Effects of water quality on deactivation and toxicity of Mexacarbate (Zectran®) to fish

Mexacarbate (Zectran®) is a carbamate insecticide that has been considered for controlling forest insect pests, particularly in northern conifer forests. The effects of different water characteristics (temperature, hardness, and pH) and aging (deactivation) on its toxicity to coho salmon(Oncorhynchus kisutch), Atlantic salmon(Salmo solar), brown trout(Salmo trutta), fathead minnows(Pimephales promelas), bluegills(Lepomis macrochirus), and yellow perch(Perca flavescens), were determined. Identification and toxicity of the breakdown products were also investigated. Mexacarbate was less toxic to eggs than to other life stages of fish. Water temperature and hardness did not affect its biological activity, but pH did—the insecticide was was 38 times more lethal to bluegills at pH 9.5 than at pH 6.5. Solutions of mexacarbate hydrolyzed to more toxic breakdown products during the first 3 weeks of aging; the rate of hydrolysis increased as alkalinity increased. The breakdown product 4-amino-3,5-xylenol was 70 times more lethal to bluegills than mexacarbate in 96-hr bioassay tests at pH 7.5. However, this breakdown product was apparently not present in sufficient concentrations or long enough to cause fish mortality when the insecticide is applied at rates recommended for control of forest insects.