Is It Japanese Beetle in the Field or Something Else

June 30, 2009
ICM News

By Erin Hodgson, Department of Entomology

A few reports of metallic adult beetles have been coming my way this week. I thought it might be too early to see adult Japanese beetles in Iowa. But literally as I am writing this article, a Japanese beetle flies into my office through an open window - guess that answers my question! Some of you may have questions regarding identification of scarab beetles, or beetles in the insect family Scarabaeidae.

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.

japanese beetle larvae

Grubs are creamy white with a brown head capsule.1

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).

Japanese beetle life cycle.

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 damage

Japanese beetle causes leaves to be bronzed and lacy. Adults often mass on plants.1

japanese beetle life cycle

The annual cycle of Japanese beetle is like other scarab beetles in Iowa. 2


Table 1. Japanese beetle and other commonly mistaken scarab beetles in Iowa.beetle chart

Photo credits

1. David Cappaert, Michigan State University (

2. M. F. Potter, D. A. Potter, and L. H. Townsend, University of Kentucky (

3. Marlin E. Rice (

4. Jerry A. Payne, USDA-ARS (

5. Steven Katovich, USDA Forest Service (

6. K. V. Makarov (

7. Peter J. Bryant (


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 30, 2009. The information contained within may not be the most current and accurate depending on when it is accessed.


Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...