Agriculture and Environment Extension Agriculture and Natural Resources Publications

11-2012 Current challenges in managing corn rootworm Erin W. Hodgson , [email protected]

Aaron J. Gassmann Iowa State University, [email protected]

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Recommended Citation Hodgson, Erin W. and Gassmann, Aaron J., "Current challenges in managing corn rootworm" (2012). Agriculture and Environment Extension Publications. 94. http://lib.dr.iastate.edu/extension_ag_pubs/94

Iowa State University Extension and Outreach publications in the Iowa State University Digital Repository are made available for historical purposes only. Users are hereby notified that the content may be inaccurate, out of date, incomplete and/or may not meet the needs and requirements of the user. Users should make their own assessment of the information and whether it is suitable for their intended purpose. For current publications and information from Iowa State University Extension and Outreach, please visit http://www.extension.iastate.edu. Prepared by Erin Hodgson and Aaron Gassmann, Photo by Purdue University Department of Entomology

orn rootworm is a major pest of corn in the United This research reports the first time a has evolved CStates. In the last decade, Iowa farmers have seen many resistance to a Bt crop in the field. Although we expected changes for managing corn rootworm. With the commer- corn rootworm to eventually adapt to transgenic technolo- cial release of transgenic Bt-rootworm corn in 2003, many gies, finding resistant populations within six years after the farmers reduced or eliminated soil-applied commercial release of Bt traits was a surprise. Since then, to manage larvae. Western corn rootworm is adapting to resistance to Cry3Bb1 has expanded geographically in Iowa transgenic technologies throughout the Corn Belt. Some (i.e., northern third of Iowa). Resistance to Cry3Bb1 has continuous cornfields have larvae that can now survive at also been confirmed in Illinois, and is suspected in Nebras- high numbers and cause severe root injury to Bt corn. We ka, Minnesota, and North Dakota. Recent data indicate that encourage farmers to use multiple strategies to protect corn. rootworm populations with resistance to Cry3Bb1 also are less susceptible to mCry3A. Western corn rootworm resistance to Bt In 2009, the Department of Entomology at ISU began Management recommendations receiving reports of severe corn rootworm injury in Bt corn. Western corn rootworm is an adaptable pest, and will even- In all cases, western corn rootworm was the predominant tually overcome any strategy that is used continuously. The species in these fields. The Gassmann Lab at ISU compared longer a field is in continuous corn, the more inputs will be survivorship of western corn rootworm collected from these needed to protect yield. In order to keep growing profitable problem fields versus from control fields not associ- corn, farmers must use multiple tactics to prolong the ef- ated with performance issues. The results were recently fectiveness of commercially available tools. published in 2011 (see Online References #1). Survival on Cry3Bb1 corn was significantly higher for larvae from prob- The single most effective tool in Iowa is to rotate corn to lem fields compared to control fields. All problem fields soybean or another crop. Breaking the cycle of continuous were growing continuous corn with the same Bt trait for corn, even once every 4-6 years, will dramatically improve at least three years. The publication also showed no cross- corn rootworm management. Corn rootworm cannot resistance with Cry34/35Ab1 for fields sampled in 2009. survive on non-corn crops, and therefore crop rotation will break the life cycle of this pest. A farmer should expect nearly zero root injury after rotating back to corn, and could significantly reduce the management costs in first- year corn. If crop rotation is not an option, farmers should incorporate a diversified management strategy, including the following options:

• Use pyramided corn rootworm transgenic traits, like SmartStax® (Cry3Bb1 and Cry34/35Ab1) or Agrisure® 3122 (Cry 34/35Ab1 and mCry3A). But understand if root- worm populations have developed resistance to one trait, the second trait is likely controlling most of the larvae. Us- ing pyramided traits continuously will increase the chances of developing resistance to both traits, which is the “worst case scenario” and something that we are trying to avoid at An Iowa cornfield, including Cry3Bb1 and mCry3a corn, with all costs. We would strongly encourage rotating pyramided severe lodging due to corn rootworm larvae. traits every 2-3 years.

Pub. No 290-12 November 2012 • Use a soil on non-rootworm Bt corn. Re- Scout every field, every year! member soil insecticides only protect a small area of roots. Iowa farmers should monitor fields for larval corn root- Transgenics, on the other hand, will be expressed through- worm injury, regardless of which management strategy out the root system and will come in contact with the entire they use. Evaluate root injury on the ISU’s 0-3 rating scale rootworm population. So if a field has high larval numbers to determine corn rootworm pressure and product efficacy and you use a soil insecticide, there may still be root prun- (see Online References #3, 4). To help properly time when ing. Soil insecticides will not necessarily eliminate resistance to scout, subscribe to ICM News (see Online References #2) because individual larvae (resistant and susceptible) will be for an annual update of predicted egg hatch. able to survive outside the insecticide-treated zone. Larval scouting. Root pruning by larvae causes yield loss. • Rotate among single Bt corn rootworm traits. For exam- Under drought-stressed conditions, measurable loss can oc- ple, if you are noticing root injury >1 node destroyed with cur at just 0.25 nodes destroyed (i.e., 2-3 roots). So ideally, Cry3Bb1 (Genuity® VT Triple Pro) or mCry3A (Agrisure® a root rating of <0.25 nodes is highly desirable, and indi- RW), then consider switching to Cry34/35Ab1 (Herculex® cates the product is adequately protecting the roots. Having XTRA). We recommend rotating single Bt traits annually, 1 node destroyed or greater can significantly impair plants or at least every two years. from taking up water and nutrients, and make plants more susceptible to lodging. • Adult control to reduce the egg load going into the winter is actually very difficult to do well. It takes serious scouting Adult scouting. We also strongly recommend scouting for effort (2 times a week for about 6-8 weeks) to monitor for adult corn rootworm in corn, especially in continuous corn, adult emergence. You must be able to distinguish sex and non-Bt fields and suspected Bt-resistant fields. The distribu- gravidity (females with fertile eggs). An effective adult con- tion of adults within and between fields is not uniform. The trol program will take 2-3 foliar sprays timed to match peak adults are constantly moving and creating hotspots that abundance of females when they are in the egg laying stage. can quickly dissipate. This is due, in part, to uneven corn silk emergence within an area. Adults will “chase” greening silks, their preferred food source, in the summer. Monitor- ing adult populations will provide information about the larval density belowground. It will also help to make man- agement decisions for the next growing season.

Online references 1. Gassmann, A. J., J. L. Petzold-Maxwell, R. S. Keweshan, and M. W. Dunbar. 2011. Field-evolved resistance to Bt by west ern corn rootworm. PLoS ONE 6(7): e22629. http://bit.ly/pn5rJ3

2. ICM News, www.extension.iastate.edu/cropnews/

3. Iowa State University Corn Rootworm, www.ent.iastate.edu/ Photo by Jon Tollefson dept/faculty/gassmann/rootworm A heavily damaged root (on right) is an indication of high corn rootworm larval densities. Assessing larval injury, or root 4. Interactive Corn Rootworm Node-Injury Scale, www.ent.iastate. pruning, will help determine future management strategies. edu/pest/rootworm/nodeinjury/nodeinjury.html

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