Toxicity and Accumulation of the Insecticide Imidan in Freshwater Invertebrates and Fishes

Abstract
Toxicity tests showed that aquatic invertebrates and fish have a broad range of sensitivities to Imidan, an organophosphate insecticide applied agriculturally and possibly to forests. Invertebrate 48‐h EC50ˈs (concentration immobilizing 50% of the test organisms) or LC50ˈs (concentration lethal to 50% of the test organisms) ranged from 2.4 μg/liter for amphipods (Gammarus pseudolimnaeus) to 3,200 μg/liter for larval midges (Chironomus plumosus). Among fish tested, chinook salmon (Onchorhynchus tshawytscha) and smallmouth bass (Micropterus dolomieui) were the most sensitive (96‐h LC50, 150 μg/liter) and channel catfish (Ictalurus punctatus) the most resistant (96‐h LC50, 11,000 μg/liter). Toxicities of the active ingredient to fishes were similar for the technical grade Imidan and the 50% wettable powder formulation. Toxicities to fish increased with the increasing temperature and decreasing pH, but were unaffected by water hardness. Eyed eggs and yolk‐sac fry of rainbow trout (Salmo gairdneri) were more resistant to Imidan than were fingerlings. Aging of Imidan in water of 20 C and pH 7.2 decreased its toxicity. Four‐day‐old solutions were less than 0.04 as toxic as fresh solutions. Seven species of aquatic invertebrates and three species of fish exposed to 14C‐Imidan in slightly alkaline water accumulated radioactive residues in 48 h that ranged from 1 to 10 times (based on wet weight of whole organism) the water concentration of 1.2 ± 0.2 μg/liter.

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