The functional integrity of northern leopard frog (Rana pipiens) and green frog (Rana clamitans) populations in orchard wetlands. II. Effects of pesticides and eutrophic conditions on early life stage development

Premetamorphic northern leopard frogs (Rana pipiens) and green frogs (Rana clamitans) were evaluated at seven wetland sites, four of which were within apple orchards, to determine if environmental conditions associated with orchard management in southern Ontario, Canada, affected frog early development. Synchronous with breeding events (May-July), embryos and tadpoles were exposed, in situ and in the laboratory, to study site pond water for 2 to 3 weeks. Six pesticides commonly applied in apple orchards (Guthion 50WP, Imidans 50WP, Thiodan 50WP, Dithane DG, Nova 40W, and Basudin 500EC) and technical grade diazinon were also evaluated for direct toxicity to green frogs using continuous and discontinuous toxicity tests. Embryos and larvae exhibited poor hatching success and survival at some orchard sites during in situ assays, but no specific water source produced consistently poor developmental success. Reduced tadpole growth occurred at several study sites, showing no clear distinction between reference and orchard sites. Despite the collection of substantial environmental data, only surface water temperature appeared to be correlated with growth rates. Basudin 500EC, technical grade diazinon, and Dithane DG caused mortality, deformities, or growth inhibition during early development at environmentally relevant concentrations (<0.01 mg/L), whereas Imidan 50WP, Guthion 50WP, and Nova 40W produced survival or growth effects at much higher levels (5-10 mg/L). Dithane DG (mancozeb) was acutely toxic during prolonged exposures, and sublethal growth and deformity responses occurred at nominal concentrations greater than 0.01 mg/L. Diazinon was the most toxic to green frogs, with median lethal concentrations of 2.8 to 5 micrograms/L and median effective concentrations of 6 to 14 micrograms/L for formulation and technical grades, respectively. The combined results of in situ and laboratory tests of green frog and leopard frog developmental success suggested that embryo-larval development of these two species could be accomplished at all study sites, independent of the association with apple orchards.