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Corn rootworm research uncovers root feeding

Yield impacts are unclear, confusing.


February 16, 2008
By Ralph Pearce


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It may surprise some growers to learn how long research into corn rootworm and its impact on
yield has been taking place. Long before terms such as Bt and CRW became part of the farming vocabulary, researchers at the University of Illinois, among others, were researching seed and soil applied insecticides and their efficacy rates against corn rootworm. In short, the trials have been going on for decades.

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Root pruning and a smaller root ball can lead to lodging and of root development from the nodes.
Photo Courtesy Of Dekalb/Monsanto.

That work has continued as more Bt and subsequent stacked hybrids have been brought to market, and as variant corn rootworm has an increasing impact on yields, mostly through the US Corn Belt. Dr. Kevin Steffey, a professor and extension entomologist with the University of Illinois at Urbana-Champaign, investigated root evaluation ratings for corn rootworm products in 2006 and 2007, and found the results to be inconclusive in several regards.

Steffey and his colleagues evaluated the effectiveness of granular and liquid soil insecticides, seed-applied insecticides and Bt corn hybrids against corn rootworm larvae. Three hybrids with CRW traits were used in the research including those from Dekalb (DKC 61-69 with YieldGard VT), Mycogen (2T787 with Herculex XTRA) and Pioneer Hi-Bred (33T59 also with Herculex XTRA). Each company also featured an untreated, non-Bt check in the evaluations, which were conducted at DeKalb, Monmouth and Urbana, Illinois.

Steffey emphasizes that this work has been ongoing for years and in no way tries to shed an unfavourable light on any one company’s hybrids. In fact, he acknowledges the fact that in the past, all of the treatments were applied to the same hybrids. “Before we had Bt corn hybrids, we were looking at soil insecticides and we applied them to the same hybrid throughout the plot,” explains Steffey. “One of the difficulties of doing these plots now is that we’re dealing with different genetic backgrounds for the different rootworm Bt corn hybrids, so we may be comparing apples with oranges when we examine yield data.”

The study’s summary and data tables are available, courtesy of Dr. Steffey and the University of Illinois at Champaign-Urbana, at ipm.uiuc.edu/bulletin/print.php?id=871

Results may vary because of varied genetic make-up
Overall, the yield results did not necessarily reflect the level of larval feeding on the roots. Those hybrids with the worst nodal feeding did not necessarily suffer the lowest in terms of yield. “In 2007, based solely on root ratings, the Mycogen and Dekalb rootworm Bt hybrids had the most rootworm larval damage at the Urbana site,” says Steffey. “There was noticeable damage on the Pioneer Herculex, almost half a node of roots was destroyed and that’s significant. But the other two hybrids had more damage.”

Where the results become cloudy is trying to relate the damage levels to yield, which has little logical
progression to it. “The yields of the YieldGard hybrid are higher by a fair margin than the yield of Pioneer’s Herculex, even though rootworm larval damage to YieldGard was significantly greater than rootworm larval damage to Pioneer’s Herculex,” details Steffey. “We’re not really sure what the
relationship between rootworm damage on Bt corn and eventual yield is. Our data on that portion have not been very clear in the past two or three years.”

One possible explanation may point to the variant western corn rootworm, as has been stated by Steffey and Dr. Mike Gray, also from the University of Illinois. “We don’t really have any hard evidence to back that up, but there is evidence in the literature that the variant may be more difficult to kill with Bt than the non-variant,” says Steffey.

Asked if Ontario growers might avoid or delay a widespread arrival of corn rootworm due to more diverse rotations, Steffey responds that such work was investigated several years prior to 2007 by some of Gray’s graduate students. “In Illinois, the acreage is devoted primarily to corn and soybeans, so the problem with rootworm in corn following soybeans gives the impression that the variant western corn rootworm is locked in on soybeans,” says Steffey. “But the truth is, the variant is leaving corn and seeking other places to lay eggs and if they can’t find soybeans, they will lay eggs in weeds or in alfalfa and possibly other crops that we have not studied.”

The timing is wrong, adds Steffey, for small grains. By the time rootworms lay eggs, wheat or oats or barley have been harvested, so the rootworms would lay eggs only if there were some other vegetation in the field, like a cover crop, perhaps or weeds.

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Although corn rootworm is common to the US Corn Belt, the insect pest has yet to land in Ontario with the same frequency or consistency, however growers must remain vigilant. Photo Courtesy Of Tracey Baute, OMAFRA.

Ontario situation quite different for now
As for whether Ontario growers need to worry about corn rootworm, and the efficacy of various hybrids, Tracey Baute suggests there is still time before corn rootworm becomes as serious a problem in the province as it is in the Corn Belt. “They do deal with a lot heavier populations there, a more consistent presence of the rotation variant and more pressure for using the technology. But our use rate for
corn rootworm traits so far has been pretty low here in Canada, so we still have a learning curve,” concedes Baute, the field crops entomologist for the Ontario Ministry of Agriculture, Food and Rural Affairs.

Although newer stacks forecast to be released in the next four or five years may create some confusion about refuges, it is also a fact that the greater number of traits stacked with different modes of action to control rootworm, the more unlikely it is for corn rootworm to adapt and build resistance. “Corn rootworm is a much more challenging insect and it has the ability to overcome resistance: its biology and behaviour are completely different than corn borer.”

One of Baute’s big concerns about the development of resistance within rootworm is the technology imparted to Bt corn hybrids has been so good, it is causing some growers to become complacent. Some might even take that activity for granted, or that it has been imparted into hybrids to provide the same level of control against rootworm. “We have to be very diligent in ensuring we follow the resistance management practices in place so that we can at least increase the time frame prior to resistance occurring,” says Baute. “In a way, it’s been to our disadvantage that the corn borer technology and resistance management strategies have worked so well, because we’ve had it for 10 years out there and no resistance has taken place.”

Yet it is not as though using Bt technology as an analogy works either. Baute says there are growers
who just want to plant their Bt hybrids and then not worry about the insect after that. “But you still need to scout and you also have to provide a refuge to allow these insects to mate and reduce the risk of resistance developing,” reasons Baute. “That’s why I’m concerned with Bt and the seed treatments. It’s getting so simple that we forget there is still a human element that has to be applied to ensure the insects don’t develop resistance.”

And the same management practice will have to apply if continuous corn acreage increases and results in an increase in rootworm activity in Ontario. Patience may be a virtue, but diligence has its rewards. -end-