EFFECT OF CHEMICAL SYNERGY AND LARVAL STAGE ON THE TOXICITY OF ATRAZINE AND ALACHLOR TO AMPHIBIAN LARVAE

We determined the acute toxicity of field-grade formulations of atrazi ne, alachlor, and a 50:50 mixture of the chemicals to early and late l arval stages of northern leopard frogs (Rana pipiens) and American toa ds (Bufo americanus). We conducted identical tests with rainbow trout (Onchorhynchus mykiss) and channel catfish (Ictalurus punctalus) to co mpare their sensitivity as bioindicators. The acute toxicity (96-h med ian lethal concentrations [LC50s]) of herbicides to amphibian larvae r anged from 47.6 mg/L for early-stage R. pipiens exposed to atrazine to 3.3 mg/L for late-stage B. americanus exposed to alachlor. The toxici ty of a 50:50 mixture of atrazine and alachlor after a 96-h exposure w as greater than additive (chemical synergy present) for most species a nd life stages tested, with 96-h LC50s as low as 1.5 mg/L for late-sta ge B. americanus larvae. Older amphibian larvae appeared to be more se nsitive than younger larvae for both species and chemicals tested. Ove rall, rainbow trout and channel catfish appeared to be less sensitive than amphibian larvae. The 96-h LC50s ranged from 23.8 mg/L for channe l catfish exposed to atrazine to 9.1 mg/L for rainbow trout exposed to alachlor. A noteworthy sublethal effect observed in amphibian larvae exposed to atrazine was edema, probably caused by renal disfunction. C hronic predicted no-observed-effect concentrations (PNOECs) for lethal ity after a 30-d exposure were estimated from mortality data at 3, 6, 24, 48, 72, and 96 h and found to be approximately 10- to 20-fold lowe r than 96-h LC50s. The calculated PNOECs are close enough to concentra tions actually found in the environment to warrant concern and further tests. Additional research on environmental concentrations, chronic e ffects, and the effects of chemical synergy is needed before definitiv e conclusions can be drawn. Chemical synergy and life-stage sensitivit y should be addressed to properly assess the toxicity of herbicides to nontarget organisms.