Integrated Crop Management

Dogbane beetles are not pests

It happens at least once or twice every year; someone finds a brilliant metallic green beetle in a corn or soybean field and asks if it is a pest. The dogbane beetle is fairly large for an Iowa beetle, about 3/8-1/2 inch in length, but it feeds only on dogbane and milkweed. The larvae feed on the roots of these two plants; therefore, this insect is fairly common along roadsides, in pastures, and grassy filter strips in fields where these plants grow.

Dogbane beetle.

One of the most intriguing characteristics of the dogbane beetle is the kaleidoscope of brilliant colors. The Illinois State Museum [2] explains this phenomenon. The dogbane leaf beetle has a special type of color that shines and changes as the insect changes position or as we change position while looking at it. This changing color is called iridescence. The beetles' iridescence is produced by special body structures and light. The surface of the body parts of this beetle is made up of stacks of tiny, slanting plates, under which is a pigment (substance that produces color). Some light rays reflect from the surface of the plates, and other light rays reflect from the pigment underneath. At different angles, the light reflects at different speeds, causing interference that result in our seeing different colors that shine.

If you can find some dogbane plants, now is the time of year to stop, look, and appreciate a beautiful insect that does no harm.

This article originally appeared on pages 133-134 of the IC-490(18) -- July 28, 2003 issue.

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