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.

Effects of Aquatic Herbicides and Housing Density on Abundance of Pond-Breeding Frogs

“…Passive avoidance may have been the result of respiratory distress (coughing). Respiratory distress has been displayed in yellow perch exposed to concentrations of 1 to 5 ppm diquat (Bimber et al, 1976 (Wilson, 1967). Similar work by Hilsenhoff (1966) Diquat applications to fully aquatic (lentic and lotic) systems may be toxic to some aquatic animals ( Table 2, 18 and 19).…”

“…Although the risk assessment scheme promoted by Peterson et al (1994) (Bain and Boltz, 1992;Olaleye et al, 1993;Johnson, 1962;Cooke, 1977, andGilderhaus, 1967 (Wilson, 1967) Hilsenhoff (1966) found that reduction in habitat due to elimination of Elodea canadensis may be the reason for reduction in numbers of Hyalella azteca in the field (Hilsenhoff, 1966) rather than direct toxicity of diquat. Therefore, the inclusion of artificial substrate like conservation webbing may prevent these population reductions (Berry et al, 1975 (Gilderhaus, 1967, Berry et al, 1975, Hilsenhoff, 1966and Bond, 1967 , Seawall (1969), Gilderhaus (1967), Rogers et al (1992) and Sedor (1997) Diquat -ENVIRONMENTAL EFFECTS (Potamogeton richardsonii) may not be effectively controlled and duckweed, curlyleaf pondweed, and Eurasian watermilfoil may not be controlled for sufficient periods of time for this relatively expensive aquatic herbicide to be of maximum utility.…”

“…Since both amphipods and cladoerans are important in the food chain, the ecological relevance of adversely affected species may be high. However, Wilson (1967) indicated that the affected species may be replaced by species of a similar size class and therefore, the impact of the few species which may be eliminated from a water body, is likely to be minimal.…”

“…Although a number of authors (Wilson, 1967, Hilsenhoff, 1966and Berry et al, 1975 have monitored a wide diversity of species in the field, no adverse impact was noted on the SEIS Swingle (1950 and) has found that while a certain amount of foliage cover and filamentous green algae (foodstuff) is necessary to achieve the ideal forage to predatory fish ratio of 3 to 6, the ideal amount of forage and foodstuff algae varies significantly between water bodies. Swingle did not specify what might cause those variations but it can be conjectured to be due to size and depth of the water body, and possibly due to other factors like transparency, and size distribution of predator and prey animals.…”

“…1 -Page 8 Diquat -LABEL DESCRIPTION & HISTORY level (RQ = 0.003 = 0.021 ppm c.e./6.8 ppm c.e.). These results are important since striped bass, largemouth bass and amphipods are considered to be extremely sensitive to most pesticides including diquat (Hughes, 1969, Wellborn, 1969, Wilson and Bond, 1969and Shearer and Halter, 1980. Therefore, for all practical purposes, the level of concern (0.1) is not exceeded in acute risk assessment for protection of the biota.…”