Performance of corn rootworm insecticides

Two integrated pest management strategies are used widely in Iowa to protect corn roots from corn rootworm injury: crop rotation and insecticides. The biological basis for crop rotation is simple. When corn is rotated with soybeans, the larvae that hatch during the soybean crop can't survive on soybean roots and therefore starve. Crop rotation is a highly successful method of preventing corn rootworm injury in corn the following year in most situations. The one notable exception is in northwestern and north central Iowa. Here some northern corn rootworm eggs survive the soybean rotation and don't hatch until two years after they are laid. This two-year delay in the hatch is called extended diapause.

Corn rootworms can injure corn roots.

If corn is not rotated, or if extended diapause has been documented to occur in a particular field, then a soil insecticide might be necessary to protect the roots. I say "might" be necessary because many fields do not have a rootworm population of a sufficient size to cause economic damage. Believe it or not, there are thousands of continuous corn fields across the state in which a rootworm insecticide is not necessary. However, fields should have been scouted last August to determine the size of the beetle population and the necessity of an insecticide in 1997. It is extremely difficult to know whether you need an insecticide if the fields were not scouted.

Heavy feeding injury caused by rootworm larvae.

Insecticide tests. Iowa State University has evaluated corn rootworm insecticide granules in side-by-side field trials for many years. These field trials have been conducted at Ames, Cedar Rapids, Chapin, Nashua, Newell, and Sutherland during the last five summers. The field trials allow us to measure the performance of these insecticides in protecting corn roots under a wide range of environmental conditions. Performance is measured two ways: root ratings and percentage consistency.

Root ratings

Injury by corn rootworm larvae can result in a significant amount of the roots being injured or removed from the plant. This injury can be rated on a numerical scale from 1-6 (shown in Table 1). A low root rating, usually a 3 or less, is highly desirable. This low root rating indicates that the insecticide adequately protected the roots from injury. Each insecticide was measured under moderate to heavy feeding pressure. The roots from the untreated plots always averaged a root rating larger than 3.0 and have averaged 4.55 during the last five years.

Table 1. Iowa 1-6 root rating scale.
1 no injury or only a few minor feeding scars
2 feeding injury evident, but roots not eaten back to within 1 1/2 inches in length
3 at least one root eaten to within 1 1/2 inches in length, but never an entire node (ring of roots) destroyed
4 one node (ring of roots) eaten to within 1 1/2 inches in length
5 two nodes (two rings of roots) eaten to within 1 1/2 inches in length
6 three nodes (three ring of roots) eaten to within 1 1/2 inches in length

Percentage consistency

Looking at root ratings and how these really translate into the performance of an insecticide can be difficult to understand. Therefore, I prefer to measure the performance of an insecticide by how consistent it is over time. Consistency is measured by the percentage of tests in which the insecticide-treated roots averaged a rating of 3.0 or less when they were attacked by moderate or large populations of corn rootworm larvae. An easy way to understand percentage consistency is to think of it as being similar to a baseball batting average; the higher the number, the better the performance.


No insecticide was 100 percent consistent in providing adequate protection (a root rating of 3.0 or less) during the last five years. Even the best products sometimes failed. However, most products gave very good root protection most of the time, based on the percentage consistency. From a statistical standpoint, everything in Table 2 from Counter 20CR down to Dyfonate 15G provided similar levels of consistency. Thimet 20G and Fortress 2.5G t-banded did not perform well under heavy rootworm pressure.

Table 2. Five-year summary of corn rootworm insecticide performance,

1992-1996, Iowa State University

Insecticide Placement Ounces of product per 1,000 row ft. Percentage consistency(2) Root rating
Counter 20CR t-band 6 95a 2.05
Force 3G(3) t-band 4 93a 2.30
Fortress 2.5G(4) furrow 6 93a 2.32
Counter 20CR furrow 6 93a 2.40
Force 3G furrow 4 93a 2.42
Lorsban 15G t-band 8 91a 2.47
Aztec 2.1G furrow 6.7 90a 2.45
Aztec 2.1G t-band 6.7 88a 2.42
Dyfonate 15G(5) t-band 8 79ab 2.61
Thimet 20G t-band 6 69b 2.89
Fortress 2.5G t-band 6 67b 2.85
untreated check - - 0c 4.55


(1) data from 13 tests, 58 replications

(2) numbers followed by the same letter are not significantly different (P=0.05)

(3) Force 1.5G tested 1992-1995, Force 3G tested in 1996 (same rate of active ingredient used all four years)

(4) Fortress 5G tested 1992-1993, Fortress 2.5G tested 1994-1996

(5) Dyfonate II 20G tested 1992-1993, Dyfonate II 15G tested 1994-1996

Final thoughts

Performance is just one factor to consider when using a corn rootworm insecticide. With so many products having similar levels of consistency, other factors worthy of consideration might be cost, pounds of active ingredient being applied per acre, ease of handling, application equipment needed, other pests controlled, restricted use labeling, and potential hazards to surface water.

Thanks to Jim Oleson, research associate, Department of Entomology, for the root rating data in this article and his dedication to the scientific evaluation of insecticide performance.

This article originally appeared on pages 1-2 of the IC-478 (1) -- February 3, 1997 issue.

Updated 02/02/1997 - 1:00pm