Corn Rootworm Larvae Found in Many Fields

June 18, 1999
During the past week, we've received a number of calls that indicate corn rootworm larvae can be found in some cornfields. Joe Spencer, an entomologist with the Illinois Natural History Survey, indicated that corn rootworm larvae could be found with relative ease in experimental plots near Champaign. Although it's too early to assess the full extent of root injury, it's not too early to begin examining some root systems for potential problems that may be just around the corner. By mid-July, corn rootworm larvae are typically finished with their root-feeding activities. In 1996 and 1997, egg hatch was delayed, and rootworm development was slowed considerably, resulting in larval injury through the end of July. We don't anticipate a prolonged feeding period this season. Bottom linea month from now, rootworm larval feeding should be nearly over for central Illinois and nearing completion in northern counties.

Each year at about this point in the season, we begin to receive a number of inquiries concerning the performance of soil insecticides for corn rootworms. Evaluating soil-insecticide performance for corn rootworm larval control requires some effort! Of utmost importance is the fundamental use of check strips (untreated rows) in a producer's field. Doing this means shutting off some insecticide boxes at planting in a few areas of a cornfield. Although this requires very little extra time during planting, many producers fail to perform this important step. Comparing root injury in treated and untreated strips throughout a field is the only way to determine adequately the value of your soil-insecticide investment for rootworm control. The following questions and answers are designed to provide some additional input on this topic.

I find very few rootworm adults in my cornfield. Doesn't this suggest that my soil insecticide worked pretty well?

No. Soil insecticides were designed to provide only one important functionroot protection. Research in Illinois and other states clearly indicates that, in certain years, more corn rootworm adults may emerge from treated than untreated areas of cornfields. If you don't find many corn rootworm adults in your cornfield, it may simply mean that you didn't have much of an infestation to begin with. Also, corn rootworm adults are very mobile (particularly the females) and may have left your field and flown to a nearby later-planted and more generously pollinating field.

If I don't have any severe lodging in my field, is it safe to assume that my soil insecticide performed adequately?

No. Corn plants lodge for a variety of reasons that may have nothing to do with corn rootworm larval injury. Plants that are top heavy (tall with large ears) may topple over in severe thunderstorms. Saturated soils may predispose plants, even those with no root injury, to lodging due to the weakened soil structure around the root system. Conversely, if soils are extremely dry and hardened, plants with severe root pruning may not lodge because they are held in rigid "concrete-like" growing conditions.

Aren't corn rootworm larvae almost impossible to find?

Locating corn rootworm grubs in early to mid-June requires some effort. First instars are quite small, and they tend to burrow into root tissue and feed internally. As rootworms reach the third and last larval instar, they become easier to find; however, a full-grown grub will not grow much beyond 1/2 inch in length. Figure 1 provides some information that can be used to determine larval instars for corn rootworms.

If I find corn rootworm larvae, how concerned should I be?

Don't be alarmed. Soil insecticides will not kill 100 percent of corn rootworm larvae within a field. Remember that soil insecticides are placed in-furrow or in a narrow band during planting. Untreated areas between rows effectively serve as a rootworm refuge. Because the entire rootworm larval population within a treated field is not exposed to a soil-insecticide application, resistance to commonly used products has not occurred for decades. Corn rootworm resistance to methyl parathion (*Penncap-M) has been confirmed in Nebraska. However, this resistance developed due to the widespread and long-term use of broadcast applications of *Penncap-M for egg-laying suppression programs. Those products preceded by an asterisk are restricted-use products and may be applied only by certified applicators.

When should I be concerned if I find corn rootworm larvae?

If you find three or more larvae per plant (7-inch cube of soil and roots from base of plant) and root injury is evident, a rescue cultivation treatment may be warranted. An insecticide applied during cultivation does not generally perform as well as a planting-time treatment. This difference is especially true under very dry soil conditions. Given the earlier and earlier planting we've witnessed in recent years, the application of rescue treatments at cultivation is increasingly no longer an option for many producers the plants are too tall.

How do I grade the performance of my soil insecticide?

To get an accurate picture of how well your soil insecticide worked, you should dig several (5 to 10) plants from about 10 different areas of your field. Following the digging, wash off the soil from the roots and look for any general feeding (brown scars); or, more importantly, examine the plants for pruned roots. Most entomologists across the Corn Belt suggest that a soil insecticide has done its job if it keeps root injury below a rating of 3.0 (several roots pruned to within 1.5 inches of the plant; never an entire node pruned) on the Iowa State root-rating scale. The economic root-injury index is static and varies according to the hybrid selected and the amount of precipitation that occurs throughout the growing season. Rainfall after the larval-feeding period is very important. The level of root regeneration from mid-July through mid-August may significantly affect yield.

Let's assume that I follow your suggestions and find very little root injury. Can I now assume that I got my money's worth for using a soil insecticide?

Perhaps, but only if roots also were removed from a check strip(s) and you found they were injured. Unfortunately, many producers are unwilling to leave some rows untreated at planting. If you don't leave some untreated areas in your cornfield, you'll never be able to estimate the real value of your soil insecticide purchase.--Mike Gray

Author: Mike Gray