Spread of Western Bean Cutworm in the U.S. Corn Belt

• The spread of western bean cutworm (WBC) appears to have coincided with the widespread use of hybrids with the YieldGard® Corn Borer (YGCB) trait, which has activity against corn borer and corn earworm (CEW), but not WBC.
• Researchers wondered if the reduction in competition from corn borer and CEW allowed WBC to thrive and become a significant pest of corn in the central Corn Belt.
• Studies to investigate competition between larvae of WBC and CEW showed that survival of WBC vs. CEW was low, but increased when larvae were fed a YGCB silk diet.
• Other studies were conducted to determine differences in ear feeding damage between hybrids with the YGCB trait, hybrids with the Herculex® 1 (HX1) trait, and non-transgenic hybrids. In these studies:
• The hybrids with the HX1 trait had very little kernel damage at any of the locations.
• At most locations, more ear-feeding damage occurred in the YGCB hybrid than in the non-transgenic hybrid.
• Researchers concluded that controlling other insects created a void for WBC to fill. In the absence of its competitors, WBC was able to thrive and create more damage than the original pests on non-transgenic corn
  

In recent years, western bean cutworm (WBC) has spread eastward from its historic habitat in Colorado and Plains states as far east as Ohio and Michigan (Figure 1). The timing of this spread has coincided with the widespread use of hybrids with the YieldGard® Corn Borer (YGCB) trait, which has activity against corn borer and corn earworm, but not western bean cutworm. Entomologists wondered if there was a direct cause and effect relationship between widespread use of hybrids with the YGCB trait and emergence of WBC as a new pest of corn in the Corn Belt of the U.S. To help answer this question, research studies were conducted by Dr. Marlin E. Rice and David L. Dorhout at Iowa State University (Rice and Dorhout, 2006).

Figure 1. Spread of WBC in the midwestern U.S.a

Western bean cutworm on corn ear tip.

In recent years, western bean cutworm (WBC) has spread eastward from its historic habitat in Colorado and Plains states as far east as Ohio and Michigan (Figure 1). The timing of this spread has coincided with the widespread use of hybrids with the YieldGard® Corn Borer (YGCB) trait, which has activity against corn borer and corn earworm, but not western bean cutworm. Entomologists wondered if there was a direct cause and effect relationship between widespread use of hybrids with the YGCB trait and emergence of WBC as a new pest of corn in the Corn Belt of the U.S. To help answer this question, research studies were conducted by Dr. Marlin E. Rice and David L. Dorhout at Iowa State University (Rice and Dorhout, 2006).

Different insects that feed on the same plant parts at the same time often compete with each other for the available food source. On corn ears, simultaneous feeding by European corn borer (ECB), corn earworm (CEW), and WBC is common. Hybrids with the YGCB trait have activity against ECB and CEW. Researchers wondered if the reduction in competition from these insects allowed WBC to thrive, spread and become a significant ear-feeding pest of corn in the central Corn Belt. Therefore, a study to investigate competition between larvae of WBC and CEW was conducted.

Corn earworms normally develop from hatching to largest instar stages in about 19 days - WBC require about 56 days to grow to approximately the same size. This gives CEW a clear competitive advantage, as large larvae are more competitive against small larvae. However, the Cry1Ab protein in the plants with the YGCB trait is lethal or detrimental to CEW, which researchers believed would eliminate or reduce their competitive advantage.

To test this theory, eggs were collected from WBC and CEW adults and the larvae were reared on an artificial diet until they were large enough for the experimental treatment. Then each WBC larva was paired with a CEW larva in a container and placed on one of three diets:

• Common artificial diet
• Silks of corn hybrid with the YGCB trait
• Silks of non-Bt isoline to YGCB hybrid

In the second year of the study, corn ears in the field were added to the list of diets.

For check treatments (controls), a WBC larva was paired with another WBC larva. Most experiments were replicated about 25 times in 2006. In 2005, the number of replications was lower due to more limited availability of larva, and averaged approximately seven for each experiment. Small (S) and large (L) sizes of larvae were tested in all combinations:

Table 1: Paired larvae treatment combinations.

2005 Study: In the artificial diet, survival of the controls (pairs of WBC larvae) was very high, at 90-100%. When western bean cutworms were paired with a corn earworm, survival of the WBCs was significantly lower (Figure 2).

Figure 2. Survival of WBC larvae in the presence of corn earworm larvae on three diets, 2005. Treatments not significantly different from the check represented by *, Chi-square, P=0.01. Source: Iowa State University.

Survival was 18% in the S vs S treatment, 0% in the S vs L treatment, 25% when the WBC larva was large and the CEW larva was small, and 0% when both larvae were large. When larvae were fed non-Bt corn silks (isoline to YGCB hybrid), all WBCs were killed by the CEWs. In contrast, when larvae were fed silks from plants with the YGCB trait, western bean cutworm survival was 100%.

2006 Study: In 2006, several new treatments were added to the study - a diet of corn ears was added, and large vs. small larvae comparisons were added to the silk diets. Survival trends for WBC were similar to 2005 (Figure 3).

