Issue No. 5, Article 5/April 27, 2007
Some People Are Finding Lots of White Grubs
In addition to the report of survival of Japanese beetle grubs in last week's issue of the Bulletin (no. 4, April 20, 2007), we have received several reports of people turning up lots of white grubs, especially Japanese beetle grubs, during field cultivation in preparation for planting. So what do we do with this type of information? Well, if the grubs being turned up are Japanese beetle grubs, I wouldn't get terribly concerned. Our experience over the past few years has been that they don't seem to cause much noticeable injury to corn seedlings, at least not if there is plenty of soil moisture. On the other hand, white grubs with 3-year life cycles (Phyllophaga species) are definite threats to corn seedlings by their feeding on the roots, so the use of a seed- or soil-applied insecticide to control these grubs is warranted.
The type of white grubs being found in fields can be determined by examining the underside of the rear end of the grubs. The pattern of hairs on the raster, the name given to the underside of the last segment of the abdomen, signifies the type of white grub being examined. Japanese beetle grubs have a V-shaped arrangement of hairs, and Phyllophaga grubs a zipperlike arrangement. There is no apparent pattern of hairs on the rasters of Cyclocephala grubs, the masked chafers, which do not cause economic damage to corn roots. Figure 1 compares the rasters of these three types of white grubs. Also, Marlin Rice, extension entomologist at Iowa State University, has published several very good photos of rasters of white grubs in issues of Integrated Crop Management over the years.
Figure 1. Raster patterns of (A) Popillia japonica (Japanese beetle), (B) Cyclocephala (masked chafer), and (C) Phyllophaga ("true" white grubs) grubs.
As requested for black cutworms and preventive control measures, we would appreciate receiving any reports of efficacy, or lack thereof, of products (especially seed-applied insecticides) used for control of white grubs. There is very little data on efficacy of seed-applied insecticides for control of white grubs in field situations, so we often have to rely on experiences to address concerns or answer questions.--Kevin Steffey