Japanese Beetle in Corn and Soybean

Encyclopedia Article

Japanese beetle, Popillia japonica, is a member of the Scarabaeidae (scarabs) family of beetles. This beetle has been expanding westward after an accidental introduction in New Jersey in 1916. The first confirmation in Iowa was in 1994, and it has been confirmed in 75 Iowa counties as of 2021. Japanese beetle has a wide host range, but corn and soybean can be fed upon, primarily by adults.


Adult: Adults are 5/16 inches long and have metallic green bodies with bronze forewings (elytra) and clubbed antennae (Photo 1). The forewings do not completely cover the end of the abdomen, and there are six white tufts of hair on each side of the abdomen. Males have large spikes on the front tibia while females will have spoon-like paddles (Photo 2).

Japanese beetle adult
Photo 1. Adult Japanese beetle. Photo by Marlin E. Rice.

sexing JB adults
Photo 2. Left: Male Japanese beetles have spikes on the tibia. Right: Female Japanese beetles have paddles on the tibia. Photos by Tom Hillyer.

Egg: Japanese beetle eggs are 1-2 mm around and creamy white in color. Usually, eggs are laid singly in the soil up to a depth of 4 inches.

Larva: The larvae are typical C-shaped grubs that are almost 1.5 inches long when fully grown. Japanese beetle grubs have an orange-brown head and a creamy white body that is covered in brown hairs. Typically, the end of the abdomen is dark-colored. This area is where the raster can be found, which are hairs on the underside of the tip of the abdomen. Japanese beetle grubs have two rows of six or seven hairs arranged in a V-shape; this can be used to distinguish Japanese beetle grubs from other species. Grubs also have three pairs of thoracic legs but do not have any fleshy abdominal legs (Photo 3).

Japanese beetle grub
Photo 3. Japanese beetle grubs are always C-shaped. Photo by Erin Hodgson.

Pupa: Pupae resemble the adult beetle. Typically, they are about ½ inch long and ¼ inch wide with cream-tan bodies that turn metallic green prior to emergence.

Be aware of look-alikes! Japanese beetles have many look-alikes in both the larval and adult stages. Identification of larvae can be tricky and often requires a microscope to see the raster patterns. If larval identification is desired, refer to this factsheet on identifying white grubs in turfgrass.

Adult Japanese beetles have many look-alikes in Iowa. Most often, it is confused with other scarab beetles. Diagnostic features of the Japanese beetle are the metallic green body with bronze wings combined with the six white tufts of hair on the abdomen. Some common look-alikes include the false Japanese beetle, masked chafers, and May/June beetles (Photo 4). The false Japanese beetle is the same size and shape as Japanese beetle, but the body is not metallic and there are no tufts of white hair. Masked chafers and May/June beetles are similar in shape (all scarab beetles) but are usually larger in size and yellow, tan, brown, or red in color. These other species are also not considered pests of field crops in Iowa.

japanese beetle, false japanese beetle, june beetle and masked chafer
Photo 4. Top left: Japanese beetle. Photo by USDA ARS Photo Unit. Top right: False Japanese beetle. Photo by Jim Kalisch. Bottom left: May/June beetle. Photo by Steven Katovich, USDA Forest Service. Bottom right: Northern masked chafer. Photo by Mike Reding and Betsy Anderson, USDA-ARS.


Distribution: Japanese beetle is an invasive insect in North America. It was introduced in New Jersey in 1916, presumably on imported Japanese iris rhizomes. Since then, Japanese beetle has spread westward throughout the United States. Currently, it is established in 28 states and 5 provinces, and other states are partially infested. In Iowa, 75 counties have reported Japanese beetle. Adults are highly mobile and are excellent flyers, but Japanese beetles have also been known to hitchhike to new areas on aircraft.

Host Plants: Part of the success of Japanese beetle can be attributed to its wide host range. Japanese beetles can feed on more than 300 plant species, including important horticultural and agricultural plants such as turfgrass species, fruit trees, corn, and soybeans.

Life Cycle: Japanese beetles have one generation per year in Iowa (Figure 1). Japanese beetles overwinter as nearly fully grown grubs in the soil. In the spring, grubs become active once soil temperatures exceed 50°F, move upward in the soil profile, and feed on grass roots before entering the pupal stage. Adults typically emerge in late June and immediately begin to feed on low-lying plants such as roses and shrubs. Adults eventually move up on trees and field crop foliage to feed and mate. It is assumed that female Japanese beetles cycle through feeding, mating, and laying eggs throughout the summer months. Eggs are laid singly in the soil, around 2-4 inches deep, and take roughly two weeks to hatch. The eggs hatch into small grubs that feed on roots until late fall when the temperature cools. The almost fully-grown grubs move deeper in the soil profile, usually up to six inches, and remain inactive all winter.

Japanese beetle life cycle
Figure 1. Japanese beetle life cycle. Image by Joel Floyd, USDA-APHIS-PPQ.

Plant Injury

Japanese beetle larvae consume root tissue of grass plants, which can cause economic losses in turfgrass or home lawns. However, larval feeding is unlikely to cause economic injury in field crops. Adults use their chewing mouthparts to feed on the foliage, flowers, and fruits of host plants. Some plants are preferred to others; in field crops, soybeans seem to be preferred over corn. Feeding on their host plants can appear severe especially when aggregations of adults are present in certain areas of the field, but economic injury by Japanese beetle is not common in Iowa corn and soybean fields.

