Japanese beetle, Popillia japonica, is a member of the Scarabaeidae family and order Coleoptera. This beetle was accidentally introduced into the United States in the early 1900s on horticultural nursery stock in 1916 in New Jersey. Since that time, it has been expanding westward. The first confirmation in Iowa was in 1994 and now has been reported in 61 counties.
Adults are 5/16 inches long, have metallic green heads with clubbed antennae and bronze wing covers (Fig. 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 hind tibia while females will have spoon-like paddles (Figs. 2-3). Read more about look-alikes in the last section.
Japanese beetle eggs are 0.5-0.8 inches (1-2 mm) around and white in color. The larvae are typical white, C-shaped grubs that are 0.5 - 1.2 inches (1.5 - 32 mm) long depending on the instar (Fig. 4). Grubs have three pairs of thoracic legs, but do not have any fleshy abdominal legs. Grubs have a orange-brown head and a creamy body. Usually, the raster (end of abdomen) is dark in color.
Japanese beetles have one generation per year in Iowa (Fig. 5). Adults emerge from grass 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. Mated females move back to grass in August and September to lay small egg masses in soil cavities. Eggs are laid about 5-10 cm into the soil and take roughly two weeks to hatch. The eggs hatch into small grubs that feed on roots underground until late September when the temperature cools. The almost fully-grown grubs burrow down in the soil and remain inactive all winter. In the early spring, grubs become active again and feed until turning into resting pupae. The pupae hatch into adults and emerge from the soil.
Injury and Management
This beetle has a large and diverse host range including woody trees such as elms to annual crops such as corn and soybean and even golf course greens. The Japanese beetle apparently will feed on about 300 plant species. Economic injury by Japanese beetles is not common in Iowa corn and soybean. Japanese beetles release a strong aggregation pheromone, and are commonly seen feeding and mating in clusters. Adults are also highly mobile and move frequently in the summer.
In soybean, injury can identified by the skeletonization of leaves (Figs. 6-7). The beetles eat the softer leaf tissue but usually avoid feeding on even the smallest leaf veins, thereby leaving a leaf “skeleton.” Injury to Iowa soybeans is most likely to occur in fields with sandy soils. The treatment threshold for Japanese beetle in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom. Most people tend to overestimate plant defoliation. Because adults are highly mobile, remember to continue scouting through to seed set. Migrating adults could reinfest the field in after knocking down an initial population.
In corn, Japanese beetles can feed on leaves, but the most significant injury comes from clipping silks during pollination (Fig. 8). Consider a foliar insecticide during tasseling and silking if: there are 3 or more beetles per ear, silks have been clipped to less than 1/2 inch, AND pollination is less than 50% complete. Migrating adults could reinfest the field in after knocking down an initial population.
Japanese Beetle Look-alikes
Japanese beetle has many look-alikes in Iowa. Most often, it is confused with other scarab beetles in the landscape. Remember, this species has a metallic green head and bronze wings. The tufts of hair are also diagnostic.
False Japanese beetle, Strigoderma arboricola, has a black and orange head (Fig. 9). Also known as a sandhill chafer, false Japanese beetles closely resemble Japanese beetles. They are about the same boxy shape and size (½ inch in length). The body is dark brown and shiny, but not metallic. They can have white hair along the side of the abdomen, but the hair is evenly spread out instead of in tufts. These scarabs have one generation per year. False Japanese beetles feed on the flowers, foliage and fruit of many plants, but they are not typically considered field crop pests.
Masked chafers are common in Iowa and are sometimes confused with Japanese beetles. These scarabs have one generation per year. Adults are about ½ inch in length and oval in shape. Masked chafers can be dark yellow or tan in color with dark markings on the head (Fig. 3). The body, legs and wings can be hairy. Adult masked chafers are not known to significantly feed on plants.
May and June beetles are also common in Iowa. Most have a two- to four-year life cycles, but some have one generation per year. Adults are 1 inch in length and oval in shape. Body color can range from chestnut brown to red (Fig. 11). Adult May/June beetles feed on a wide variety of tree foliage and are not considered field crop pests.
Originally prepared by Marlin E. Rice. Updated by Erin Hodgson in 2017.