Replanting Necessitated by White Grub and Wireworm Damage, and a Note About Grape Colaspis

May 5, 2000
In previous issues of the Bulletin, we have discussed these insects in some detail. Obviously, insecticides to prevent damage caused by these insects should have been applied in high-risk fields either before or at planting. However, as everyone knows, anticipating white grub or wireworm problems is difficult. Reports from a few weeks ago suggested that white grubs might be quite numerous in some areas this spring, but assessments of individual fields are required for making decisions about whether to treat for these subterranean pests.

Duane Frederking with Pioneer informed us that several cornfields have already been replanted as a result of white grub damage. As all of you know, there are no rescue treatments for either white grubs or wireworms. 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. And the insecticide you decide to use during replanting, if you believe an insecticide is necessary, should be selected based on its control of either white grubs or wireworms, or both, depending on the insects in the field that is being replanted.

If you decide to use a soil insecticide during replanting, 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.

And as you think about white grubs and wireworms, don't forget grape colaspis, a pest that caused significant problems in west-central counties last year. The finding of grape colaspis larvae by Dale Bermester and Ria Barrido in southern Illinois indicates that the larvae are active and could begin causing noticeable damage soon. Remember that plants injured by grape colaspis larvae resemble plants injured by white grubswilted 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. I'll provide more information about this pest in next week's Bulletin.--Kevin Steffey

Author: Kevin Steffey