EFFECTS OF LOW CONCENTRATIONS OF FOREST-USE PESTICIDES ON FROG EMBRYOS AND TADPOLES

Management of coniferous forests of eastern Canada may involve sprayin g with the insecticide fenitrothion and the herbicides triclopyr and h exazinone. Because ranid frogs breed in ponds that are unavoidably con taminated by spraying, we measured the toxicity of these chemicals to embryos and tadpoles of Rana pipiens (leopard frog), Rana clamitans (g reen frog), and Rana catesbeiana (bullfrog) under lab conditions. Embr yos were exposed during late neurula stage and tadpoles within 48 h af ter hatching to fenitrothion (24 h; 0.5-8.0 ppm), triclopyr (48 h; 0.6 -4.8 ppm), and hexazinone (8 d; 100 ppm). We measured hatching success of embryos, and for tadpoles, mortality, ability to swim away when pr odded, and total body length one week after exposure. Hexazinone had n o effects on embryos or tadpoles, even at the unreasonably high levels to which they were exposed. Hatching success of embryos and subsequen t avoidance behavior were unaffected in all species by exposures to tr iclopyr and fenitrothion. Newly hatched tadpoles of all species were v ery sensitive to 2.4 and 4.8 ppm triclopyr and to 4.0 and 8.0 ppm feni trothion, either dying or remaining paralyzed following exposure. Tadp oles initially affected by exposure to lower concentrations of fenitro thion or triclopyr usually recovered within 1 to 3 d. Bullfrog and gre en frog tadpoles appear to be more sensitive than leopard frog tadpole s, and bullfrog tadpoles were consistently more sensitive than green f rog tadpoles.