Several calls and samples from eastern Iowa counties would suggest this is another "banner year" for the false Japanese beetle, Strigoderma arbicola. The false Japanese beetle is a member of the Junebug family with a vague resemblance to the famous imported pest, the Japanese beetle. See the table below for a comparison of diagnostic characteristics.
|Descriptions||Japanese Beetle||False Japanese Beetle|
|Size / Shape||7/16" / nearly square||7/16" / longer than wide|
|Colors||thorax: shiny green||thorax: brown to slightly green|
|wing covers: coppery brown||wing covers: dark tan to brown|
|overall: colorful and shiny||overall: drab|
|Abdomen||five small, white tufts of hair along each side||no white tufts of hair|
|Distribution||limited to 5 eastern Iowa counties as of 1999||statewide, but more common in EC Iowa|
False Japanese beetles are occasionally a significant pest in soybean fields and flower gardens. The beetles are leaf feeders and cause large, irregular holes in foliage, flowers and buds. Damage is especially common on white and yellow roses and other flowers.
The adult beetles are usually present for only 2 weeks in late June and early July, during which time they mate and lay eggs in the soil. The larvae are white grubs found in the soil of grassy, sandy areas where they feed on plant roots. They have not been reported as a soil pest in Iowa. There is only one generation per year.
Handpick the beetles in the garden if infestations are light or consider fabric covers for high value plants (for example, screening over individual rose bushes). Garden insecticides can be used but results may be disappointing, as insecticide residues may not control new beetles that fly in. Sprays will probably be more effective than dusts. Check the labels of home garden and landscape insecticides for the list of plants that can be sprayed. Avoid treating flowers that are being visited by bees.
This article originally appeared in the July 2, 1999 issue, p. 92.