Issue No. 1, Article 8/March 18, 2004
Seed Treatments and Bt Corn Rootworm Hybrids: Perspectives on Control and Influence on Nontarget Populations
Considerable interest this spring surrounds the anticipated increased use of systemic insecticidal seed treatments Poncho (clothianidin) and Cruiser (thiamethoxam) for corn rootworm (1.25 mg per kernel rate for both products) and secondary soil insect control (0.25 mg per kernel for clothianidin and 0.125 mg per kernel for thiameth-oxam). These nicotinoid products, as the name suggests, are chemically related to the toxin found in tobacco known as nicotine. They are effective and persistent nerve poisons that interfere with the transmission of nerve impulses. Results from our corn rootworm insecticide efficacy trials last year suggest that Poncho and Cruiser, when used at the higher corn rootworm rates, performed at acceptable levels in Urbana. Results for these products in DeKalb and Monmouth were somewhat inconsistent. To date, our message regarding the use of seed treatments for corn rootworm control has been straightforward regarding their inconsistency as compared with some of the traditional granular soil insecticides. However, we recognize that because of convenience and the perception that these seed treatments provide root protection under most field conditions, producers are eager to embrace these products. We will continue to learn more about their performance under a wide variety of environmental conditions. In 2003, our insecticide efficacy trials were planted late by producers' standards, and we received generous rainfall throughout the growing season. It remains uncertain how well the nicotinoid seed treatments will perform under drier soil and earlier planting conditions.
An article published in the December 2003 Journal of Economic Entomology reported the results of a 5-year (1997-2001) experiment conducted in northeastern Spain on the impact of imidacloprid (Gaucho) on nontarget arthropod populations in corn. Imida-cloprid is a nicotinoid insecticide. Corn seed was treated with Gaucho 35FS (4.9 g [AI]/kg). Nontarget arthropod densities were estimated by visual counts as well as pitfall traps in corn plots in which seed was treated with imidacloprid. Estimates of arthropod densities also were determined in untreated corn plots. The check plots had not been treated with soil insecticides since 1992. The corn plots in which imidacloprid was used had been previously treated with carbofuran. Based on visual counts, densities of spiders (Araneae), lady beetles (Coccinellidae), and rove beetles (Staphylinidae) were not affected by imidacloprid. Ground beetle (Carabidae) numbers were greater in untreated plots in only one year of this study. Rove beetle densities were lower as measured by pitfall traps in plots treated with the seed treatment. The numbers of ground beetles and spiders caught in pitfall traps did not significantly differ between treated and untreated plots. We need to continue studies of this type and generate additional data from multiple locations, but these initial results are encouraging.
Ground beetle larva.
Ground beetle adult.
In addition to increasing our knowledge of potential effects of nicotinoid products on beneficial insects, we need to continue gathering field data regarding the use of Bt corn rootworm hybrids and their potential influence on nontarget arthropods. In August 2003, a team of entomologists at Kansas State University published an article in Environmental Entomology titled "Effect of Bt Corn for Corn Rootworm Control on Nontarget Soil Microarthropods and Nematodes." In 2000, Bt (MON 863, Cry3Bb1 toxin) and non-Bt corn hybrids were established at eight experimental sites in Kansas. The following season, three experimental locations in Kansas were used for this study. Soil samples were taken in Bt plots and non-Bt plots at three different points in the season (early, mid, and late season). The researchers reached the following conclusion: "In general, numbers of soil mites (Prostigmata, Mesostigmata, and Oribatei), Collembola, and nematodes were similar in soil planted with Bt corn and soil planted with its isoline." Although we believe that many additional long-term ecological studies are warranted with respect to the potential influence of transgenic corn rootworm hybrids on nontarget arthropods, results from this Kansas study, similar to the previously discussed paper, also are encouraging.
Please share with us your experiences with these new seed treatments and transgenic corn rootworm hybrids during the 2004 growing season. We have much to learn with these relatively new technologies.--Mike Gray