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Map of the distribution of Japanese beetles in Iowa. Hover your curser over the map to view county names and the number of reports.
Description of Japanese beetles
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 beetle in the U.S. and Canada
The Japanese beetle did not appear in Iowa until 1994 but is now found in over 3/4ths of the state.
The Japanese beetle is the "worst landscape pest in America" because it damages plants in both the adult and larval stages. As an adult beetle in mid-summer, the beetles are known to devour the foliage, flowers, and fruit from more than 300 species of plants. The rest of the year, the larvae are white grubs that live in the soil and eat the roots from trees, shrubs, and turfgrass. Damaged turf wilts, turns tan, and dies during August or September after the roots are eaten. Click here to read more about white grubs and their control in turfgrass.
Adult beetles emerge in mid-June through July. They are similar to other Junebugs 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.
Life cycle of Japanese beetles
Japanese beetle larvae are annual white grubs. The grubs are C-shaped and approximately 1.25 inches when fully grown. Adult females lay eggs in moist sod in July. Larvae live in the soil, where they feed on plant roots (especially turfgrass) and organic matter. Grubs are underground from August until the following June when the larvae pupate and transform to the adult stage, then emerge as adults to repeat the cycle. Japanese beetles are highly variable from place to place and from year to year.
Damage caused by Japanese beetles
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.
It is typical of Japanese beetles and other invasive species to go through an "invasion cycle" where the first discovery of the pest is followed by a few years of low abundance and then rapid or "explosive" or exponential growth of the population. Each location with Japanese beetles goes through the same cycle but at different times in different places. Often there is huge variation within a city block or neighborhood. Some Iowans are seeing the Japanese beetles for the first time, some are experiencing the explosive growth and severe defoliation, and some are past the peak and see the natural variation in abundance from year to year.
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.
There are limited alternatives to traditional synthetic garden insecticides. One option is to use insecticidal soap or pyrethrin insecticide such as Pyola® as somewhat effective contact insecticides. These products have no residual effect, and frequent (if not daily) application will be required. Two organic feeding deterrents are available. Feeding deterrents do not kill beetles; they move them to eat somewhere else. Kaolin Clay (Surround®) or Neem Oil, sprayed every few days and after rain will reduce the numbers by discouraging the feeding. Sprays containing Bacillus thuringiensis galleriae are also effective and avalable under a couple of different trade names.
Systemic Insecticides. A systemic insecticide is taken up by the tree and moved in the sap to all parts of the tree. Beetles feeding on the leaves of a treated tree will be killed. Systemic insecticides can be applied as a soil drench, granule, or trunk injection. Application must be made before the arrival of the beetles (April or May) to give the plant enough time to absorb and transfer the product to the leaves.
Systemic treatments avoid the possibility of pesticide drift and are only one application is needed per year. However, systemic insecticides can be toxic to insects that feed on the nectar and pollen. Do not apply to soil near blooming plants that may attract pollinators. It is a violation of the label directions to apply imidacloprid systemic insecticide to lindens and basswoods.
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.
Plant Selection. One way to limit the impact of adult Japanese beetle defoliation may be to select plants that the Japanese beetles tend to avoid. Click here for a list of plants preferred by Japanese beetle adults.
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