“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.