No. 4 Article 2/April 29, 2011

Black Cutworms, Bt Corn, and Insecticidal Seed Treatments: Recent Research Developments

Concerns over potential black cutworm damage to corn have increased in recent weeks due to the continuing delays in planting across the Corn Belt, impressive winter annual weed populations in some fields, and heavy moth flights reported in many areas. Because the use of Bt corn has become the norm for many producers, questions often surface regarding the efficacy of these transgenic hybrids against pests such as black cutworms.

An article in the April 2011 issue of the Journal of Economic Entomology (Vol. 2, No. 104, pp. 484-493, DOI: 10.1603/EC10360) sheds some light on the interactions of black cutworm larvae with Bt corn and one insecticidal seed treatment. Authors Sigrun A. Kullik, Mark K. Sears, and Arthur W. Schaafsma are researchers with the School of Environmental Science, Ontario Agricultural College, University of Guelph and the Department of Plant Agriculture, Ridgetown Campus, University of Guelph.

The entomologists used greenhouse studies, leaf bioassays, and field strip trials in their investigations. Five treatments were examined: a non-Bt hybrid; a non-Bt hybrid and clothianidin (25 mg [AI] kernel-1); a non-Bt hybrid and clothianidin (125 mg [AI] kernel-1); a Bt hybrid; and a Bt hybrid and clothianidin (25 mg [AI] kernel-1). The Bt hybrid used was Pioneer Hi-Bred 38P04 (transformation event TC1507, expressing Cry 1F protein). The near isoline non-Bt hybrid was Pioneer Hi-Bred 38P05.

The authors offered the following conclusions (p. 491):

"The results of this study suggest that the low rate of clothianidin is not sufficient for effective cutworm control when applied to non-Bt corn, but that performance is improved when combined with Bt corn. In the two field trials the Bt corn treatments consistently had the highest plant populations and yields, regardless of whether they were treated with the low rate of clothianidin or not. Bt corn alone or in combination with the low rate of clothianidin seems suitable as a means of suppressing black cutworm in no-tillage cornfields, although rescue treatments may still be necessary for severe infestations. This is particularly true knowing that mortality in the laboratory was very low when larvae were fed corn seedlings containing either the Cry1Fa2 protein, clothianidin, or both. Similarly, the high rate of clothianidin gave some protection against black cutworm larvae but may not be economical for field corn and should be monitored in the event that rescue treatments are required. The low rate of clothianidin applied to non-Bt corn should not be recommended in fields where black cutworms are expected until its effects on larval physiology and field performance are better understood."

This research is particularly useful at a time when many questions are surfacing on the effectiveness of Bt hybrids and insecticidal seed treatments against black cutworms. Black cutworm and armyworm moths continue to surge northward on successive storm fronts this spring. University of Kentucky entomologists have reported impressive captures of armyworm moths during the past several weeks. No-till corn planted into cover crops, especially rye, is susceptible to severe armyworm infestations.--Mike Gray

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