Western bean cutworm (WBC) larvae are being reported in corn ears throughout the North Central region, including Wisconsin. As readers can recall from recent Wisconsin Crop Manager newsletter articles [Western Bean Cutworm Reported – Monitor Field & Sweet Corn for Egg Masses and Small Larvae, August 7^th, 2008;  New UWEX Pub-Western bean cutworm: A pest of field and sweet corn, May 21^st, 2008] , this corn ear pest is expanding in range and intensity since 2005 in Wisconsin, as it moves eastward from the Western Corn Belt states.

 

WBC does overwinter in Wisconsin. There is one generation per year. At the present time (mid-September) large instar larvae are in corn ears, joining the late-season corn ear insect pest complex which can also include corn earworm and second-generation European corn borer. WBC larvae feed on kernels in corn ears until nearing the 6^th or 7^th larval instar. As we get later into September, larvae will drop from the ear to pupate in the soil, forming an overwintering soil chamber. Spring development begins when temperatures exceed 50°F, at which time WBC pupate in the soil chamber. Adult moths emerge in late June, with peak egg-laying in corn in the Midwest typically occurring between the second and third weeks of July. The August 7^th, 2008 WCM article above alerted readers to the WBC moth flight and field scouting recommendation to monitor for WBC egg masses and small larvae before they enter the corn ear.

 

This article summarizes WBC larvae reports from around the state; provides diagnostic pictures of WBC larvae and damage; discusses how to distinguish WBC larvae and feeding from corn earworm and/or European corn borer; and summarizes economic impact and yield-loss relationship data for WBC in field corn. Late-September, and into October, you may see damaged ears, but the causative larvae (WBC, corn earworm or corn borers) will have left the ear – making diagnosis difficult. Now is a good time to become familiar with the extent of WBC infestation (or lack thereof) on your farm or in your region as you think about harvest plans and grain quality, and insect scouting needs for 2009.

 

Dave Cole, Crop Consultant/WI Association of Professional Agricultural Consultants (WAPAC), reported WBC this week from two of his WAPAC variety trials. Both locations were at least 20% infested. One location in Southeastern Sauk County in a trial at the US Dairy Forage Research Center conducted with Rick Wallenbach; and another location in Southwestern Columbia County in West Point Township.

 

Dean Volenberg, UW-Extension Door County Agriculture Agent; Todd Schaumburg, Polenske Agronomic Consulting Crop Consultant; and Ron Prodell, CCA Rio Creek Feed Mill; all report WBC in Northern Door County. Volenberg reported heavy WBC pressure, with most severe infestations along Lake Michigan. Schaumburg reported WBC from Egg Harbor, Door County. Prodell estimated a field with 60% infestations in Northeastern Door County, in Liberty Township near Ellison Bay, as well as a second field in the area although he did not know the infestation %.

 

From Central Wisconsin, Rob Shields, Agronomist, Wisconsin River Agronomy, sent in photographs of WBC larvae and ear damage from a field in Southern Juneau County. Larvae were easy to find in his field inspection.

 

In Southern Wisconsin, I visited a field in Northern Columbia County with approximately 20% infestation. Together with Roger Schmidt, UW Nutrient & Pest Management Program, I filmed a WBC diagnostic, scouting and control recommendation video in this northern Columbia County field. The video will be shown throughout the state at eight locations during the November Pest Management Update meeting series. Additionally, Gary Brandt, CCA, Quality Crop Care, Darlington, WI reported a couple of infested corn fields near central Lafayette County.

Western bean cutworm larva.  Photo Credit: Mark Moore, Moore Communications

 

 

 

Western bean cutworm larvae and feeding damage from corn field in southern Juneau County, WI, September 2008.  Photo Credit: Rob Shields, Agronomist, Wisconsin River Agronomy

 When diagnosing corn ear damage in fall, keep in mind that WBC larvae can enter the corn ear from the tip through silks, or from the side of the ear by chewing a hole in the husk and entering. Therefore WBC feeding can be found at the ear tip and along the sides of the ear, depending upon its entry route and movement within the ear. By contrast, corn earworm moths lay eggs on corn silks and corn earworm larvae enter the ear through the tip and begin feeding at the tip, with some movement and feeding damage from the tip downward.

 

Hole in side of corn ear where WBC larva entered side of ear from a field sample in Central Lafayette County, early September 2008.  Photo Credit: Gary Brandt, CCA, Quality Crop Care, Darlington, WI

 

 

Western Bean Cutworm. Note broad dark stripes behind head capsule.  Photo Credit: Gary Brandt, CCA, Quality Crop Care, Darlington, WI

 

Under heavy infestations it is possible to find more than one WBC larvae per ear (e.g., at tip and sides). By contrast, corn earworm larvae are cannibalistic, resulting in typically one corn earworm larva per year, usually near the tip or upper-third of the ear.

Second-generation European corn borer (ECB) larvae can feed on the side of the ear, near the tips (although they do not enter the ears exclusively through the tip), or shank. European corn borer is the only one of these three larvae (WBC, CEW, ECB) that bores into the stalk. Therefore, you may find frass and entry routes into the plant by ECB.

 

Below is a summary of yield-loss relationships for WBC and field corn.

 

(Gary Hein, Univ of Nebraska): 

Field average 1 larvae/ear at dent stage = 3.7 bu/A loss. (At approximately 5% infestation)

 (Marlin Rice, ISU):

Field average of 1 larvae/ear resulted in yield loss range of 4-11%. Multiple larvae per year have caused up to 40% (e.g. 39%) yield loss under severe infestations, (i.e., western Corn Belt).

 These data were presented during a recent Distance Education Web Seminar on Western Bean Cutworm presented by entomologists from WI, IL, NE and IA. You can access the full web course on WBC anytime (all presentations and voice over) from the Extension link on the UW Entomology Cullen lab web site: www.entomology.wisc.edu/cullenlab

(Click on Extension link, and then 'Field Crop IPM Educational Webcasts').

http://www.entomology.wisc.edu/cullenlab/site%20pages/extension/ext_webcast.html  

 

(The WBC Distance Education Course was sponsored and archived by the North Central IPM Center).

 

Please visit the following UW Extension Resources for more information.

*Includes biology, identification, field scouting, economic thresholds, and control recommendations.

*Includes a Bt Corn Trait Chart listing which Bt corn hybrids offer WBC control