The Effects of Dursban® Insecticide on Pocillopora damicomis (Cnidaria: Scleractinia)

Abstract

The effects of Dursban®, a chlorpyrifos-based pesticide, on reef-building corals was investigated. Short-term (96 hours) static bioassays with renewal of toxicant every 24 hours were conducted with Pocillopora damicomis colonies. Two sets of experiments were conducted. The first examined the toxicity of the commercial pesticide mixture made up of filtered seawater (0.45 µm) and the manufacturer's recommended dose for the treatment of lawns and gardens (0.91 mI/l). The second determined the toxicity of effluent seawater obtained from a soil column 24 hours after it was treated with Dursban® mixture (0.91 mi/l) at the manufacturer's recommended level of coverage (1.53 ml/12.6 cm2). In both experiments, coral branches were exposed to logarithmic dilutions of the toxicant mixture for up to four days. The 96 hour median lethal concentration (96 h LC50) for the pesticide mixture was found to be 1.2 x 10-7% of the original solution while the soil effluent mixture had a 96 hour LC50 of 7.0 x 10-8% of the effluent solution. Gas chromatographic analysis of the pesticide stock solution showed that the chlorpyrifos levels remained relatively stable for the duration of the experiment. Pesticide levels were monitored in the experimental test water at each dilution level prior to exposure of corals to determine actual pesticide concentration although several of the lower dilutions yielded concentrations below the analytical detection limit of 2 µg/l. Data gathered from the bioassay tests revealed high sensitivity of the coral Pocillopora damicomis to the two toxicant preparations. The soil effluent water was appreciably more toxic to the coral than the straight pesticide mixture. This was thought to reflect formation of more toxic breakdown products derived from chlorpyrifos after application to the soil column. Other factors, like the interactive effects between the chemical binders and dispersants within the commercial formulation and the soil may have contributed to the increase in toxicity of the soil effluent solution. Effluent water from pesticide-treated areas may be more toxic to corals than previously suspected.