Farmers Caught Unprotected From Western Bean Cutworm

Outbreaks of this relatively new insect pest are catching many corn growers off-guard in Iowa.

Published on: Aug 29, 2006

An outbreak of western bean cutworm in parts of the Western Corn Belt, particularly in Iowa and Nebraska, has caught many corn farmers by surprise, leaving them scrambling to make rescue insecticide applications and re-considering their in-plant trait selection for the 2007 production season.


"Many growers are finding worm levels above economic thresholds for the first time since western bean cutworm moved into Iowa several years ago," says John Long, district agronomist for Mycogen Seeds. He's located at Sydney, in southwest Iowa. "If corn producers haven't scouted their fields, yet, they need to get out there immediately. If they've found that the worms have reached the silks and are feeding on ears, it may be too late and very difficult to control them with an insecticide at that point."


Long adds that corn producers who planted hybrids containing the Herculex I or Herculex XTRA Insect Protection in-plant trait have protection against WBCW. However, you need to keep in mind that hybrids containing the YieldGard in-plant trait do not offer protection against WBCW.


Western bean cutworm outbreak


"There are a lot of corn producers who were considering the impact that western bean cutworm might have on corn this year, but chose YieldGard as a result of recommendations they received or a belief that western bean cutworm would not impact them this year," Long says. "Needless to say, they are wishing they had selected Herculex I or Herculex XTRA."


As of August 4 of this year, 63 of the 80 counties in Iowa where moths are being monitored have seen moth captures at levels of greater than 100 per week. Of those, 38 have seen moth captures of greater than 500 per week. (Visit to see the latest moth trap counts throughout the Midwest.)


In addition, seven counties in northwest Illinois and one in northwest Indiana have moth captures of greater than 100 per week. WBCW moths have been trapped as far east as northwest Ohio, which suggests that WBCW is rapidly moving eastward from its origins in western Nebraska and has grown to a pest of importance to Corn Belt producers.


"It's safe to say western bean cutworm is here to stay in the Corn Belt," says Long. "I also think it would be wrong to characterize it as just a nuisance pest. It certainly has an attitude bent toward hurting yields and profitability. We need to be aware of its movement."


Scouting and rescue sprays


Maynard Ochs, product development agronomist for Dow AgroSciences, says that as growers move into fields to scout for WBCW that they need to be aware that corn holds a much lower threshold to WBCW than what growers are accustomed to when evaluating European corn borer (ECB) infestations. While thresholds for ECB run at levels of 25% to 50%, thresholds for WBCW are 8% for field corn and 5% for popcorn or sweet corn. With many pests, one larva per plant rarely causes economic injury.


"It takes a high percentage of infested plants to do serious damage. However, studies show that just one western bean cutworm larva per plant can reduce yield by 3.7 bushels per acre. In severe infestations, multiple larvae per ear may be common, as western bean cutworm is not cannibalistic like corn earworm. Under heavy feeding pressure, 50% to 60% of an ear's kernels may be damaged, resulting in serious losses," says Ochs.


Timing of insecticide is tricky


The timing of an insecticide treatment to control WCBW is also key. "It's important to make rescue sprays before the larvae reach silks. Otherwise, they find protection from the insecticide under the husks," Ochs says.


So long as the worms have not found cover within the silk and under the husks, they may be controlled with an insecticide like Lorsban 4E insecticide or any number of available pyrethroid insecticides, Ochs adds.


"Typically, you want to be scouting for egg masses on the upper side of the upper leaves beginning in mid-July when peak moth flights occur to determine if you have economic threshold levels of western bean cutworm," Ochs says. "If eggs have hatched, insecticide applications can begin once 95% of the plant tassels have emerged, but before the larvae move to the silks."


There are numerous university sites available to learn more about scouting and treating for WBCW, including:

Nebraska (

Iowa State (

Illinois (

Purdue (    


Seed considerations for 2007


As a result of WBCW becoming a pest of economic importance, Ochs says corn farmers seeking protection against WBCW need to consider the following as they make their seed and in-plant trait decision for 2007:


If you want protection from an in-plant trait, they need to go with hybrids containing either Herculex I or Herculex XTRA Insect Protection. The YieldGard in-plant traits do not provide activity against WBCW, while Herculex will provide very good protection against WBCW.


Corn rootworm (CRW) is a determining factor in which Herculex trait to select. Herculex I controls above-ground pests, including WBCW, ECB, black cutworm, fall armyworm, southwestern corn borer and corn earworm. If corn a farmer also needs control of CRW, Herculex XTRA Insect Protection is their choice because it contains both Herculex I and Herculex RW Rootworm Protection, which offers control of CRW.


"If you are a corn grower and you are uncertain whether western bean cutworm will be a problem pest for you in 2007, your simplest approach is to purchase a corn hybrid with either the Herculex I or Herculex XTRA in-plant trait," says Ochs. "You're essentially buying peace of mind that you won't need to be out there scouting next summer like you would if you use the YieldGard trait.


"Otherwise, you can go the route of keeping an insecticide treatment in your back pocket. You just need to be certain you will be proactive about scouting for the egg masses and not procrastinating to the point you miss the window of opportunity to control those larvae with an insecticide treatment."


Available hybrids for 2007


Long says Mycogen Seeds will offer 58 new grain corn hybrids next year, including several stacked-trait hybrids containing Herculex XTRA Insect Protection, Roundup Ready and LibertyLink technologies. The majority of hybrids from Mycogen Seeds contain Herculex I Insect Protection.


"These new corn hybrids feature elite genetics, so growers get top-performing hybrids, excellent standability and ultimately higher yields," Long adds.


Since European Union (EU) import approval is pending, Herculex RW Rootworm Protection and Herculex XTRA are Market Choices products. Grain containing either product must be marketed domestically, fed on-farm, or delivered to elevators or other markets where neither the grain nor its processed products (e.g., gluten) will be shipped to the EU.


Grain from hybrids containing Herculex I only is fully approved for food and feed use in the European Union; however, this approval does not apply to grain (or processed products from this grain, e.g. gluten) from Herculex I Insect Protection hybrids stacked with Roundup Ready Corn 2. Grain from hybrids containing Herculex I and Roundup Ready corn is a Market Choices product and must be marketed accordingly. For information, contact a Mycogen Seed representative.