Growing Green

By Robin Trott, Extension Educator

The Japanese beetle (Popillia japonica) is the most destructive insect pest in the landscape and garden. It was first found in this country in 1916, near Riverton, New Jersey, after arriving in nursery stock from Japan. Japanese beetle infestations slowly expanded southward and westward and are now found from the Atlantic Ocean to the Rocky Mountains. Japanese beetles were first discovered in Minnesota in 1968. At first, only a few beetles were found. By 2001, they occurred in much higher numbers. In one year, the Minnesota Department of Agriculture (MDA) trapped more than one million beetles. Since then, Japanese beetle numbers have fluctuated from year to year.

Adult beetles emerge in mid-June through July. They are similar to other June bugs in general appearance, and 3/8-inch-long and 1/4 inch wide. The head and thorax are shiny metallic green, and the wing covers are coppery red. The row of five tufts of white hairs on each side of the abdomen is a distinguishing feature.  Adult beetles eat the foliage, fruits, and flowers of over 300 plants. Foliage is consumed by eating the tissue between the veins but leave the veins, a type of feeding called skeletonization. Flowers and fruits may be devoured completely, often by a horde of a dozen or more beetles at a time.  Trees, shrubs, grapes, and other plants often turn brown in August because of the feeding damage.

Management of Japanese beetles

Control of adult beetles is difficult because they emerge every day for several weeks.  Also, there are no easy answers or at least no popular answers. There also is no one correct answer.  Persistence, diligence, and repeated efforts are necessary because beetles emerge every day for several weeks. By mid-July, beetle emergence is at its peak, and anything done after a majority of beetles have emerged, and plants are already damaged will be too-little-too-late. Available control options all have limitations and variable applicability. Options include tolerating the damage, handpicking and screening, insecticide application.

Tolerate the damage. Japanese beetle defoliation is not fatal to otherwise healthy plants. Linden, crabapple, cherry and other trees and shrubs look terrible when defoliated, but they are not dead.  The trees are stressed but usually recover with new leaves in the fall or the following spring. Don’t cut down a defoliated tree unless you want to remove the tree and plant something else. Young, newly planted, or unhealthy trees should be protected from severe defoliation to prevent stress, stunting, or death.

Handpicking and screening. Mechanical controls may be useful in isolated situations with small plants and a limited number of beetles. There is little appeal to handpicking in large gardens or on large plants.  Even for a small number of small plants, handpicking becomes a laborious hobby to keep up with beetles that appear daily, if not hourly. Remove beetles early and often to preserve the beauty of the plant and to reduce the attraction of more beetles. Remove beetles early in the morning while temperatures are cool and the beetles are sluggish. Collect or shake beetles into a bucket of soapy water and discard.

Insecticide spray. Spraying infested foliage of high-value ornamental plants with a residual insecticide will keep the green leaves on treated plants, but spraying must be thorough and frequent and started in early July. There is no one-and-done spray. It will take 3-5 sprays over the 6 weeks of beetle presence, more if it rains. Most garden insecticides will control the beetles present at the time of application and offer some residual protection. Common examples include permethrin, bifenthrin or cyfluthrin. Read and follow label directions concerning rate and timing of application, the maximum number of treatments per year, and the interval between sprays or between spraying and harvest of food crops. Do not spray plants that are in bloom or when bees and beneficial insects are present.

Insect Traps. Several Japanese beetle traps using a floral lure and sex attractant are available. However, traps do not protect your plants and may increase the feeding damage. Japanese beetle traps attract more beetles than they catch. That is, they may attract more beetles into a yard than would occur otherwise.  The only benefit of JB traps is the emotional satisfaction of seeing and smelling hundreds (thousands?) of dead, decaying beetles. If you do use a trap, place it as far from the favored plants as possible.

Grub Control. Japanese beetles develop as white grubs in turfgrass, and there is speculation that controlling your white grubs in your lawn in August will reduce the number of adult beetles on your ornamental plants the following year. Unfortunately, this is not the case.  JB adults are highly mobile, and by some accounts, they may travel up to 15 miles in the adult stage. Therefore, eliminating white grubs from your limited turfgrass area (compared to the turfgrass area of the county) will not prevent damage to trees, shrubs, and flowers the following year. Similarly, spraying Japanese beetle adults on ornamental plants in July does not prevent white grub damage in the lawn in September.

For more information about Japanese beetles visit https://extension.umn.edu/yard-and-garden-insects/japanese-beetles or call the Douglas County Extension Office at 320-762-3890.

Until next time, happy gardening.