Economic Injury Levels for Sorghum Midge, Stenodiplosis sorghicola1, and Corn Earworm, Helicoverpa zea2, Feeding on Panicles of Sorghum, Sorghum bicolor
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Sorghum midge, Stenodiplosis sorghicola (Coquillett), and corn earworm, Helicoverpa zea (Boddie) are important insect pests of sorghum, Sorghum bicolor (L.) Moench. Both pests feed on the developing kernel and directly reduce grain yield. Economic injury levels (EIL) that respond to pest management costs, commodity values and pest density are developed for each pest based upon published reports on grain loss per insect stage and stage-specific mortality. The economic injury level for sorghum midge is M = C 33,256 / V F where M = mean number of adult sorghum midge per flowering panicle, C = cost of management to control sorghum midge per unit area ($/acre), V = market value per unit of production ($ per 100 lbs of sorghum grain), and F = number of flowering panicles per acre. The EIL for sorghum midge is expressed as the number of adult sorghum midge per flowering panicle because only flowering panicles are susceptible to oviposition by sorghum midge females. Corn earworm larval stage must be considered in the EIL because grain consumption per corn earworm larva increases and larval mortality rate decreases with increasing larval size. The EIL for 5-6th instar corn earworm is L = C 9,754 / V P where L = mean number of corn earworm larvae per panicle, P = number of panicles per acre and C and V are as above. The EIL for 3rd-4th fourth instar larvae is calculated by dividing the EIL for 5-6th instar larvae by 0.19 to adjust for the 81% mean mortality reported for this stage. First and 2 nd instar corn earworm larvae consume very little grain and experience high mortality. These stages contribute little to economic loss and are not included in calculating the EIL. Economic thresholds are presented for each pest for a range of typical treatment costs/acre (C), market value per 100 lbs of grain (V) and number of flowering panicles (F) or all panicles (P) per acre.
author list (cited authors)
Knutson, A. E., & Cronholm, G.