Japanese Beetles

Identification:

Adult: Adults are 5/16 inches long and have metallic green bodies with bronze forewings (elytra) and clubbed antennae. 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.

Lifecycle:

Larvae (grubs) feed on roots of many hosts, and overwinter. Adults emerge mid-summer and feed on hundreds of hosts, including soybeans. Adults may persist into fall. Eggs laid in the soil in July-Sept.
1 generation per year

Damage:

Injury to corn: Although Japanese beetle adults can feed on the leaves of corn plants, the main concern is silk clipping. 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.

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. 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.

Scouting:

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.

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.

Economic Threshold:

Corn: Foliage feeding in corn is almost never economic, though economic damage from silk clipping is possible (though rare).  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.

Soybeans: While the damage might look startling, it is very rare that this reaches economic levels from Japanese beetle.  A rescue treatment is advised when defoliation levels reach 30% in pre-bloom stages, and 20% in bloom to pod fill.  These defoliation levels apply to the plant as a whole, not just certain leaves, and can also be used for general defoliation from more than one kind of leaf-feeding insect in soybean.

Control Options: 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.

Corn Insect Control Table

Soybean Insect Control Table

Soybean defoliation guide: