Issue No. 22, Article 2/September 1, 2006
Planning to Manage Western Bean Cutworm in Corn in 2007? Know Your Insects
The continued eastward spread of the western bean cutworm's range has been a topic of significant conversation in Illinois, Indiana, Michigan, Ohio, and Wisconsin. After its somewhat "lonely" discovery in Warren County in Illinois in 2004, a network of pheromone traps has documented the western bean cutworm's rapid spread eastward and southward through Illinois. As of August 4, 2006, western bean cutworm moths had been captured as far south as St. Clair and Washington counties in Illinois and as far east as western Ohio. The numbers of moths captured in pheromone traps have been particularly large in Illinois in northwestern counties--Jo Daviess, Stephenson, Winnebago, Carroll, Ogle, Whiteside, and Bureau.
Figure 1. Captures of western bean cutworm moths in pheromone traps as of August 4, 2006 (from Iowa State University's "Western Bean Cutworm Monitoring Network").
Although the numbers of western bean cutworm moths captured this year in Illinois have been noticeable, they pale in comparison with the numbers captured in Iowa. In Illinois, only seven counties with traps have experienced captures of 100 to 500 moths, and none have exceeded 500. In Iowa, by contrast, most of the counties where traps are located have experienced captures of at least 100, and many of them more than 500. So we need to keep the situation in perspective.
Even so, western bean cutworms will deserve management attention in some areas of Illinois in 2007. Some producers will want to consider planting a Herculex Bt corn hybrid that will control western bean cutworms (i.e., Herculex I, Herculex XTRA), whereas others may decide to address management next year by regular scouting and application of an insecticide if needed. Regardless, it is very important that overreaction not be part of the management plan.
Although western bean cutworm appears to be relatively widespread in Illinois in 2006, it is unlikely that the larvae have caused much damage to corn in most areas where the pest has been found thus far. We also are aware that some other ear-damaging insects have been misidentified as western bean cutworm. An individual who had examined some cornfields in Livingston County during the last week of August discovered fall armyworms and corn earworms feeding in about 20% of the ears in one area of the field. Although the damage to corn ears caused by corn earworms, fall armyworms, and western bean cutworms is relatively similar, the insects can be distinguished from one another with certain characteristics.
Fall armyworm worm larvae, Livingston County, Illinois, 2006.
Fall armyworm larva, Livingston County, Illinois, 2006. (Note the four black spots on top of each body segment.)
As we have stated before, the western bean cutworm larva is pale tan, brown, or pink-gray and lacks conspicuous markings, except for short, dark stripes on the top of the first segment behind the head. The fall armyworm larva is slightly more colorful than the western bean cutworm, and it also has some distinguishing body markings--three yellow-white lines along the back and a wider dark stripe on each side of these lines. Below the dark stripe on each side is a wide, wavy, yellow stripe with red splotches. In addition, a fall armyworm larva has a dark brown head with a white inverted Y on the front. Corn earworm larvae are usually more colorful than fall armyworm larvae. A corn earworm larva may be green, yellow, brown, red, or pink with longitudinal white stripes. The skin of both a fall armyworm and a western bean cutworm is smooth, whereas the skin of a corn earworm is covered with microspines. The head of a corn earworm larva usually is yellow-brown or yellow-orange and stippled, and it lacks the white inverted Y of the fall armyworm. Marlin Rice, extension entomologist at Iowa State University, published some excellent photographs of fall armyworm and corn earworm larvae in the article "Insect Injury to Mid-Season Corn" in the June 28, 1999, issue of Integrated Crop Management. Also, Kansas State University has an excellent identification key, "Identifying Caterpillars in Corn and Sorghum".
Being able to distinguish among caterpillars that may have damaged corn ears this year may save producers money next year. Although western bean cutworms have captured a lot of attention because of their newness, mistaking corn earworms or fall armyworms for western bean cutworms will not aid management plans.
As the 2006 chapter on western bean cutworms in Illinois closes, we will begin to compile information from our and other states' efforts to develop educational materials and programs. In the meantime, sharpen your insect-identification skills, and let us know what you are finding in your area.--Kevin Steffey