In 1990 and 1991, the efficacy of reduced application rates of soil insecticides registered for com rootworms, Diabrotica spp., and the prevalence of economic injury caused by these key pests were determined in the fields of growers in northern Illinois. Fifty-eight experiments were conducted in 10 Illinois counties during this 2-yr period. Root injury ratings from these on-farm experiments indicated that the reduced application rate (three-fourths of labeled rate) of several commonly used soil insecticides provided root protection comparable to labeled rates. Differences in yield between the labeled and lower insecticide application rates were statistically negligible in each year of the study. However, for 1990, when yield information was averaged across all 29 fields, these data could be used in a misleading fashion to support the use of soil insecticides for continuous com production in northern Illinois at labeled (3.70/ha) and three-fourths (5.24/ha) application rates. For instance, in 1990, the majority of growers (20) would have gained marginal profits by applying a labeled rate (I.73Iha) and lost money by using the lower rate (2.64/ha). In 1991, growers would have lost 6.14/ha and 0.67/ha by applying labeled and lower application rates of soil insecticides, respectively. Only 26 of the 58 experiments (45%) had root injury at or above the economic injury index of 3.0. If an economic injury index of 4.0 is used, then not a single trial in 1991 had average injury in the untreated control at the economic level. Regardless of the economic injury index that is used, these data suggest that farmers in these experiments, and perhaps in northern Illinois, may be using soil insecticides on more acres of continuous corn than necessary.
Document Type: Research Article
Publication date: October 1, 1993
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