European Corn Borer Threshold Calculations

With the recent increase in commodity prices it is probably a good time to reconsider treatment thresholds for these pests. In 1985 we published a corn borer management guide that contained the following equation to calculate an economic threshold for the European corn borer: Economic Threshold (# Larvae per Plant) = Control Cost / (Yield Loss per Borer * Market Value*Yield * Proportion of Larvae Killed) European corn borer threshold using 1984 Data* Control Cost ($ per Acre) 12.50 (Insecticide plus application cost.) Yield Loss per Borer 0.044 (This number varies depending on the stage of corn growth: 0.044 for late whorl, 0.066 for pre-tassel, 0.044 for pollen shed, 0.030 for blister and 0.020 for dough). Market Value of Corn ($/Bu) 2.50 Expected Yield (Bu/A) 150 Proportion of Larvae Killed 0.95 Thus the equation becomes:$12.50 / (0.44*$2.50*150*0.95) And Economic Threshold (# Larvae per Plant) = 0.80 Then to convert this threshold to the # Egg masses per plant needed to reach economic threshold take (Economic Threshold * Proportion of Eggs Laid)/(Proportion Surviving * Eggs per Mass) Economic Threshold (Larvae per Plant) 0.80 Proportion of Eggs Laid 0.50 (Estimates are based on three assumptions. First that fields are being scouted regularly. Second, that 5% of oviposition has occurred by the time the first egg masses are found, Third, that the egg laying period will last about 3 weeks. Thus, about 8 days after the first eggs are found about 50% of the eggs should have been laid.) Proportion Surviving 0.20 (Based on studies in Kansas a value of 0.20 is recommended) Eggs per Mass 20 Thus the equation becomes: (0.80*0.5)/(0.2*20) = Egg Masses per plant need to reach the economic threshold = 0.10 or 10% of the plants with an egg mass. If we plug in some 2008 prices the threshold will change significantly. European corn borer threshold using 2008 Prices Control Cost ($ per Acre) 14.00 (This cost could vary greatly depending on products used, however one might also need to adjust the proportion of larvae killed if less expensive lower rates are used) Yield Loss per Borer 0.044 Market Value of Corn ($/Bu) 6.50 Expected Yield (Bu/A) 205 Proportion of Larvae Killed 0.95 Thus the equation becomes:$14/ (0.44*$6.50*205*0.95) And Economic Threshold (# Larvae per Plant) = 0.25 Then to convert this threshold to the # Egg masses per plant needed to reach economic threshold: Economic Threshold (Larvae per Plant) 0.25 Proportion of Eggs Laid 0.50 Proportion Surviving 0.20 Eggs per Mass 20

Thus = (0.80*0.5)/(0.2*20) = Egg Masses per plant need to reach the economic threshold = 0.03 or 3% of the plants with an egg mass.

Treatments should be made after one reaches the economic threshold and as eggs begin to hatch and larvae are visible in the leaf axils. Then if fresh eggs are still observed seven to 10 days after the first application a second application may be justified. In general fields that exceed treatment thresholds early in the oviposition period are more likely to benefit from a second application, and if the treatment threshold is not reached until late in the ovipostion period then often one application may give adequate control. Keep in mind that once borers enter the stalk, ear tip, or ear shank, insecticides will not be effective. At three percent the threshold is so low that it is tempting to say: Why scout? Why not just treat every field? It is also tempting to conclude that with thresholds this low; why not add some insecticide every time any other treatment is applied? Obviously if you remove the application cost then the threshold would be even lower. There are several reasons why treating without knowledge of current pest populations is a bad idea. One is that repeated insecticide applications can lead to resistance. Another is that insecticides can have significant impact on non-target insects and beneficial organisms which can lead to secondary pest outbreaks. Plus there are potential health hazards to applicators and field workers. However, possibly the biggest reason is because if the timing is wrong the application may have little or no impact on the target pest. Note that the economic threshold is based on getting 95% control. If the timing is off this could fall dramatically and in many cases there would be little or no economic benefit from making a treatment. Worse yet, if treatments are applied too early, the majority of the eggs could hatch after the residual of the insecticide is gone and one could suffer severe corn borer injury even though one paid for an insecticide treatment. This can especially be true when insecticides are added to fungicide or herbicide applications without regard to proper timing for corn borer or other potential pests. So even though the thresholds are much lower given the commodity prices, proper scouting and timing are need to collect the benefits from any insecticide applications.

If you would like to try to calculate an economic threshold based on your own economic information try down loading the following excel spreadsheet and inserting your own data: Corn Borer Threshold Calculation Spreadsheet

Calculations based material in the publication: The European Corn Borer - Part Two: Reaching a Management Decision, Sloderbeck et. al. 1984, Kansas State Cooperative Exentsion Service. MF-725.