Despite the fact that early-season insect pests of corn should finish feeding soon, discoveries of the feeding results are still being made. Dave Feltes, Extension IPM educator in the Quad Cities, visited a field on June 6 and observed uneven plant heights in a field that had looked fine just a week earlier. A couple of good corn-growing days last week allowed uninjured plants to outpace the grub-injured plants, so a height differential became obvious. Dave identified the grubs in the field as a combination of Phyllophaga grubs ("true" white grubs) and Japanese beetle grubs. Interestingly, a quick overview of some of the data we have gathered from one of our Japanese beetle grub trials in Piatt County revealed no pattern in damage among the treatments in the trial. In other words, the untreated check plots didn't seem to have any more or less grub damage than most of the insecticide-treated plots in the trial. This is contrary to reports we have received this spring of significant damage caused by Japanese beetle grubs. Keep in mind, however, our data are preliminary. The data may reveal some trends upon further investigation.
Mike Hellmer, field sales agronomist with Pioneer Hi-Bred International, continues to find the consequences of grape colaspis injury in some cornfields in east-central Illinois. In a 40-acre field near Hope (Vermilion County), Mike observed 10 to 12 grape colaspis larvae per plant. About 25 acres of the 40-acre field were damaged severely enough that replanting was warranted, even at this late date. Injured plants were dead or dying.
Warmer temperatures should help corn plants injured by white grubs or grape colaspis grow past the injury. As long as the plants live and seem to recover, they probably will produce ears. However, we are not certain what effect these early-season setbacks in growth might have on plant yields. One of the objectives of our study is to address that uncertainty. We'll let you know what we learn.--Kevin Steffey