The effects of the herbicides Diquat and Dichlobenil on pond invertebrates Public Deposited

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  • The acute toxicity of Diquat (1:1-ethylene-2:2' dipyridylium dibromide) and Dichlobenil (2, 6-dichlorobenzonitrile) to six selected pond invertebrates was determined using the median tolerance limit estimation method. Diquat was more toxic to the amphipod, Hyalella azeteca than to the aquatic insects Callibaetis, Limnephilus, Enallagma, Libellula, and Tendipedidae. The addition of mud to the test vessels reduced the toxicity to H. azeteca over 96 hours. The invertebrates were generally more tolerant to diquat than the fishes, Micropterus salmoides and Lepomis macrochirus. Dichlobenil formed a concentrated layer on the bottom of the test vessels and was more toxic to Tendipedidae than to the free swimming invertebrates. Dichlobenil appeared to have a narcotizing or turnover effect which was recorded as an IC₅₀ immobilization concentration. The addition of mud to the test aquaria did not change the toxicity. Formulation differences were apparent with the four percent granules exhibiting a 48-hour "lag" before an effective concentration was obtained equalivalent to the toxic effect of the 50 percent wettable powder. There was a greater toxic action with time for the invertebrates when compared to the fishes, M. salmoides and L. macrochirus. A field study was designed to determine the effects of the two chemicals at the recommended field application rates to pond invertebrates and aquatic plants. A split-plot statistical design was incorporated by dividing ponds into plots with polyethylene sheeting. Diquat killed all submerged plants within two weeks; however, the control was only temporary with regrowth beginning immediately. A reduction in dissolved oxygen was noted for ten days after treatment followed by an increase associated with heavy algal blooms. Diquat caused a reduction in H. azeteca associated with acute toxicity. Tendipedidae, Coenagrionidae, Libellulidae, Baetidae, and Sialidae were not reduced by the chemical. Dichlobenil acted slowly over a period of weeks to reduce submergent and emergent plant growth. Filamentous algae was completely controlled. Nine months after treatment less than a five percent regrowth of plants was noted in the plots. No major reduction in dissolved oxygen was noted following treatment. A reduction in Tendipedidae occurred associated with acute toxicity. Alteration of preferred habitat resulting from treatment reduced Coenagrionidae and Callibaetis. Libellulidae and H. azeteca were unaffected. Statistical evaluation of the experimental design indicated that pond divisions were effective in reducing needed replications and provided for a better estimate for treatment comparisons. The "power of test" formula for detecting differences indicated that the sample size could be reduced in future experiments for detection of a 50 percent difference at the 95 percent level.
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