No. 3/April 10, 1998
Discovering White Grubs During Spring Tillage Operations
Each spring, some producers discover white grubs in their fields during spring tillage passes. Finding white grubs churning out of the soil beneath a field cultivator does cause some justifiable concern. White grubs typically cause damage to less than one percent of the corn acreage in Illinois; however, when present in economic densities within a given field, white grubs can cause significant stand reductions.
Should I use a soil insecticide at planting if I discover white grubs during a spring tillage pass? Maybe. If you determine that you have true white grubs, the answer is "yes." If you have annual white grubs, the answer is "no." True white grubs have a 3-year life cycle, and grubs have the potential to feed all summer long on developing root systems. Annual white grubs complete one generation each year and, by May or early June, pupate and emerge as May or June beetles. Bottom line--annual white grubs are not around long enough during the growing season to worry about.
In 1994, Marlin Rice, Extension entomologist with Iowa State University, published the results of his research on this topic. The results of his investigation clearly support our recommendation not to treat for annual white grubs. In his experiment, he infested corn and soybean seedlings with annual white grubs in a greenhouse. In this study, individual corn or soybean plants were grown for 27 days and infested with three, six, or nine annual whitegrubs per plant. Uninfested checks (no grubs) also were included as a treatment. The results were clear for soybeans: Seedling emergence, dry root weights, total dry weights, and leaf areas were not affected by annual white grubs. The results for corn were almost identical, with the exception that, for one year of the experiment, corn plants infested with nine grubs per plant caused corn seedlings to be smaller, as compared with the control plants (no grubs). The conclusion of this study: Don't use a soil insecticide to protect corn or soybean plants from annual white grubs.
Is it difficult to separate true white grubs from annual white grubs? No (if you've done it before). Yes (if you're attempting it for the first time). With a little practice, you can easily impress your neighbors with the relative ease by which you separate injurious from noninjurious white grubs. Figure 3 depicts the pattern of bristles on the undersurface of the last abdominal segment of true and annual white grubs. True white grubs appear to have a "zipper" comprised of two parallel rows of bristles. Annual white grubs lack a "zipper" and instead have bristles in no particular pattern.
Figure 3. White grubs are characterized by a C-shaped, milky white body (left) that is 1/2 to 1-1/4 inches long, and a brown head. The underside of the last abdominal segment distinguishes annual white grubs (middle) from true white grubs (right).
Checking ventral surface of last abdominal segment of white grub.
Are there any established thresholds for white grubs in commercial corn? No. Entomologists from Purdue University have suggested that, for seed-productions fields, a density of "two or more live white grubs per cubic foot of soil prior to planting may signal a potential problem."
Should a field coming out of sod and planted to corn be treated with a soil insecticide? Yes. Having economic infestations of white grubs and wireworms at this stage is almost a given.
Which soil insecticides are recommended for white grubs control? Insecticides listed in the Illinois Agricultural Pest Management Handbook include *Aztec 2.1G, *Counter CR, *Force 1.5G/ 3G, *Fortress 2.5G/*Fortress 5G, and *Thimet 15G/20G. (* = Use of product restricted to certified applicators only.) Lorsban 15G also is listed for white grub control; however, it is not known to be an exceptional white grub insecticide. Lorsban 4E is labeled for use as a broadcast pre-plant incorporated product and may offer an option for some growers who do not have insecticide boxes on their planters.
Does placement of an insecticide make a difference when it comes to white grub control? My preference for white grub control is to go with an in-furrow treatment. This rules out the use of Thimet 15G or 20G, which can be used only as a band application.
Are there any rescue treatment options for white grubs? No. If white grubs begin to cause a significant stand reduction, replanting and the use of a soil insecticide during the replant operation is the only option (other than doing nothing).
Mike Gray, (firstname.lastname@example.org), Extension Entomology, (217)333-6652