Adult Japanese beetles first emerged in some areas of Iowa around the end of May. This is very early compared to a normal year, says Erin Hodgson, Iowa State University Extension entomologist. At the same time, many other beetles in this large insect family (Scarabaeidae) are becoming active and causing confusion with identification.
"In general, scarabs are stout beetles that are boxy in shape, have clubbed antennae and thick legs adapted for digging," explains Hodgson. "Adults can be active during the night or day depending on the species; regardless, they are clumsy fliers. Some species are scavengers that feed on dung, carrion or decomposing organic materials; others can be significant plant pests. In most years, adult scarabs emerge mid-June and can be active until August."
The following information and scouting recommendations are provided by Hodgson, along with her suggestions for managing this insect pest.
Can you tell difference between Japanese beetle and look-alike insects?
Japanese beetle adults are just over ½ inch in length. These scarabs have one generation per year. Probably the most diagnostic features are the white tufts of hairs along the sides of the abdomen and metallic green head and bronze forewings (see photo). The forewings do not fully cover the tip of the abdomen. Adults have a very wide host range (more than 300 different types of plants)--including roses, fruit and shade trees, grapes, corn and soybeans. Japanese beetles are skeletonizers and feed between leaf veins. View this online slideshow for more information on Japanese beetle management.
Here's what a "False Japanese beetle" looks like; it differs from a true one
Also known as a sandhill chafer, false Japanese beetles closely resemble true Japanese beetles. They are about the same boxy shape and size (½ inch in length). The body is dark brown and shiny (see photo), but is not metallic green and bronze. 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.
How to distinguish "masked chafers" from true Japanese beetles
There are several masked chafers in Iowa. 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 (see photo). The body, legs and wings can be hairy. Adult masked chafers are not known to significantly feed on plants.
May or June beetles are also sometimes confused with Japanese beetles
There are several scarabs with the common names of May or June beetles in Iowa. Most have a two to four year life cycle, 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 (see photo). Adult May/June beetles feed on a wide variety of tree foliage and are not considered field crop pests.