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Dr. Joseph L. Spencer
Assistant Professional Scientist
Center for Ecological Entomology
Illinois Natural History Survey
(217) 244-6851; firstname.lastname@example.org
Extension Specialist – Entomology IPM
Department of Crop Sciences
Research Specialist in Agriculture
Department of Crop Sciences
21st Century Corn Rootworm Management: Old Dogs and New Tricks
Down through the years, corn rootworm beetles have been a continual concern for corn producers. Prior to the arrival of the western corn rootworm (WCR) in the early 1960s, the northern corn rootworm (NCR) was the long-time, primary beetle threat to Illinois corn production. However, adult fidelity to cornfields for oviposition and larval dependence on corn roots during development made the NCR (and later the WCR) susceptible to annual crop rotation which disrupted the year after year availability of corn roots that both species depended upon. Along with chemical control options, crop rotation could be used to effectively manage corn rootworm injury in corn. Unfortunately, both species developed resistance to crop rotation. By the mid-1980s, the NCR had evolved a prolonged egg diapause that endowed a growing proportion of NCR eggs with the ability to diapause in the soil for more than one winter. By the mid-1990s, WCR in east-central Illinois began to lay many of their eggs in the crops rotated with corn (the WCR had also acquired a high level of resistance to a number of insecticides years before they entered Illinois). Both adaptations to crop rotation allowed these rootworms to circumvent the disruption of their life cycles.
Throughout the history of corn rootworm problems in Illinois, growers have repeatedly embraced new rootworm management recommendations when the adaptations of their long-time foes demanded change. Equipment companies such as John Deere no longer manufacture corn planters capable of delivering granular insecticides. This trend indicates that the use of granular insecticides are, and will continue to decline. The latest weapons in the rootworm management arsenal: transgenic corn for rootworms and seed treated with potent systemic insecticides, both reduce grower exposure to insecticides and potentially simplify the management process. Transgenic corn provides protection from root injury equivalent to that provided by many soil insecticides. Efficacy of YieldGard Rootworm, nine granular insecticides, two liquid insecticides, and two seed treatments will be evaluated in our 2004 field research trials.
Whereas as the decision to use insecticides as part of an integrated pest management (IPM) program should be based on pest monitoring and application of economic thresholds, proper stewardship of transgenic corn technology by law must include measures to provide non-transgenic refuges (20% of field area as adjacent blocks or in-field strips) as part of federally mandated ‘insect resistance management’ (IRM) plans. Refuges become a source of susceptible beetles that will move into surrounding transgenic corn and dilute the impact of resistance by mating with rare, potentially-resistant insects produced in transgenic cornfields. Failure to plant refuges may lead to rapid development of resistance to transgenic corn.
Studies indicate that an average Illinois WCR adult moves approx. 12 meters per day within cornfields and just 6 meters per day between corn and rotated crops. Refuges need to be close enough to transgenic fields that transgenic field and refuge populations can thoroughly mix during the mating period. We are learning that some crop environments, like wheat stubble, may not be as attractive to egg-laying females as soybean or double-cropped wheat and soybean. We have also learned that interfield WCR movement is dependent on local conditions, especially late in the growing season. If new types of resistance evolve, understanding how far beetles can move will help in the design of plans to prevent widespread expansion of resistance.
By paying attention to new and established developments in rootworm management, monitoring pest populations, and adhering to principles of IPM and IRM, we can improve the chances that our ‘new tricks’ will keep the ‘old dogs’ guessing for years to come.