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Performance problems surface again with Bt corn rootworm traits

By Ken Ostlie and Bruce Potter, University of Minnesota Extension

Bt-RW problem field with lodged corn
Bt-RW problem field with lodged corn

Calls over the last two weeks indicate Bt-RW trait performance problems may be expanding in scope. Field observations suggest corn rootworm populations have increased markedly in corn after corn fields since 2011; recent calls indicate a major expansion of the geography of performance problems into SC and WC Minnesota. Unfortunately the drought has masked the primary tip-off to severe corn rootworm injury—lodging. With injury largely completed and corn rootworm emergence peaking, now is the time to check fields for signs or symptoms of performance problems with your Bt-RW traits. Getting a handle on Bt trait performance is critical before making seed purchases for 2013. You may need to change your corn rootworm management strategy/strategies.

corn rootworm map
2009-11 problem fields

Since the first reports surfaced in 2009, scattered performance-problem fields have been reported in a broad arc from NW Illinois through NE Iowa, W Wisconsin, SE Minnesota, SW Minnesota, E South Dakota, NW Iowa, and NE Nebraska. The map at the right indicates where Bt-RW performance problems have been previously reported in Minnesota (2009-2011 map). Field visits and phone calls in 2012 suggest an increase in the geographical scope of performance problems, especially in SC and WC Minnesota, and prospective resistance to more Bt-RW traits. Your help is needed in identifying where, and to which traits, these problems are occurring!

Drought this summer has been both a blessing and a curse when it comes to identifying performance problems. Typically, the most visual tip-off to corn rootworm injury and performance problems is lodging. But lodging is an imperfect indicator since besides corn rootworm injury, it also requires thunderstorm activity. The moist soils loosen root grip and strong winds generate strong force on the corn. Under drought conditions, thunderstorm prevalence has been minimal and offer little insight into status of the resistance problem. However, corn rootworm survival and injury is enhanced under drought conditions and above-ground, stress symptoms of this root injury (stunting, leaf roll) are magnified. For many growers this year, it's these symptoms and unusually high numbers of corn rootworm beetles that have been the tip-off to performance problems.

Click an image below for a closer look at symptoms.

If you identify fields with Bt-RW performance problems, and prospective resistance to a Bt-RW trait, it's critical to report the field ASAP to the seed company (whether or not the field is in compliance with respect to refuge)! Confirming resistance involves both field and lab verification. The process begins by contacting your seed dealer to report the problem. This page outlines this reporting process and what to expect. Field visits will gather field history, verify trait presence and corn rootworm injury of sufficient magnitude to report the problem, and possibly collect adult beetles to evaluate their progeny for resistance.

If you detect a performance problem field, we'd appreciate notification as well: These reports will enable us to get a better handle on the geography of the problem and the traits involved so we can tailor our extension and research efforts, serve as an independent source of information, and improve transparency on the critical issue of Bt-RW trait resistance!

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