Figure 3. Survival of western bean cutworm larvae in the pre-sence of corn earworm larvae on three diets, 2006. Treatments not significantly different from the check are represented by *, Chi-square, P=0.01. Source: Iowa State University.

On the artificial diet, pairing WBC and CEW larvae resulted in no or low WBC survival, except when WBCs were large and CEWs small. This same trend was evident on the non-Bt silks, where WBC survived in high numbers only when they were large and CEW larvae were small.

When larvae were fed silks from plants with the YGCB trait, small WBC larvae were able to survive against small CEW larvae. However, when CEW larvae were already large upon starting this diet, WBC survival was low, at 0% and 20% for small and large larvae, respectively. This is different than 2005 results that showed 100% survival for large WBC vs. large CEW on this diet.

In the field experiments with caged larvae on corn ears, there were no differences in survival between WBCs paired with WBCs or CEWs. The authors attributed these inconclusive results to high levels of naturally occurring WBC larvae that confounded the results.

To further test the effect on WBC when corn borer and CEW are controlled by the YGCB trait, field trials were conducted. The plots included hybrids with the YGCB trait, hybrids with the HX1 trait, and a non-transgenic isoline hybrid. The HX1 trait controls WBC in addition to corn borer and corn earworm.

Three locations were included in the study. Ears were divided into four quadrates and the longest amount of injury in each quadrate was measured. The four values were then totaled to arrive at a damage value for the entire ear.

Western bean cutworm damage to corn husks and kernels.

Results
The hybrids with the Herculex® I trait had very little kernel damage at any of the locations (Figure 4). At Laurens, Iowa, damage in the hybrids with the YGCB trait was not significantly different from the non-transgenic hybrids. However, at the Mallard and Rembrandt locations, the hybrids with the YGCB trait had significantly more kernel damage than the non-transgenic hybrids.

Another three-location study was conducted using three different Pioneer brand hybrids from the same genetic platform - one with the YGCB trait, one with the HX1 trait, and the base genetics hybrid with no traits added. Ear feeding damage was scored using a 1-9 rating system where 1 indicates extreme damage and 9 indicates no damage. At the Blackhawk County location, the HX1 hybrid had the best rating, followed by the YGCB hybrid and then the non-transgenic isoline (Figure 5). However, at the other two locations, more damage occurred in the YGCB hybrid than in the non-transgenic hybrid.

Figure 5. Western bean cutworm ear damage ratings using Pioneer hybrids with the HX1 trait, YGCB trait and non-Bt isoline hybrids. Source: Iowa State University.

These studies suggest that by removing certain damaging species, the hybrids with the YGCB trait created a "void" for a new insect species to thrive. In these studies, the damage from the new insect on hybrids with the YGCB trait was worse than damage from the old insect species on the non-transgenic hybrid. This phenomenon of "pest replacement" may help explain the recent, rapid range expansion of WBC across the Corn Belt from Nebraska to Ohio.

Dorhout, D. L., and M. E. Rice. 2004. First report of western bean cutworm, Richia albicosta (Noctuidae) in Illinois and Missouri. Crop Mgmt doi:10.1094/CM-2004-01-BR. Online: (http://www.plantmanagementnetwork.org/pub/cm/brief/2004/cutworm).

Rice, M. E., 2000. Western bean cutworm hits northwest Iowa. Integrated Crop Mgt., IC-484 22: 163. Iowa State Univ. Ext., Ames, IA. Online:
http://www.ipm.iastate.edu/ipm/icm/2000/9-18-2000/wbcw.html

Rice, M.E and D.L. Dorhout, 2006. Western bean cutworm in Iowa, Illinois, Indiana and now Ohio: Did biotech corn influence the spread of this pest? In 2006 Integrated Crop Management Conference Online. pp. 165-172. Iowa State University, Ames, IA.

^aData compiled by Pioneer Marketing Department from the following university sources:

• www.ent.iastate.edu/trap/westernbeancutworm
• www.plantmanagementnetwork.org/pub/cm/brief/2004/cutworm
• http://nerec.unl.edu/ipm/lighttrap2006.htm
• http://nerec.unl.edu/ipm/lighttrap2005.htm
• http://hamilton.unl.edu/2004.htm (also see 2003.htm and 2002.htm)
• http://westcentral.unl.edu/entomology/lighttrap.htm
• http://scal.unl.edu/Research/Insect/2006_Light_Trap_Report.shtml
• http://entomology.unl.edu/scal/Lighttrap/2005_lighttrapdata.htm
  (also see 2003_lighttrapdata.htm and 2004_lighttrapdata.htm)
• http://plantsci.sdstate.edu/ent/entpubs/wbc2006_bere.htm
  (also see other locations and years.)
• www.vegedge.umn.edu/2005/wblist.htm
• www.vegedge.umn.edu/2006/WBC/MNwbc.htm
• http://pestbulletin.wi.gov issues 45-52, 66-71.