During the past several years, producers throughout Illinois have reported increasing problems with grubs (white grubs, wireworms, grape colaspis) in corn. Speculation to date suggests that factors such as early planting, mild winters, and poor insecticide performance may be responsible for the escalating grub-related damage in some cornfields. Some questions and answers are provided and should shed some light on the biology and management of white grubs. |
What types of white grubs are most frequently found in Illinois cornfields?
Annual and true white grubs easily can be found in any producer's cornfield throughout Illinois. The adults of annual white grubs are called southern masked chafers, whereas the adults of true white grubs are called May or June beetles. Southern masked chafers are approximately 1/2 inch in length and have yellowish brown wing covers, and the head is "mask-like" in appearance. True white grub adults are somewhat larger (3/4 inch long) and reddish brown to nearly black. Along the eastern edge of Illinois and in pockets of fields surrounding metropolitan areas of the state, Japanese beetle grubs also can be found. Japanese beetle adults are the most "handsome" of the commonly observed grub species in Illinois cornfields. The adults are shiny metallic green with bronzed wing covers. On each side of the main body are numerous tufts of white bristles.
How easily can larvae of white grub species be identified?
At first glance, separating species of white grub larvae may seem daunting. With a little practice, however, this task can be accomplished easily with a magnifying glass, and dividends can be reaped for making the correct identification and resulting management decision. Collectively, white grubs (as the name suggests) are creamy white and strongly C-shaped, possess three pairs of long legs, and have orange-brown heads.
The posterior end of grubs is typically darkened by fecal material in the hindgut. By examining the arrangement of bristles on the lower surface of the last abdominal segment (raster), white grub larvae can be identified correctly. Annual white grub larvae have scattered hairs on the raster, in contrast to the two parallel rows of bristles that can be observed (some magnification helps) on true white grub larvae. Japanese beetle larvae display a V-shaped pattern of bristles on their rasters (Figure 2).
Why is it important to separate species of white grubs?
The potential to inflict economic losses varies considerably among the grub species. Annual white grub larvae typically cause only minor injury to seedling corn plants. In experiments conducted by entomologists at Iowa State University, densities of nine annual white grubs per plant did not prevent emergence or reduce stand counts. In recent years, we have speculated that annual white grub larvae may be inflicting more of an economic "punch" because of the early planting dates and the lengthening of the feeding period. True white grubs are often implicated in stripping root hairs from corn plants. Seedling plants that are injured may become yellowish, wilt, and die when infestations are severe. In some instances, corn plants may become purple due to their inability to adequately absorb phosphorus. Like annual white grubs, larvae of the Japanese beetle are generally not considered of economic importance to corn production. However, in summer issues of the Bulletin, we will discuss the potential economic impact of silk-clipping activities by adult Japanese beetles.
Why are true white grub larvae of more importance than annual or Japanese beetle larvae?
Annual white grubs and Japanese beetles have 1-year life cycles in the Corn Belt. Pupation of annual white and Japanese beetle grubs most often takes place from mid- to late May, and in some years, early June. Because feeding ceases during pupation, annual white and Japanese beetle grubs are unable to inflict much injury to seedling corn plants. However, as mentioned previously, the trend toward earlier planting has changed this scenario to some extent. True white grubs require 3 years to complete a single generation. During the second summer of a true white grub's life cycle, severe injury can be dealt to a corn plant's root system. For this reason, accurately identifying the grub species in a producer's cornfield is essential in making the correct management recommendation.
Are some cornfields at greater risk to true white grub infestations?
Yes. Researchers who conducted experiments in the northern plains revealed that cornfields with borders planted to cottonwood or willow trees supported greater densities of true white grubs. Apparently, true white grub adults were fond of feeding on leaves in the windbreaks during the evening. During the day, females were prone to leave trees and travel to adjacent cornfields to deposit their eggs in the soil. Over time, densities of true white grub larvae in these cornfields increased to economic levels. In addition, fields that will be coming out of sod and devoted to corn production also are at great risk for economic infestations of white grubs.
Are there any good scouting approaches and thresholds for true white grubs?
Not really. Researchers have recommended taking spring soil samples (1 foot by 1 foot by 6 inches) and determining the number of larvae per sample. Twenty soil samples per 40 acres are suggested. Stands may be significantly reduced by as few as one grub per square foot of soil. Rescue treatments are not an option, so making the correct decision regarding the use of a soil insecticide at planting is crucial (Table 1).
We encourage individuals to report white grub damage by email to firstname.lastname@example.org. When you report damage, please include date of discovery, field location (county, township, section number), level of damage, and grub species.--Sue Ratcliffe and Mike Gray