Injury to corn: Although Japanese beetle adults can feed on the leaves of corn plants, the main concern is silk clipping (Photo 5). Silk clipping may interfere with pollination and lead to reduced seed set. Additionally, adults may feed on exposed kernels, but this injury is less concerning than silk clipping. Drought stress can exacerbate the effect of silk clipping by Japanese beetles. Silk clipping usually only causes reduced pollination when a majority of corn plants have silks clipped back to less than ½ inches.

Japanese beetle silk clipping injury on corn
Photo 5. Japanese beetles often aggregate and feed on corn silks. Photo Mark Licht.

Injury to soybeans: In soybeans, Japanese beetle is part of the complex of defoliating insects. Japanese beetles cause skeletonization, which is characteristic of this species because adults feed on the leaf tissue but leave all veins intact (Photos 6 and 7). Aggregations of Japanese beetle adults on soybean plants can cause heavy defoliation in a particular area, but beetles are usually only feeding in the upper canopy and on a few trifoliates.

Japanese beetle defoliation on soybean
Photo 6. Japanese beetles cause skeletonization of soybean leaves. Photo Mark Licht.

Japanese beetle injury on soybean
Photo 7. 10% (left), 16% (center), and 25% (right) defoliation caused by adult Japanese beetles. Photo by Marlin E. Rice.

Risk Factors

In general, soybean is preferred over corn, and the edges of fields can experience severe defoliation or silk-clipping. Soybean fields with sandy soils seem to be at higher risk of infestation by Japanese beetle. Corn fields may be at higher risk of Japanese beetle feeding if they follow sod, cover crops, or soybean (to a lesser degree than sod or cover crops). Japanese beetles prefer to lay eggs where grass is present since larvae primarily feed on grass roots.


Japanese beetle traps or trap designs are readily available to people; however, it is NOT recommended to use traps as a means of scouting or control in field crops. The lures used in Japanese beetle traps are a mixture of a floral attractant and a sex pheromone that is highly attractive to adults. Traps often overflow, leading to spill over into adjacent plants. If placed at the edge of a crop field, it is likely that the plants surrounding the trap will experience severe injury.

Corn: Obtain a representative sample of silk clipping in corn by assessing silk clipping for five random plants in five locations of the field. It is especially important to scout for silk clipping during the first five days of silking. A foliar insecticide is warranted if three conditions are met:

  1. Three or more beetles are present per ear;
  2. Silks have been clipped to less than ½ inch; AND
  3. Pollination is less than 50% complete.

Because adults are highly mobile, remember to continue scouting until pollination is complete. Adults can continue to reinfest fields even after an insecticide application.

Soybean: Scouting for Japanese beetle in soybean involves estimating percent defoliation across the entire field and throughout the entire plant canopy. Management decisions are often made for the entire complex of defoliating insects in soybean (caterpillars, beetles, grasshoppers) since it is often difficult to distinguish between types of defoliation. Use the scouting plan in Figure 2 to estimate field-wide defoliation. Remember it is important to scout the entire field because defoliation may be concentrated at field edges, and it is important to scout the entire canopy because Japanese beetle exhibits top-down feeding behavior. Make sure that defoliating pests are still present in the field before making an insecticide application by visually looking for pests or using a sweep net.

soybean defoliation scouting plan
Figure 2. Soybean defoliation scouting plan.

The treatment threshold for Japanese beetle in soybean is 30% defoliation before bloom and 20% defoliation after bloom. Because adults are highly mobile, remember to continue scouting through seed set. Adults can continue to reinfest fields even after an insecticide application.

Most humans tend to overestimate defoliation on plants. You can train your “defoliation eye” before scouting by using this Soybean Insect Defoliation Training Tool from the Crop Protection Network.


It is unlikely that Japanese beetle adults will cause economic injury to corn or soybean in Iowa. However, sometimes silk clipping or defoliation could warrant treatment.

Tracking development: Although tracking development is not essential for making management decisions, tracking growing degree days (GDD) can inform us of when adults begin to emerge in Iowa. Adult Japanese beetles tend to emerge when 1,030 to 2,150 GDD (base 50°F) have accumulated since January 1. Use the Pest Forecasting Page on the Mesonet website to track Japanese beetle development.

Cultural: There are many landscape and soil factors that influence Japanese beetle populations that could be exploited to decrease the risk of infestation; however, recall that adults are highly mobile and attracted to soybean foliage and corn silks. Some examples of situations Japanese beetles particular like:

  • Weeds: smartweed and evening primrose seem to be preferred by adults upon emergence
  • Corridors near uncultivated land
  • Cover crops: adults prefer to lay eggs in ryegrass and clover
  • Soil: females lay eggs in moist soil and soils with high organic matter

Biological: Many biological control agents, including nematodes, parasitic wasps, and bacteria, have been identified for the control of Japanese beetle grubs and adults. However, these options are still very expensive and are not recommended for farmers to use in field crops since there is a low risk of economic injury from Japanese beetle in Iowa corn and soybean. Generalist predators for grubs/eggs (ground beetles, rove beetles) and adults (birds) occur in Iowa but likely do not play a large role in suppression of Japanese beetles.

Chemical: Many insecticides are labeled for Japanese beetle grubs and adults and are very effective. However, Japanese beetle adults are highly mobile and may reinfest fields even after an insecticide application.

Additional Resources

Shanovich, Dean, Koch, and Hodgson. 2019. Biology and Management of Japanese Beetle (Coleoptera: Scarabaeidae) in Corn and Soybean. Journal of IPM.


Originally prepared by Marlin E. Rice. Updated by Erin Hodgson in 2017. Updated again by Ashley and Erin in 2022.