Performance of terbufos on corn rootworm (Coleoptera: Chrysomelidae) in the corn belt

During 1984, soil insecticides used for the control of the western corn rootworm, Diabrotica virgifera virgifera LeConte, and the northern corn rootworm, D. barberi Smith & Lawrence, appeared to provide substandard protection of maize (Zea mays L.), based on farmer complaints across the corn belt. One theoretical explanation assumed that all of the insecticidal products were subject to enhanced microbial degradation regardless of their chemical class. The efficacy and behavior of terbufos were examined under grower-use conditions in Illinois, Iowa, Nebraska, and Wisconsin. The study involved 67 field trials conducted with calibrated grower equipment from 1985 through 1987. Fields where farmers considered terbufos performance to have been suspect in 1984 were selected as soils with a history of terbufos use to be compared with fields with soils that had no history of terbufos use. Soil samples from each field trial were bioassayed with southern corn rootworms, Diabrotica undecimpunctata howardi Barber, and chemically analysed for residues of terbufos and its metabolites. Corn root damage ratings were taken at the field trial sites. No significant differences between mean values of root ratings on corn from the history versus nonhistory soils were found. Mean concentrations of terbufos and its metabolites 60 d after treatment equaled or exceeded LC50's for Diabrotica spp. over the 3-yr period. Root ratings exceeded 3.0 in 6 of the 67 field trials. In the six field trials, initial terbufos deposits were found to substantially below 6 parts per million because of flawed applications at planting. There was no trend to indicate that terbufos and its metabolites degraded more rapidly in soils with a history of terbufos use than they did nonhistory soils, which would have been expected with the occurrence of enhanced microbial degradation.