I have a new daily ritual in the garden: Picking Japanese beetles off ornamental plants and dropping them in soapy water.
A month ago, I was squeamish about touching them with ungloved hands. Now I know this is the only way to capture two, three, six or more beetles on a single leaf. Each day, 25 to 50 Japanese beetles go for a swim in our yard. A squirt of dish soap in a plastic container half full of water is a safe, effective way to dispose of this copper-colored pest.
Adult Japanese beetles damage scores of fruits, vegetables and ornamental plants. They generally feed on foliage, leaving holes or a lace-like skeleton. They eat 300 host plants, including birch, linden, Norway and Japanese maple, crabapples, purple-leaf plums, mountain ash, pole beans, apples, grapes and roses. They can defoliate the upper canopy of trees.
We have picked them from leaves of fruit trees, hibiscus, Boston ivy, Virginia creeper, asters, morning glories and cannas. We have seen them feed on the flowers of zinnia and anise hyssop. Fortunately, we pick most of them off weeds – notably smartweed, or persicaria.
Broad feeding and flight patterns make Japanese beetles a frustrating foe. Adults feed for two months. The presence of beetles on or near a plant attracts more beetles, so it’s not uncommon to see several in orgy-like fashion on leaves. (I try not to think about this.) They can fly up to one-half mile to breed.
The quantity of Japanese beetles is new to many gardeners in Portage County as these insects spread north. They are also new in other areas of Wisconsin in 2012 and have appeared as far north as Oconto and Barron counties, according to University of Wisconsin-Extension (UW-Extension) reports.
Japanese beetle adults are nearly one-half inch long, and shiny, metallic green. Bronze-brown wing covers do not entirely cover the abdomen. Six white patches are visible along the sides and back of the body. Adult beetles typically appear about July 1.
Beetle counts have declined in some areas of the state, Phil Pellitteri, an entomologist at the University of Wisconsin and Cooperative Extension, told Portage County Master Gardener Volunteers last fall. Based on experience in eastern states where beetles first were found, they come on strong, then subside, Pellitteri said. His neighbor in the Madison area counted 130,000 Japanese beetles in 2007, which dropped in half the next year. In 2010, he had 12,000 beetles.
Those who live near large irrigated turf, such as a golf course, will see more Japanese beetles. That’s because adult beetles prefer lush sod to lay eggs. Beetles in the grub stage can cause serious damage to turf. They feed on the roots of grass and ornamental plants, reducing plants’ ability to take up enough water and nutrients to withstand the stress of hot, dry weather. Patches of dying grass can expand until turf can be rolled back like carpet.
The Japanese beetle, Popillia japonica, is considered the most important turfgrass-infesting pest in the United States, according to R. Chris Williamson, UW-Extension turf and ornamental specialist. It was first discovered in the United States in southern New Jersey in 1916. Japanese beetles occur in every state east of the Mississippi River except Florida.
They have one generation per year. When they emerge in mid-June, adult females mate and begin laying eggs, burrowing two to four inches into the soil under turf. Eggs hatch in about two weeks. Larvae are approximately one-eighth inch long and translucent, creamy white. As grubs begin feeding on the roots of turf grass, their hindguts appear gray to black. By late-August, they are nearly one-inch long.
Eggs and newly hatched grubs need adequate soil moisture to prevent drying out. Older grubs are more drought-tolerant and will move deeper into the soil if conditions become dry. They overwinter in soil, two to six inches below the surface. They become inactive when soil temperatures fall below 50°F, Williamson said. In the spring, grubs move up to the root-zone to resume feeding for about three to five weeks before pupate into adults.
Because both adult beetles and grubs cause damage, controlling one life stage will not preclude problems with the other.
Removing beetles by hand is adequate for most gardeners. Adult beetles are most active in the afternoon in full-sun, so that’s a good time to remove them. It’s easy to pick them off plants, although some fly away or drop to the ground. Monitor and repeat daily, as needed.
Traps, which use a female hormone, actually attract more beetles, and about one-quarter of these visitors find their way to plants, not the trap. (I heard a speaker suggest the best use of traps is convincing your neighbor to place one in his yard.)
Combating beetles with insecticidal spray is a high-maintenance option because sprays are effective for only five days. If you do use chemical control, apply in the afternoon when beetles are most active. Several insecticides are labeled for use against adult Japanese beetles. Always follow label directions.
See UW-Extension bulletins for more details about meaningful chemical controls of grubs. The optimal timing for this “curative” control is early to mid-August, Williamson said. A preventive systemic control needs to be applied before egg lay, in June.
Pellitteri recommends treatment only if lawn has 15 to 20 grubs per square yard. “Given time, they will settle down.”
Because Japanese beetle eggs and young grubs have difficulty surviving dry soil conditions, withholding irrigation during peak adult beetle flight may help to reduce grub populations, Williamson said. Keep in mind that adequate soil moisture in late-August and September can help turf recover from grub damage.