White grubs found prior to planting in rotated corn or continuous corn that will not receive a soil insecticide may cause concern. True white grubs and annual white grubs look similar, but both groups do not cause stand loss in corn. Here is a brief description of three types of grubs in Iowa corn and suggested recommendations.
The good. Annual white grubs and Aphodius grubs have not been reported to cause stand loss to either corn or soybean. Experiments I conducted in a greenhouse showed that as many as nine annual white grubs per soybean or corn seedling did not result in plant death. Annual white grubs are close to changing from larvae to pupae when corn planting occurs so this may be part of the reason we dont see stand loss. Annual white grubs are identified by the absence of two parallel rows of short, stiff hairs on the bottom side of the tail. Aphodius grubs are most easily identified by their small size. Annual white grubs and Aphodius grubs feed on dead organic matter in the soil, making them good insects. No insecticides are recommended if only annual white grubs or Aphodius grubs are found.
The bad. True white grubs will kill seedling corn. The larvae live in the soil for part of three years and can damage corn during their last two summers. True white grubs can be identified by the parallel row of short, stiff hairs on the bottom side of the tail, which is commonly called the zipper. Annual white grubs do not have this zipper. If true white grubs are found prior to planting, a soil insecticide should be used, preferably with some of the chemical placed in the furrow.
The ugly. They're all ugly!
This article originally appeared on page 35 of the IC-476 (5) -- April 15, 1996 issue.