The Effects of the Herbicides Diquat® and Dichlobenil (Casoron®) on Pond Invertebrates Part I. Acute Toxicity

Abstract
The median tolerance limits (TLm) of Diquat (1:1‐ethylene‐2:2′ dipyridylium dibromide) and Dichlobenil (2,6‐dichlorobenzonitrile) to one amphipod and larvae of five species of aquatic insects were determined. Diquat was over 300 times more toxic to the amphipod (Hyalella azeteca) than to the mayfly (Callibaetis sp.) while the caddisfly (Limnephilus sp.), tendipedid (Tendipedidae), damselfly (Enallagma sp.), and dragonfly (Libellula sp.) in that order, were more resistant. The addition of pond mud to the test vessels changed the 96‐hour TLm of Diquat for the amphipod from 0.046 to 6.8 mg/liter. Dichlobenil was less toxic than Diquat to the amphipod but more toxic to the aquatic insects. Dichlobenil had a narcotizing effect that was recorded as an immobilization concentration (IC50). The addition of pond mud did not reduce the toxicity of Dichlobenil to the caddisfly. The 4% granular formulation of Dichlobenil had a 72‐hour “lag” before an effective concentration was obtained equivalent to the toxic effect of the 50% wettable powder. Dichlobenil formed a strong layer at the bottom of the test vessel and was more toxic with time to the invertebrates than to the fishes Micropterus salmoides and Lepomis macrochirus.

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