Western Bean Cutworm
July 26, 2011 by Andrew Westhoven, CCA
European corn borer, corn rootworm, corn earworm, armyworm, black cutworm, stalk borer...the list of insects that can harm corn plants seems never ending. Management of some of these insects, especially European corn borer and corn rootworm, has become easier with use of Bt corn traits. However, it is crucial to become cognizant of new insect pests that may damage corn in the field. Experts in some states in the Northern Corn Belt have recognized western bean cutworm as the most destructive insect to corn in Indiana in 2009 and 2010. Please continue reading for more information on this insect that is relatively new to the Northern Corn Belt.
Western bean cutworm (WBC), Striacosta albicosta (Smith), was traditionally a pest of dry beans in West Mountain states and Western Corn Belt states. Economic damage was found in Minnesota and Iowa for the first time in 2000. WBC continued its eastern movement in following years (see Figure 1). Even though its name doesn't suggest it, WBC is a pest of corn, feeding on tassels, pollen, silks, and kernels. Damage can cause reduced yields and provides an entry point for mold or disease infection, reducing grain quality.
Anything that damages corn can be described as ugly. Please see Figures 2-6 that show diagnostic features of WBC.
Figure 2. - Purple eggs about one day before hatch (Rice from Rice and Pilcher).
Figure 3. - Hatched larvae consuming shells (J. Obermeyer from Michel et al.).
Figure 4. - Larvae color can vary, but two dark square blocks are always present behind the head after the 4th instar (J. Obermeyer from Michel et al.).
Figure 5. - Larvae in an earthen nest (J. Obermeyer from Michel et al.).
Figure 6. - Adult moth with diagnostic identification features (J. Obermeyer from Michel et al.).
In the Upper Midwest, WBC adults (moths) are first seen in late June with peak flights occurring in mid-July. Adult females lay egg masses within a week of mating, usually on one of the upper few corn leaves. Moths are attracted to fields that are near tasseling but have not yet tasseled. Freshly laid eggs are white and after 5 to 7 days will gradually darken to purple just before hatch. After hatching, larvae usually consume their shells before moving to feed on the tassel, pollen, silks, and tissue behind leaf axils. As pollen shed declines and tassels desiccate, larvae move to the ear and enter from the tip or bore a hole through the husk (Figure 7). WBCs are not cannibalistic, so it is not uncommon to find several feeding on the same ear. In late summer, larvae exit the ear, drop to the ground, burrow down to a depth of 5 to 10 inches, build a nest, and overwinter. The cycle starts over again the next summer with adults emerging from the soil in June.
Silk feeding can reduce kernel set if done extensively, however most WBC damage is a result of feeding on the ear. Yield reductions range from minor to as high as 40% in heavily infested fields. Studies have shown an average WBC count of one per plant reduced yield by 3.7 bu/acre. Feeding during corn stage R3 (milk) can cause ears to deform. Feeding at any time provides an entry and infection point for diseases and molds (Figure 8), reducing grain quality and possibly enabling accumulation of mycotoxins.
An active scouting program for WBC will ensure that control efforts can be made if necessary. Live traps can be placed near corn fields to help determine when moths are in flight. Purdue University has an extensive trapping network and the moth flights for 2011 can be found here (http://extension.entm.purdue.edu/pestcrop/2011/issue15/WesternBeanCutworm.html). High moth counts does not always equate to high field damage. Once flights have been detected, scout corn fields for egg masses that have been laid on the upper leaf surface of upper corn leaves. Dr. Christian Krupke from Purdue posted a series of videos on YouTube in 2010 that explain scouting techniques for WBC. Links to these videos are listed below.
Early July egg scouting: http://www.youtube.com/watch?v=BFXRELD6T9E
Mid-July small larvae scouting: http://www.youtube.com/watch?v=gLeTgEo_jiQ
Early August larvae in the ear: http://www.youtube.com/watch?v=QB6RMMyNXHo
Genuity SmartStax, Herculex, and Agrisure Viptera hybrids with the Vip3A event provide good control of WBC and offer peace of mind. Insecticides are effective, but timing is critical. The WBC is a rather weak pest and great control can be accomplished if the insect comes into contact with the insecticide. Additionally, insecticides do have some residual properties, which will also provide a larger window for control. Once larvae enter the ear, insecticide effectiveness is greatly reduced. An economic threshold of the treatment when 8% or more plants have egg masses or larvae has been used in the past. With current corn prices, treatment might be warranted at a lower threshold of 5%. An evaluation of a spray treatment completed after a heavy WBC infestation was found by Dr. Christian Krupke can be found at: http://www.youtube.com/watch?v=IGVr0mLG8hA.
Moth flight is in peak flight. Moth captures started out slowly, but have since spiked rapidly in some areas. Scouting should begin immediately in high-risk fields (those with WBC problems in the past). If a field is scheduled to receive a fungicide application, it might be warranted to include an insecticide for WBC control. This is only warranted if egg masses are detected.
Michel, Krupke, Baute, and Difonzo. Ecology and Management of the Western Bean Cutworm in Corn and Dry Beans. J. Integ. Pest Mngmt. 1(1): 2010; DOI: 10.1603/IPM10003.
Rice and Pilcher. USDA-CSREES. Western Bean Cutworm.