Dimethoate is the common name for the organophosphorous insecticide, o,o-dimethyl-S- (N-methylcarbamoyl- methyl)phosphorodithioate. This compound, which has been in use since 1956 as an insecticide and acaricide, is registered for insect and mite control on agricultural crops and ornamental plants. It is also registered as a residual fly spray in animal quarters.
A bioassay of the carcinogenicity of technical-grade dimethoate was conducted using Osborne-Mendel rats and B6C3F1 mice. The test material was administered in feed to groups of 50 rats of each sex at either of two concentrations for 80 weeks, followed by 35 weeks of observation. Initial doses were not well tolerated; therefore, they were reduced during the study. The "time-weighted average doses" for rats were 155 and 310 ppm for males and 192 and 384 ppm for females. All surviving rats were killed between 113 and 115 weeks.
Dimethoate was administered in feed to groups of 50 male and 50 female mice at two concentrations. Female mice received diets containing 200 and 500 ppm of dimethoate for 80 weeks; male mice received the same dosage. However, high-dose males were returned to the control diet at 60weeks, and low-dose males at 69 weeks. All surviving mice were killed between 93 and 94 weeks.
Tremors and hyperexcitability, both indications of dimethoate toxicity, were observed in the treated animals. However, it is considered that the low-dose group of rats and both dose groups of mice survived long enough to permit an evaluation of carcinogenicity. Pathologic evaluation revealed no statistically significant increase in tumors associated with dimethoate treatment in either species of animal, and it is concluded that there was no carcinogenic effect under the conditions of the experiment.