Time of Insecticide Applications in European Corn Borer Control1

Experiments to study time of insecticide application in relation to control of the European corn borer ( Pyrausta nubilalis (Hbn.)) were carried out at Ankeny, Iowa, in 1951-56. Two applications of DDT gave significantly better borer control than one application in all experiments. Each year the best control was obtained following applications when egg laying was almost completed. The minimum points on regression curves represented the estimated optimum treatment dates. Criteria used for predicting time of insecticide application, as they existed on the dates of optimum treatment, were compared with the recommended or suggested criteria. In 5 out of 6 years the number of borer-degree days on the date of optimum treatment was below the suggested 1100, averaging about 900. A tassel bud-plant height ratio of 35 appeared to be more suitable than the recommended range of 30 to 50. Except for 1 year plants in plots with the best borer control were consistently taller than the recommended 35 inches. Plant height and unhatched egg masses were found to be unreliable. Data on moth emergence were consistent with recommendations; however, this criterion is unsuitable for use by the average grower. The recommendation to treat when 75% of the plants show evidence of recent feeding in the whorls was the most rapid and simple to use.