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Reports of White Grub and Grape Colaspis Damage

May 11, 2001
Many of you will recall that grape colaspis larvae caused some significant damage over the past couple of years in fields of corn planted after soybean in west-central and western counties. Well, let's make it 3 years in a row. I received my first report of grape colaspis injury this morning from Don Rhodes, Burrus Power Hybrids, Arenzville in Morgan County. Don and Kevin Adams with Bayer reported aboveground symptoms in fields at the V3 and V4 development stage. Further investigations confirmed grape colaspis was responsible for the damage. Remember that plants injured by grape colaspis larvae resemble plants injured by white grubs (wilted leaves and purple coloration). In addition, the edges of the leaves may appear yellow or burned. Grape colaspis larvae chew off root hairs, and injured plants cannot take up water and phosphorus efficiently. Severe infestations may cause plant death and reduced plant populations. Injury is more severe when weather conditions retard the growth of the seedlings.

Duane Frederking with Pioneer informed us that several cornfields have already been replanted as a result of white grub damage in Menard County near Greenview. In addition to white grub larvae, the fields contained grape colaspis larvae. In fields near Beardstown in Schuyler County and Hardin in Calhoun County, Duane reported white grub adult emergence and the presence of larvae. Pete Fandel, Extension unit educator, crop systems, Woodford Extension Unit, has found white grub larval damage in numerous fields. The level of damage, on average, is 1 out of every 10 plants. Mark Hoard, Extension educator, Integrated Pest Management, Mt. Vernon Extension Center, sent an excellent photo of the raster of a Japanese beetle larva.

As all of you know, there are no rescue treatments for either white grubs or grape colaspis. After damage by these pests is discovered, the only reasonable response is to determine whether the current stand will provide the yield hoped for. If the plant population has been reduced substantially, replanting might be the right thing to do.

If you think you need to replant because of insect damage, make certain you identify the guilty insect correctly. We know that annual white grubs can cause some injury to early-planted corn, but they do not cause as much damage as "true" white grubs. For more information on white grubs, refer to the article "White Grubs: Expectations and Management Recommendations for 2001" in the April 13, 2001, issue of the Bulletin.

If you decide to use an insecticide during replanting, it should be selected based on its control of the insects in the field that is being replanted, and be aware that there are some restrictions regarding the amount of an insecticide that can be applied per season. For some insecticides, if you applied a full rate during the first planting, the insecticide cannot be applied during replanting because the amount will exceed the maximum allowable amount. Do not exceed the following amounts of specific products per acre per season: 7.3 lb of Aztec 2.1G; 6.5 lb of Counter CR; 13.5 lb of Lorsban 15G; 4.2 oz of Regent 4SC; 6.5 lb of Thimet 20G. The language on the label of Force 3G states: Use Force 3G only once per crop.--Susan Ratcliffe and Kevin Steffey

Author: Kevin Steffey Susan Ratcliffe

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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