Toxicity of Guthion and Guthion 2S to Xenopus laevis embryos

The development of Xenopus laevis (African clawed frog) embryos exposed to the pesticide Guthion (technical grade) and Guthion 2S (commercial formulation) was evaluated in modified Frog Embryo Teratogenesis Assay--Xenopus (FETAX) tests. The embryos were exposed to five or six increasing concentrations of pesticide in 10- and 100-ml exposure volumes of test solution for 96 h. Embryos exposed in 10-ml volumes of Guthion exhibited increased mortality, increased deformation, and decreased size as compared to those exposed in 100-ml volumes. LC50s for embryos exposed in the 10-ml Guthion tests ranged from 6.1 to 6.3 mg/L as compared to 10.6 to 11.9 mg/L for those in the 100-ml tests. The percentage of deformities at 3 mg/L Guthion in test survivors in 10-ml tests ranged from 73 to 89%, while in the 100-ml tests less than 2% were deformed at the same concentration. Mean control embryo lengths at test completion were 8.2 and 10.6 mm, respectively, for 10- and 100-ml tests. The LC50 for embryos in 100 ml Guthion 2S was 1.6 mg/L active ingredient, indicating a much greater toxicity of the commercial formulation. NOAEL (No Observed Adverse Effect Level) values for Guthion and Guthion 2S ranged from 0.48 to 7.96 mg/L, depending upon basis (length, deformity, mortality) and pesticide formulation, and were many times greater than the existing water quality criterion of 0.01 microgram/L.