Old enemy, new threat
An old enemy has adopted a new strategy to test the mettle of corn growers.
November 12, 2007 By Top Crop Manager
"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
"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."