D.A. Herbert, Jr.; S. Malone; R. L. Brandenburg.
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Abstract: The southern corn rootworm, Diabrotica undecimpunctata howardi Barber, is an annual soil insect pest of peanut (Arachis hypogaea L.) in Virginia, North Carolina, Texas, and other peanut growing states. Larvae feed on developing pods causing direct yield loss and indirect damage by allowing entry of secondary pathogens. Because southern corn rootworm is a soil pest, scouting is difficult and producers make preventive treatments without knowledge of actual pest abundance. A predictive index for southern corn rootworm injury was evaluated using 392 field case studies conducted in Virginia and North Carolina from 1997 to 2001. Factors influencing the index score (soil texture, soil drainage class, planting date, cultivar resistance, and field history of rootworm damage), and point assignments for predicting low, moderate, and high-risk fields were analyzed. The Goal of this project was to determine which combination of factors provided the highest percentage of correct risk predictions. The best index combination used all five factors to determine the total point score, with 70 or more points indicating a high-risk field, 55 to 65 points a moderate-risk field, and less than or equal to 50 points a low-risk field. Growers who use the index eliminate a preventive insecticide treatment in low-risk and some moderate-risk fields. The index correctly predicted the level of pod damage in 45% (177 of 392) of the field case studies. Insecticide was correctly recommended in 46 fields. Thirty-three percent (131 of 392) of the fields were correctly identified as not needing treatment. There were 209 cases where there was an overestimation of pod damage with predictions of either a high or moderate level when only a low level occurred. In these cases, an insecticide treatment would have been recommended and an average of 6.1 and 2.6% pod damage, respectively, would have been prevented. Conversely, there were very few fields that should have been treated but were not treated (6 of 392). Overall, use of the index would have protected fields from pod damage and potential loss 98.5% of the time.