The development and use of sampling and scouting procedures for corn insects and the relationship of corn rootworm densities to corn yield
A research pest management scouting program was conducted in three counties in Iowa from 1979 through 1982. Wireworms were not found in damaging numbers in any scouted fields and, therefore, were considered a rare pest. Black cutworms were an occasional pest and were much more severe in some years than in others. European corn borers were almost always present, but seldom reached damaging levels. The corn rootworms were severe pests because they almost always reached densities that were considered potentially damaging;Two types of sequential sampling plans were devised for corn rootworm adults to make sampling for that pest more efficient. Waters' method of sequential sampling classifies the population into a category and is useful for pest management decision making. Kuno's method gives an estimate of the population density present and is useful for research purposes. For corn rootworm beetles, both methods were significantly faster than sampling with a fixed sample size and Waters' method was significantly faster than Kuno's method. Waters' method reduced overall scouting time by 36% and gave more than 96% correct decisions;Regression equations were developed relating rootworm adult and egg densities with larval damage and corn yield loss. Although some of the equations were statistically significant, the correlation coefficients were quite low, indicating that most of the variation was unexplained. Calculated economic thresholds closely approximated current thresholds for adult counts and root ratings. However, a threshold could not be calculated for egg counts. Adult counts were accurate in predicting if the larval damage would reach economic levels, but were not accurate in predicting economic yield loss. Egg counts did not predict damage or yield loss accurately;A four-variable model accounted for 70% of the variation in root damage ratings. The variables were organic matter, planting date, nitrogen applied, and the yield potential of the corn variety planted. A six-variable model accounted for 42% of the variation in yields. The variables were rootworm beetle densities in week 31, nitrogen applied, plant population, phosphorous needed, soil pH, and June rainfall.