Adult Japanese beetles emerge from the soil in June to eat a variety of foods - including foliage. While they rarely cause economic damage to field crops, they can cause aesthetic damage to ornamental plants and fruit trees.
"A few reports of metallic adult beetles have been coming my way this week," said Erin Hodgson, Iowa State University Extension entomologist, on June 30. "I thought it might be too early to see adult Japanese beetles in Iowa. But a Japanese beetle flew into my office through an open window - guess that answers my question! Some people may have questions regarding identification of scarab beetles, or beetles in the insect family Scarabaeidae." Hodgson offers the following guidelines and information.
The larvae are grubs that feed underground
In general, adult scarab beetles are stout insects with a hardened body and clubbed antennae. Adults eat a variety of foods, including fungi, dung, carrion, sap, pollen and foliage. Rarely do the adults cause economic damage to field crops, but they can occasionally cause aesthetic damage to ornamental plants and fruit trees. The larvae are called grubs that feed underground or under debris. Larvae are pale yellow, gray or creamy in color, and are always c-shaped. Larvae can cause significant plant damage, particularly to grasses, as they feed on the root system.
There are several scarab beetles in Iowa, and probably the most important species is the Japanese beetle. The larvae are difficult to distinguish, but careful examination of the raster (aka, the butt) hairs will provide diagnostic details. The adults are more easily identified based on size and color (see Table 1).
Grubs are creamy white with a brown head capsule.
There is one generation per year, with adults emerging from the soil in June. Mated females lay eggs in the soil until late August. Adults have an exceptionally wide host range (more than 300 plants) and skeletonize leaves. Hatched larvae feed on the roots until temperatures begin to cool in the fall; larvae move deep into the soil to overwinter. Nearly fully-grown larvae resume feeding in the spring, pupate within the soil and emerge as adults.
Japanese beetle causes leaves to be bronzed and lacy. Adults often mass on plants.
Jim Fawcett, ISU Extension field agronomist in eastern Iowa, says his first Japanese beetles of the season in a cornfield last week on the edge of Iowa City. Japanese beetle grubs feed on corn roots, among other plant roots, notes Fawcett. The soil was very sandy in the field, which probably meant the grubs developed more rapidly in the warmer sandy soil, so it may be a few days before the beetles become more common in other areas.
He adds, "The beetles look like a small colorful June bug. They feed on many plants including soybeans. The feeding injury looks much worse than it really is, leading most people to spray soybean fields with an insecticide when the beans really don't need to be sprayed. The general threshold for insects that feed on soybean leaves is defoliation, so usually if you think there is 20% defoliation, it's probably closer to 10%."
Fawcett says it's best for people to resist the temptation to spray in early July because by late July and early August you may have soybean aphids that do need to be sprayed. Spraying too early can actually increase aphid problems by killing off the beneficial insects that are keeping the aphids in check.