directly on the beetles, but it will also harm beneficial bugs.
What you should not do is purchase Japanese beetle traps. A study by the University of Kentucky found that the traps attracted more beetles than they caught. It’s also important to confirm that the Japanese beetle is the source of the damage before using any chemicals.
While Japanese beetles can be frustrating, take heart in knowing that their numbers will vary from year-to-year, and that you can minimize damage by understanding their life cycle and being vigilant in your home garden.
Got yard and garden questions? Connect with Anoka County Master Gardeners for answers! Join us for Ask a Master Gardener Online sessions every Tuesday from 6:30 - 8 p.m. through September. Visit anokamastergardeners.org to learn more. Follow us on Facebook: University of MN Extension, Anoka County Master Gardeners.