Insect Resistance Management for Corn Rootworm Alan Reynolds, Sharlene Matten, and Tessa Milofsky

1. Product Toxin Target Pest Yieldgard Rootworm Cry3Bb CRW Yieldgard Plus Cry3Bb + Cry1Ab CRW, Lepidoptera Herculex Rootworm Cry34Ab1/Cry35Ab1 CRW Herculex Xtra Cry34Ab1/Cry35Ab1 + Cry1F CRW, Lepidoptera Insect Resistance Management for Corn Rootworm Alan Reynolds, Sharlene Matten, and Tessa Milofsky U.S. EPA/Office of Pesticide Programs, Biopesticides and Pollution Prevention Division (7511C) 1200 Pennsylvania Avenue NW, Washington DC 20460 reynolds.alan@epa.gov; matten.sharlene@epa.gov; milofsky.tessa@epa.gov Abstract As with other Bt crops, EPA has required Insect Resistance Management (IRM) strategies for Bt corn crops targeting corn rootworm (CRW, Diabrotica sp.). IRM is needed due to the risk of resistance from sustained, season-long expression of high levels of Bt toxins. The preferred strategy involves the use of refuges to produce susceptible beetles to mate with any resistant beetles from the Bt crop. For additional information, see EPA’s biopesticides web site: www.epa.gov/pesticides/biopesticides. Information Used to Formulate IRM Strategies for CRW • Dose • Dose is defined as the amount of toxin expressed by the transgenic crop relative to the susceptibility of the target pest. • "High dose" is defined as a level of toxin at least 25 times greater than the amount needed to kill all susceptible insects. • Data submitted for Yieldgard Rootworm and Herculex Rootworm show that both express less than a high dose for CRW, though they are still efficacious against CRW. • Models • Models are predictive tools to estimate the likelihood of resistance given certain assumptions (i.e. they can help evaluate the durability of an IRM strategy). • They can be used to evaluate and compare IRM/refuge options. • In the absence of field resistance, models are often the only tool to evaluate IRM strategies. • Models used to evaluate CRW IRM options: • Onstad et al. 2001 (deterministic, multiple patch model) • Storer 2003 (stochastic, spatial model) Currently Registered Rootworm Products Refuge Strategy A structured refuge is strategic planting of non-transgenic (non-Bt) crops in close proximity to the Bt crop to promote interbreeding of resistant and non-resistant insects. The goal is to reduce chances that resistant beetles mate with each other by providing large numbers of susceptible beetles from the refuge (non-Bt crop). • CRW IRM Challenges • CRW biology: • Behavior (movement, mating, oviposition) • Life cycle – both adults and larvae may be exposed to Bt toxins • Corn/soybean rotation-resistant western CRW • Extended diapause northern CRW • Resistance monitoring – adapting techniques used for lepidopteran pests • Stacked products – harmonizing IRM plans for both CRW and lepidopteran pests • Studying CRW – larvae feed underground and are difficult to observe • Mode of action for Bt corn: toxic and/or behavioral? • Treatment of refuges for non-CRW pests (i.e. Lepidoptera) Resistant Beetles Susceptible Beetles Refuge Designs TARGET = 500 susceptible to 1 resistant Bt Crop Perimeter (Border) In-field Strips Adjacent Block Non-Bt Crop