Golden Valley, Minnesota
January 31, 2008
It’s a dirty job, but someone has
to do it. In fact, entomologists are encouraging more growers to
dig corn roots this coming summer to look for signs of corn
rootworm feeding.
“Digging roots when it’s hot outside and corn is pollinating
usually isn’t something a person would choose to do,” says Kevin
Steffey, entomologist with the University of Illinois. But with
the increasing spread of western and northern corn rootworms to
rotated corn, Steffey says it’s important to monitor what’s
happening below ground.
“Corn rootworm is not a problem in every corn field in every
year,” says Steffey. “But between extended diapause in northern
corn rootworms and the variant western corn rootworm, pressure
in first-year corn is spreading.”
That’s in addition to continuous corn rotations, where rootworm
pressure is common in most areas of the Corn Belt. “Corn after
corn is the crop sequence that the corn rootworm has made its
living on for many years,” reminds Jon Tollefson, entomologist
with Iowa State University.
Whether growing first-year or continuous corn, digging and
rating roots isn’t just a good way to monitor root damage, “it’s
the only way,” says Iowa State University entomologist Marlin
Rice.
Lodging is probably the most obvious indicator of a rootworm
problem, but it’s not a reliable one. “Lodging can have nothing
to do with rootworm control,” says Rice. “You can have a strong
wind take down your corn, especially in a wet field, and it
won’t have anything to do with rootworm feeding.”
You can’t go by root ball size or stalk strength either, says
Larry Bledsoe, entomologist with Purdue University. “Those are
genetic factors that are largely determined by your hybrid,” he
explains.
“What you’re looking for is signs of root feeding, and the only
way to do that is to dig up roots and look at them – especially
during mid to late July after most of the injury has occurred,”
says Rice.
“If you suspect rootworm pressure or want to compare control
methods, don’t wait until harvest to look at your roots,”
advises Von Kaster, entomologist with Syngenta Seeds. “It’s a
hassle, but an in-season root dig will really give you the
clearest picture of what’s happening in your fields.”
To help growers rate rootworm damage, Iowa State University has
developed an interactive web site that illustrates degrees of
root feeding damage based on a 0 to 3 scale. The web site is
accessible at
www.ent.iastate.edu/pest/rootworm/.
As explained in the web site, a zero rating indicates no
rootworm damage. A rating of one indicates one full node or its
equivalent has been eaten back to within 1.5 inches of the
stalk. A rating of two indicates two complete nodes eaten, and
so forth. Damage in between complete nodes eaten is noted as the
percentage of the node missing, i.e. 1.50 = 1 1/2 nodes eaten,
0.25 = 1/4 of one node eaten, etc.
“There’s no magic number on the scale that definitively
indicates when we reach the point of yield loss,” says Kaster.
Soil moisture, root regeneration…even wind…can affect the extent
of yield loss from root feeding.
“A root rating of 1.0 might cause significant yield loss in a
dry year, but no loss at all in a wet year,” notes Bledsoe.
“There also can be big differences in root ratings when you
compare different hybrids. Fifteen larvae might destroy an
entire node in a smaller-rooted hybrid while the same amount of
feeding on a larger root system might result in only a .5 or .75
root rating.”
When evaluating rootworm control methods, Kaster advises looking
at root ratings from multiple sites. “That will give you a much
broader view of insect control over a range of weather, soil and
cultural conditions,” he explains. “For example, root ratings
from Syngenta and university trials representing 57 site years
in 2004 through 2007 demonstrate Agrisure® RW is very consistent
across locations, with an average root rating of 0.22.”
Entomologists agree that it’s important to look at a variety of
results when evaluating any corn rootworm control method.
Likewise, it’s important to monitor what’s happening below
ground in your own fields. So take a look at the ISU root
ratings web site this winter and make sure you’re out in your
fields this summer with a strong shovel and a cold water bottle.
Syngenta is a
world-leading agribusiness committed to sustainable agriculture
through innovative research and technology. The company is a
leader in crop protection, and ranks third in the high-value
commercial seeds market. Sales in 2006 were approximately $8.1
billion. Syngenta employs around 21,000 people in over 90
countries. Syngenta is listed on the Swiss stock exchange (SYNN)
and in New York (SYT). Further information is available at
www.syngenta.com.
Agrisure® is a trademark of a Syngenta Group Company. |
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