Issue No. 8, Article 2/May 16, 2008
White Grubs and Their Subterranean Comrades
In last week's issue of the Bulletin (No. 7, May 9, 2008), Mike Gray briefly addressed the potential effects of late planting and cool weather on white grubs, among other insects. Several entomologists in the Midwest also have addressed white grubs in their weekly newsletters, focusing primarily on grubs discovered during tillage or planting. On May 13, we received a report of white grubs in a field of seedling corn, although there was no apparent injury to the seedlings. Jim Donnelly, field advisor for Monsanto, observed Japanese beetle grubs (0.5 to 1 per seedling plant) in a field in Bureau County. Our past experience with trying to determine whether Japanese beetle grubs would cause significant injury to corn seedlings has been frustrating. But more often than not, Japanese beetle grubs cause more injury to corn seedlings when the weather is dry. During periods of wet weather, the grubs seem to be able to find enough food and moisture elsewhere in the soil and often leave corn roots untouched.
Japanese beetle grubs in a cornfield in Bureau County, May 2008 (photo courtesy of Jim Donnelly, Monsanto Company).
Because cool, wet weather slows the growth of corn plants, seedling corn is exposed to injury by subterranean insect pests for a longer period. The white grub species of most concern under such conditions is the so-called "true" white grub in the genus Phyllophaga. When you get opportunities to walk fields and assess corn stands, make sure you know how to identify grubs, in case you encounter some reduced stands and find grubs while digging for the cause. The arrangement of bristles and hairs (the raster pattern) on the undersides of the rear ends of white grubs should allow you to categorize the type (Figure 1). Look for the "zipper" on Phyllophaga grubs, the "V" on Japanese beetle grubs, the seeming scattering of hairs on Cyclocephala grubs, and the semicircle on Asiatic garden beetle grubs (more on these in the next paragraph.) Phyllophaga grubs can cause severe stand reductions in cornfields, whereas annual white grubs (including Japanese beetle grubs and masked chafer grubs [Cyclocephala species]) infrequently or rarely cause noticeable injury. If you detect a reduced stand and find annual white grubs easily, you might want to continue digging for other culprits. You don't want to be fooled by the presence of annual white grubs if other subterranean insect pests are at fault (e.g., black cutworm, seedcorn maggot, wireworms).
Figure 1. Raster patterns for (from left to right) Phyllophaga grubs, Japanese beetle (Popillia japonica) grubs, Cyclocephala grubs, and Asiatic garden beetle (Maladera castanea) grubs. (Illustrations from "Identification of White Grubs in Turfgrass," The Ohio State University FactSheet HYG-2510-94, author David J. Shetlar.)
The Asiatic garden beetle has been found in Indiana and Michigan recently, but not so far as we know in Illinois. Be on the lookout for this invasive species as you scout for grubs in general. In addition to the characteristic raster pattern (Figure 1), you can identify Asiatic garden beetle grubs by the characteristic white, enlarged "cheeks" on each side of the head. These are actually enlarged portions of the maxillae (part of the mouthparts). If you happen to discover this insect this spring, please let us know. If you are interested in learning more about it, refer to 2007 and 2008 articles in Purdue University's Pest & Crop Newsletter and Michigan State University's Field Crop Advisory Team Alerts newsletter.--Kevin Steffey