Transgenic Hybrids, Seed Treatments, and Soil Insecticides: Making Sense of Soil Insect Control

Michael Gray
Professor and Extension IPM Coordinator

Office: S-320 Turner Hall
Phone: (217) 333-6652


On June 21, 2000, Monsanto Company submitted a corn rootworm insect resistance management (IRM) proposal to the Environmental Protection Agency as a component of their data package for registration approval and exemption from tolerance. Claims of confidentiality regarding the IRM proposal were waived because the information fell within the guidelines outlined by the Federal Insecticide Fungicide and Rodenticide Act (FIFRA) 10(d)(1)(A)(B) or (C). The IRM plan specifically requested the "registration of Bacillus thuringiensis Cry3Bb protein and the genetic material (Vector ZMIR 13L) necessary for its production in corn." Monsanto intends to use the MaxGardÃ’ for all hybrids that express the Cry3Bb protein with the approved transformation. Monsanto had requested permission for a limited launch for the 2001 season, primarily in the western Corn Belt.

In light of the widely publicized StarLink controversy, this ambitious timetable is doubtful because of the considerable hoopla and concerns surrounding the registration of transgenic events. When the first transgenic insecticidal cultivars for corn rootworms become commercialized, they are likely to be "stacked" and also provide control of European corn borer. Other companies such as Pioneer Hi-Bred International, Incorporated, in cooperation with Mycogen Seeds and Dow AgroSciences, also are determined to register a transgenic hybrid that provides corn rootworm control. Both the corporate investment and economic risk associated with this exciting and promising technology are impressive, and the pressure to establish a "beachhead" in the lucrative corn rootworm marketplace is intense.

For the 2001 and 2002 growing seasons, producers will most likely continue to rely upon conventional soil insecticides and crop rotation (not in east central Illinois) for corn rootworm protection. In this presentation, I offer some guidance and encourage caution with respect to the adoption of seed treatments for "stand-alone" corn rootworm protection. Root protection data from University of Illinois insecticide efficacy trials for the 2000 growing season are provided for DeKalb, Monmouth, Urbana, and Iowa State University. These data indicate very clearly that the seed treatments marketed under the trade names ProShield and Prescribe do not satisfactorily prevent corn rootworm larval injury under moderate to heavy infestations. In addition, data are provided on soil insecticide performance against wireworms from extensive University of Missouri trials conducted from 1991 to 1998. We have considerably more data regarding the performance of conventional soil insecticides against wireworms and some other secondary insect pests than we have currently for the new seed treatment products.

ProShield Technology with Force ST (Syngenta) and Gaucho and Prescribe (Gustafson) are now registered for the control of several soil insect pests. In addition to corn rootworms, ProShield is labeled for use against white grubs, wireworms, and seedcorn maggots. Performance data regarding the efficacy of ProShield against white grubs and wireworms is scarce. The active ingredient of ProShield (tefluthrin) has performed well against both these soil insects when applied as Force 3G. ProShield is likely to prevent direct wireworm injury to germinating seeds. Less is known about the performance of ProShield against wireworms when extensive injury occurs to the below-ground portion of stems. Efficacy of ProShield against white grubs is more suspect because these insects do not feed directly on the seed and instead graze root hairs. Gaucho (0.165 mg of imidacloprid per seed) and Prescribe (1.34 mg of imidacloprid per seed) are labeled for control of seedcorn maggots and wireworms. Prescribe also is targeted at corn rootworms. Imidacloprid is a systemic insecticide (translocated in plant) and is likely to provide control of wireworms that feed on seeds as well as stem tissue. However, collection of more performance data for both Gaucho and Prescribe is warranted for secondary insect pests.

Many entomologists, myself included, 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 percent 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 upon 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, is 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.

In conclusion, 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. ProShield and Prescribe are not recommended by the University of Illinois for corn rootworm control in 2001. Data highlighted in this presentation (Figures 1-8) 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 my 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.


I extend thanks to Mr. Jim Oleson, Department of Entomology, Iowa State University; Dr. Armon Keaster, University of Missouri; and Mr. John Shaw, Illinois Natural History Survey, for providing insecticide efficacy data for this presentation.

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