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Large Numbers of Corn Rootworm Larvae Reported in Some Cornfields

June 15, 2001
We have received a few reports of corn rootworm larval injury, in some cases from fields that were treated at planting with a soil insecticide. Kevin Black, technical specialist with GROWMARK, confirmed moderate to severe pruning of plants was occurring in a field located in Pike County, just south of Kinderhook. Kevin and Rich Archer, Pike County Service Company, observed numerous larvae in the soil surrounding the plant samples they took. The majority of larvae observed were second and third instars; however, they also found a few pupae. This means that emergence of adult western corn rootworms will begin across central Illinois by the end of the third week in June. As Sue Ratcliffe mentions in one of her articles in this issue of the Bulletin, those of you who intend to monitor adult populations of corn rootworms this season better make sure that you have your traps ready to go. Evaluating soil insecticide performance for corn rootworm larval control requires a little effort. The following questions and answers are designed to provide some additional input on this topic.

Aren't corn rootworm larvae almost impossible to find?

Locating corn rootworm grubs in 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% 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 because of the widespread and long-term use of broadcast applications of Penncap-M for egg-laying suppression programs.

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 compared with a planting-time treatment. This is especially true under very dry soil conditions. Because of 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 because 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 (treated and untreated strips) 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. The use of four widely separated check strips (two to four rows per strip) works well for this purpose. Comparing root injury in treated and untreated strips throughout a field is the only way to adequately determine the value of your soil insecticide investment for rootworm control.

If I find very few rootworm adults in my cornfield in July, doesn't this suggest that my soil insecticide worked pretty well?

Not necessarily. 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 because of 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.--Mike Gray

Corn rootworm damage to roots, Pike County, June 12, 2001. (Photo courtesy of Kevin Black, Growmark.)

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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