University of Illinois

No. 18/July 24, 1998

Economic Thresholds for Insect Pests on High-Oil Corn: Recommendations Unclear

During the past two seasons, the number of questions related to economic thresholds for insect pests on high-oil corn has escalated. Numerous callers continue to seek advice on thresholds for corn rootworm adults, Japanese beetles, and corn leaf aphidsduring the silking and pollination period of corn. Unfortunately, very little research has been directed at seeking answers for these questions. Let's take a look at some fundamental definitions of the economic threshold and economic injury level.

An economic threshold may be viewed as that number of insects (pest density) at which point a management action should be taken to prevent the pest population from reaching an economic level. Occasionally, the economic threshold is referred to as the action threshold. Determination of an appropriate economic threshold is quite complex and is dependent upon the economic injury level. The economic injury level (EIL) is equal to the cost (C) of management per area ($/acre) divided by the product of the following factors: market value (V) per unit of product ($/acre), injury units (I) per insect per production unit (percent defoliation/insect/acre), and damage per unit injury (bushels lost/acre/percent injury). Theformula looks like this: EIL = C/V x I x D. A great deal of research is required to determine the factors I and D. Despite the interest and proliferation of specialty grains, it is unlikelythat economic thresholds will be established for each of the many insect pests capable of infesting the great diversity of specialty hybrids. So, what are the options in managing insect pests of specialty grains such as high-oil corn?

Unfortunately, this question can be answered only in very general terms. Because the value of specialty grains is greater than standard commercial corn, it takes lower pest densities and less overall injury to result in crop damage that warrants an insecticide application strictly on economic terms. Precise recommendations on economic thresholds likely will not be forthcoming in the near future. To complicate this scenario even more, consider that the economic injury level fluctuates according to environmental factors as well. Bottom line (with this discussion as background), when trying to decide whether or not an insecticide application is warranted on specialty grains, use established economic thresholds for commercial corn only as a starting point. After determining the economic threshold forcommercial corn, seek additional information for seed-production fields. Purdue University has published a manual entitled Seed Corn Pest Management Manual for the Midwest. For more information about the manual, contact John Obermeyer, Purdue Pest Management Program, 1158 Entomology Hall, Purdue University, West Lafayette, Indiana, 47907-1158. John's telephone number is (765)494-4563.

Mike Gray (, Extension Entomology, (217)333-6652