Products based on bacteria (here Bacillus thuringiensis, Bt) that act as parasites on insects are globally among the most important biocides. One of these biocides is Dipel ES, a product commonly used in the U.S. and Europe. In Germany, it is mainly used to combat butterfly caterpillars such as the oak processionary moth in forest areas and parks. It is also approved for organic agriculture. However, the extensive and frequent application of Dipel ES has raised concerns about potential toxicity to wildlife.
Even if the proposed minimum distance to water bodies is maintained, biocides can enter the water cycle. In laboratory experiments, scientists investigated how Dipel ES affects water fleas (Daphnia magna). Water fleas are of central importance in the aquatic food web and are actually no target organism of the pesticide. "We exposed neonates and adults of this water flea to a wide range of concentrations of Dipel ES", explained Dr. Anderson Abel de Souza Machado, guest scientist at IGB and author of the study. "By observing mortality, immobilization, and other sensitive sublethal responses (physiological and biochemical biomarkers, such as body composition and detoxification enzymes) we assessed their general fitness status."
Dipel ES is more toxic in water than expected
The researchers found that Dipel ES triggered a very unusual type of toxicity patterns on water fleas.The immobilization and mortality were affected in an inverted U-shaped fashion, i.e. the toxicity was not always increased by increasing toxicant exposure. Based on the results, this biopesticide might be tens of thousands times more toxic (calculated as EC50s) than reported by the manufacturer, the researchers assumed.
The unusual toxicity pattern cause doubts about a further assumption: regulation toxicology usually assumes that the negative effects of toxins increase with their dose. "The effect that toxicity increases with increasing amounts of the toxin is called monotony", explained Anderson Abel de Souza Machado. "The effects of Dipel ES are apparently based on other mechanisms. Higher concentrations were non-toxic, which is termed in toxicology as a non-monotonic effect or effect with a inverted U-curve", he said. The preliminary chemical analyzes indicate that these uncommon effects may be partially explained by a particular behavior of this pesticide in water solution. However, more research is necessary to investigate whether these non-monotonic effects of Dipel ES might occur under environmental conditions.
The study was conducted within the framework of the Erasmus Mundus Joint Doctorate Programme SMART (Science for Management of Rivers and Their Tidal Systems) and was financially supported by the European Union.