Old enemy, new threat

Top Crop Manager
November 12, 2007
By Top Crop Manager
42a"There's a lot of buzz in Ontario about the new corn rootworm threat from the US," says Dr. Art Schaafsma, professor at Ridgetown College, University of Guelph. "There have been no signs of the problem here yet, but it pays to be vigilant." The new corn rootworm variant appeared in Illinois in 1987. Since then it has moved slowly across the US corn belt causing significant losses. Ontario growers are asking if they have the right tools to deal with the threat when it does finally cross the border.

Crop rotation has been the most effective corn rootworm management tool. Corn rootworm eggs overwinter in the ground and hatch in early June. The larvae feed on corn roots in the early summer. If the eggs hatch into another crop, such as soybeans or wheat, they starve.

The crop rotation variant lays eggs in fields that are not planted to corn. That means when corn is planted the next season, the rootworm is there. Although the adults do feed on soybean plants, they need to go back to corn to get the right nutrition to produce eggs. So they go back and forth between soybean and corn fields. The larvae do not feed on soybean roots.

Following the rootworm trail
Schaafsma has been monitoring corn rootworm populations in Ontario since 1999. He has found some rootworm egg-laying in soybean and wheat fields but the populations have been insignificant. He believes they were a result of environmental pressures on the insect rather than genetic changes.

In 2003, Schaafsma and his team were called to a corn field that had seen wheat, corn and soybeans the previous year. There was no rootworm damage in the corn on corn and no damage on the corn on soybean. There was significant damage on the corn on wheat.

Corn rootworm larvae can only travel one metre but adult rootworm beetles can fly and are carried by wind as they search for corn silks. "The adult corn rootworm needs soil moisture to lay eggs," says Schaafsma. "It was very dry in 2002. There was no moisture in the corn field and the soybeans were suffering from drought. The only soil moisture was in the wheat stubble, so that's where they laid their eggs."

This year, Schaafsma monitored 50 first-year corn fields in a high risk zone from Woodstock to Essex. "We had a hard time finding any evidence of root damage," says Schaafsma. "There were a couple of beetles emerging from some of the root/soil samples, but it wasn't possible to tell whether they were the crop rotation variant." Work is being done to develop a molecular marker to differentiate common corn rootworm and the new variant, but a final product is at least five years away, says Schaafsma.

Ryan Brown, production issues manager with the Ontario Corn Producers' Association, believes it is only a matter of time before the new pest makes its way north. "This is one of the first situations where we know what's coming and can prepare for it," says Brown. "And Ontario growers already have access to hybrid technology that controls corn rootworm."

Hybrids now offer control
In 2004, growers in eastern Canada could choose between two Dekalb hybrids with the YieldGard rootworm trait developed by Monsanto using Bt technology. Four more hybrids will be available in 2005 with YieldGard Plus, which combines YieldGard rootworm and YieldGard corn borer traits.

All Dekalb hybrids with YieldGard Rootworm or YieldGard Plus technology will be treated with Poncho 250, the new on-seed insecticide from Bayer CropScience introduced into Ontario in 2004. Poncho 250 controls early season insects like wireworm and seed corn maggot.

More than 2.4 million acres of YieldGard rootworm hybrids were planted in the US in 2003-2004. The September 3, 2004 issue of Pest Management & Crop Development Bulletin, published by the University of Illinois, reports severe lodging in YieldGard Rootworm hybrids planted at the university's research sites in Urbana and in commercial fields in 2004. The report draws no conclusions from the incidents, stating, "As the 2004 growing season revealed, this recombinant technology is still relatively new with respect to corn rootworm control, and we have much to learn."

"These results don't match up with what we're seeing in the field," says Jamie Rickard, Monsanto's seed marketing manager. "With a 14 bushel an acre yield advantage against an untreated check, the satisfaction rating with our product among Canadian and American corn growers is very high. No doubt we'll continue to learn more about this technology over the next five years."

More options in the pipeline
Dow AgroSciences and Pioneer Hi-Bred International are jointly developing a Bt corn rootworm gene named 'Herculex RW'. The new gene, when inserted into corn hybrids, will provide protection against corn rootworm. Plans are to market it as Herculex XTRA, combining the rootworm control of new Herculex RW, and corn borer and black cutworm control of Herculex I. Herculex XTRA is the next generation of Herculex I, the gene already available in Pioneer and Mycogen Seeds corn hybrids.

Herculex XTRA hybrids are not registered yet, but growers may see them available in Pioneer and Mycogen corn hybrids for the 2006 season. Like Herculex I, the new Herculex RW will also contain the herbicide resistant trait that provides tolerance to Liberty herbicide.

Growers do not like handling insecticides. In addition, other management issues with corn rootworm are insecticide resistance among adult beetles and variable product performance.

"The currently available Herculex I hybrids and upcoming Herculex XTRA hybrids reduce the reliance on, and potential exposure concerns with, insecticide applications," says Virgil Edlin, marketing manager with Mycogen Seeds. "Corn hybrids with Herculex traits provide consistent performance and control in variable field conditions. They will protect yield potential of top-performing corn hybrids 24/7, regardless of weather conditions."

While the Bt corn rootworm hybrids are of primary interest to Ontario growers who grow corn on corn, crop rotation remains a critical tool for growers to manage corn rootworm.

Schaafsma believes that Ontario's smaller fields and three-crop rotation system make it less likely the corn rootworm variant will find its way into Ontario corn fields any time soon. "But we welcome the tools that are being developed to fight this pest," he says. "The whole picture will change if we get the variant. We'll have to rethink the situation then."

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