Issue No. 7, Article 4/May 12, 2006
Plenty of White Grubs Being Found in Cornfields
We have received numerous reports from people who have encountered relatively large numbers of white grubs in cornfields this spring. Thus far, although very little injury to the corn seedlings has been observed, the numbers of grubs being found (as many as 10 per foot of row) have people concerned. Although not all reports have included identification of the grubs to species, at least a couple of observers have confirmed that the grubs they found were Japanese beetle grubs.
Over the past several years, we have tried repeatedly to establish research trials to examine the efficacy of selected soil and seed-applied insecticides against Japanese beetle grubs. Unfortunately, we have had no luck in obtaining much useful data. Although many of the trials were established in fields where large numbers of grubs were present, we were never able to detect any differences in the amount of injury among the different treatments and the untreated check. In fact, we almost never observed injury in the untreated check plots. With counsel from Dr. Phil Nixon, a University of Illinois entomologist with experience with Japanese beetle grubs in turf, we have speculated that the grubs in our trials fed on organic matter rather than on corn roots. Although people have observed situations in which Japanese beetle grubs caused injury to corn seedlings, it is likely that Japanese beetle grubs cause very little injury to corn seedlings in most fields during most years. Nonetheless, large densities of white grubs are worth noting. If the growth of the corn seedlings is slowed, the effects of white grubs feeding on the roots become noticeable.
White grub and injured corn seedling. Note the purple stem and wilted leaves.
It's also important to determine the type of white grubs that are found. Masked chafer grubs and Japanese beetle grubs (both annual white grubs) may cause little injury to corn, but Phyllophaga grubs (3-year life cycle) can cause substantial injury by chewing off root hairs and small roots. Aboveground symptoms of grub injury include wilting and purpling and occasionally plant death, resulting in reduced plant populations. However, many other factors cause purpling, too, so make certain grubs are present before you blame purple plants on grub injury.
White grubs can be identified "to type" by examining the pattern of hairs on the underside of the last abdominal segment (raster). Japanese beetle grubs have a V-shaped pattern of hairs on the raster, and Phyllophaga grubs ("true" white grub) have two parallel rows of hairs on the raster. Southern masked chafers have neither of these patterns on the raster.
Figure 1. Line drawings of a white grub and raster patterns of the southern masked chafter, Japanese beetle, and Phyllophaga (true white grub) grubs.
We would appreciate receiving any information you have about the effectiveness of soil or seed-applied insecticides against white grubs. We have more trials established this spring, but there is no guarantee that they will generate the type of information we seek. Therefore, your input could help us get a handle on the effectiveness of different products for control of white grubs. In greenhouse studies, Marlin Rice, extension entomologist at Iowa State University, determined that both the low and high rates of Cruiser and Poncho provided very good protection of seedling corn plants when the density was two grubs per plant. However, results from field tests are few and far between. Maybe we'll get lucky this year.--Kevin Steffey