Effect of chemical synergy and larval stage on the toxicity of atrazine and alachlor to amphibian larvae

Abstract

AbstractWe determined the acute toxicity of field‐grade formulations of atrazine, alachlor, and a 50:50 mixture of the chemicals to early and late larval stages of northern leopard frogs (Rana pipiens) and American toads (Bufo americanus). We conducted identical tests with rainbow trout (Onchorhynchus mykiss) and channel catfish (Ictalurus punctatus) to compare their sensitivity as bioindicators. The acute toxicity (96‐h median lethal concentrations [LC50s]) of herbicides to amphibian larvae ranged from 47.6 mg/L for early‐stageR. pipiensexposed to atrazine to 3.3 mg/L for late‐stageB. americanusexposed to alachlor. The toxicity of a 50:50 mixture of atrazine and alachlor after a 96‐h exposure was greater than additive (chemical synergy present) for most species and life stages tested, with 96‐h LC50s as low as 1.5 mg/L for late‐stageB. americanuslarvae. Older amphibian larvae appeared to be more sensitive than younger larvae for both species and chemicals tested. Overall, rainbow trout and channel catfish appeared to be less sensitive than amphibian larvae. The 96‐h LC50s ranged from 23.8 mg/L for channel 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. Chronic predicted no‐observed‐effect concentrations (PNOECs) for lethality 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 lower than 96‐h LC50s. The calculated PNOECs are close enough to concentrations actually found in the environment to warrant concern and further tests. Additional research on environmental concentrations, chronic effects, and the effects of chemical synergy is needed before definitive conclusions can be drawn. Chemical synergy and life‐stage sensitivity should be addressed to properly assess the toxicity of herbicides to nontarget organisms.