Post by 48 on Aug 18, 2011 13:06:09 GMT -5
Rootworms Resist Bt Trait
Research Verifies Resistant Rootworms in Iowa
Kurt Lawton Crops Technology Editor
Bio
Thu Aug 18, 2011 08:30 AM CDT
EDEN PRAIRIE, Minn. (DTN) -- Ever since the first YieldGard corn hybrids with a rootworm-resistant trait were planted in 2003, entomologists and farmers have witnessed some rootworms feeding on this technology.
Western corn rootworms continue to evolve against control measures. (Photo courtesy Purdue University)
Monsanto was upfront about the fact that some feeding would occur with this low-dose trait, unlike the successful high-dose Bt traits that farmers relied on to control corn borer. However, resistance to rootworms was not projected to occur with proper stewardship of the technology.
Well, the need for better stewardship is now more apparent than ever, as the western corn rootworm continues to find ways to evade control measures. A recent published paper, by Iowa State University entomologist Aaron Gassmann, has documented the first field-evolved western corn rootworm resistance to the Monsanto single protein trait event (Cry3Bb1). And that announcement came as no real surprise to Corn Belt insect experts.
The back-story to this Gassmann paper begins in 2009 when his entomology team traveled the back roads of northeast Iowa. They prowled several fields at farmer requests to examine plants and collect rootworms to test in the lab. They confirmed abnormally heavy feeding by western corn rootworms, which, in each case, proved their ability to eat roots supposedly protected by the Cry3Bb1 trait.
Upon interviewing farmers and farm managers, Gassmann learned that a common pattern was present among fields with resistant rootworms -- highlighting the need for better Integrated Pest Management (IPM). "There was a consistent pattern of continuous corn production dating back to at least 2003, and the continuous use of the same Bt trait for at least three consecutive years."
While no questions were asked about refuge history, entomologists know lack of refuge is a big contributing factor for selecting resistant populations. In his paper, Gassmann cited that currently, only 50% of Bt corn planted in the Midwest complies with U.S. EPA requirements for refuge size and proximity to Bt fields.
Billy Fuller, entomologist at South Dakota State University, says growers won't talk about their refuge because they don't want to get in trouble. "With the very, very high value of corn in recent years and the desire to not have any damaged acres, the 20% refuge concept has always been considered a threat to the bottom line -- so growers will claim their neighbors as the refuge without any knowledge of what the neighbor planted," he said.
Gassmann said farmers need to think about using the Bt rootworm technology in a more integrated way. "They cannot rely on one Bt event. The best case scenario would be if they rotated crops. Short of that they could rotate Bt traits, or plant a pyramided (double trait) Bt event such as SmartStax. They may even consider combining the trait with a soil insecticide in fields of continuous corn with a history of rootworm pressure. A key component of effective, long-term management is to monitor performance by digging roots to evaluate feeding injury," he said.
A key point to remember with this technology is that the sky is not falling. Gassmann emphasized that this problem is not broad, yet. "We're dealing with a very small number of fields. Given the large amount of Bt corn, the technology has proven to work on the vast majority of acres," he said.
Monsanto echoed this sentiment in a July 29 statement posted on its website. It states that both YieldGard VT Triple and Genuity VT Triple PRO corn products continue to provide expected levels of western corn rootworm control on more than 99% of the acres planted to this technology.
But like many pests, they can vary in severity depending on the year. Ken Ostlie, University of Minnesota entomologist, saw a dramatic increase in performance problems in 2009 -- in an area running from Rochester, Minn., down to Waterloo or Cedar Rapids, Iowa. A second pattern of scattered pockets of damage ran diagonal from the Twin Cities to Sioux Falls, S.D.
"The wet year of 2009 helped us begin to see a pattern where the majority of fields with abnormal rootworm feeding involved hybrids with the Cry3Bb1 trait. So Aaron pulled samples in Iowa and we pulled samples in Minnesota and we went to work," he said.
In his research bit.ly/…, Gassmann also tested for resistance to another rootworm trait (Cry34/35Ab1), which is used in SmartStax along with the Cry3Bb1 trait. "Resistance was found only for the Cry3Bb1 trait, so the pyramiding of multiple Bt toxins may still provide some benefit to delay resistance to either toxin," he said.
Both researchers continue to monitor fields and collect samples to see if more signs of resistance appear -- as do entomologists in other Corn Belt states. Ostlie says they are investigating five or six field situations for performance problems in 2011, and evaluating these populations for resistance. "So far we haven't pinpointed a mechanism for resistance, but we know there are elevated levels of survival. How the western corn rootworm is accomplishing this is not quite known at this point."
Growing knowledge about pest biology may also provide further insight. Wade French, USDA-ARS research entomologist in Brookings, S.D., seeks a greater understanding on the reproductive biology of corn rootworms. "Hopefully we'll have some results soon on reproductive habits between susceptible and resistant mates, as well as more information on how Bt toxins may impact pest fitness such as egg laying, life expectancy and more," he said.
For now, Ostlie says growers should not assume the technology is working. "Go dig roots, check for unusual levels of feeding and note if beetles are unusually abundant in the field. And if you've been planting the same corn rootworm trait multiple years, make changes for next spring," he said.
Kurt Lawton can be reached at kurt.lawton@telventdtn.com
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