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Seed Treatments and Consistent Corn Rootworm Control: Not a Proven Strategy

November 3, 2000
By now many of us have seen the bewildering array of advertisements touting the ability of seed treatments to provide a satisfactory level of corn rootworm larval control. Many entomologists, including us, believe advancements in seed treatment technology offer real promise for control of many soil insect pests. In addition, the potential benefits to producers, especially the ease of use, make seed treatments an attractive option compared with the use of traditional soil insecticides. Assuming insecticidal control options are roughly equal in cost, producers would be more than willing to switch to a technology that eliminates the hassle of pouring an insecticide into planter boxes and then worrying about the accuracy of calibration settings. Despite these potential benefits that an insecticidal seed treatment offers, please consider that these pluses lose their luster rapidly if corn rootworm larvae are not adequately controlled. No producer wants to harvest acres of severely lodged corn at the end of a growing season.

For years, we have pointed out repeatedly that about 50% of the cornfields in Illinois do not have economic infestations of corn rootworm larvae. Researchers in other states have made similar observations. So, seed treatments, as well as soil insecticides, stand an excellent chance of "working" very well on at least half of Illinois cornfields in any given season. Don't be fooled with testimonials that suggest corn rootworm products worked very well unless check strips (untreated areas) also were established in fields. Root ratings need to be compared from treated and untreated sections of fields. This procedure is practiced infrequently. Over time, producers have come to rely on soil insecticides as corn rootworm "insurance plans." In essence, an investment of approximately $15.00 per acre hopefully buys some "peace of mind." Although we have recommended scouting and the use of thresholds in making more informed decisions about the use of soil insecticides, most producers still do not follow this recommendation. Now that the entire class of organophosphate insecticides, in particular some popular soil insecticides, are being scrutinized by the Environmental Protection Agency, we wonder if the same level of scrutiny would now be focused on this class of products had they been used more judiciously in the past.

We view the use of seed treatments for corn rootworm control with considerable skepticism at this point. They represent promise for the future; however, for now, their promise is unfulfilled. The insecticidal seed treatments ProShield (tefluthrin is the active ingredient) and Prescribe (imidacloprid is the active ingredient) are not recommended by the University of Illinois for corn rootworm control in 2001. Through a unique technology, Novartis is able to coat Northrup King seeds with tefluthrin (same active ingredient as Force 3G, Zeneca Ag Products). Tefluthrin is not systemic and is therefore not taken up by the plant. Limited root protection may be due to the potential repellancy of tefluthrin to corn rootworm larvae as well as to some direct toxic effects. Gustafson will market two systemic seed treatment products in 2001 with the same active ingredient (imidacloprid). The amount of active ingredient per seed dictates product identity. Imidacloprid will be applied to seeds at a rate of 0.165 milligram per seed and marketed as Gaucho for the control of secondary insects such as wireworms and seedcorn maggots. Imidacloprid also will be applied at a higher rate (1.34 milligrams per seed) to some seeds and marketed as Prescribe for the control of corn rootworms.

Root-rating data summarized previously from University of Illinois insecticide efficacy trials in DeKalb, Monmouth, and Urbana clearly point out the weaknesses in both ProShield and the 1.3-milligram application rate of imidacloprid for corn rootworm control (issues no. 17 and no. 19 of the Bulletin). Efficacy data from other land-grant institutions such as Iowa State University also point out the inconsistent nature of root protection afforded by ProShield or imidacloprid. In 2000, ProShield and imidacloprid provided only 22% and 9% consistency ratings, respectively, when averaged across six test locations in Iowa. Consistency was based on the percentage of times a product limited the node-injury rating to no more than 25% of a single node. Using this same benchmark, consistency ratings for other soil insecticides were as follows: Force 3G, T-band96%; Aztec 2.1G, T-band96%; Fortress 5G, T-band95%; Force 3G, furrow94%; Counter 20CR, T-band89%; Fortress 5G, furrow86%; Lorsban 15G, T-band83%; Counter 20CR, furrow76%; Capture 2EC, T-band75%; Thimet 20G, T-band66%; and Regent 4SC, furrow, microtube51%. Consistency in the check was 13% and not statistically different from either ProShield (22%) or imidacloprid (9%).

These data, along with data from other university trials in the Corn Belt, clearly indicate that seed treatments do not offer "top-flight" corn root protection against corn rootworm larvae. Potential buyers should be fully aware that depending on these products to provide consistent corn rootworm control could be a costly mistake. It is our opinion that these seed treatments, as currently designed and marketed, are not up to the challenge of protecting roots from injury against moderate to heavy pressure by corn rootworm larvae.

For now, if you're interested in buying corn rootworm insurance, there are better plans on the market.--Mike Gray and Kevin Steffey

Author: Kevin Steffey Mike Gray

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

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