Effects of three organophosphorus insecticides on tropical anuran tadpoles: lethality and implications to motor activity

Organophosphorus (OP) pesticides are widely used worldwide as pest controls, but they can have negative effects on non-target species such as amphibians, which are the most threatened group of vertebrates. The aim of this study was to evaluate the lethal (median lethal concentration, LC50) and sublethal effects (burst swimming speed and activity of acetylcholinesterase, AChE) of three OP insecticides, chlorpyrifos, diazinon, and monocrotophos on tadpoles of three tropical anuran species from Colombia, Rhinella horribilis, Boana xerophylla, and Engystomops pustulosus. Each species was exposed independently to each of the three OP insecticides under laboratory conditions for 96 h. Chlorpyrifos was the most lethal insecticide in all species (LC50 range: 1.18-1.85 mg L-1), followed by diazinon (LC50: 6.21-7.92 mg L-1) and monocrotophos (LC50: 419-554 mg L-1). Chlorpyrifos fully inhibited the burst swimming speed in all three species, while increased sublethal concentrations of diazinon decreased it in E. pustulosus or did not have an effect in B. xerophylla and R. horribilis. Monocrotophos reduced the burst swimming speed in both B. xerophylla and R. horribilis and inhibited all swimming activity in E. pustulosus. A parallel pattern was found in the levels of AChE activity that was reduced by the insecticides in E. pustulosus and R. horribilis, although B. xerophylla tadpoles showed a hormetic dose-response. The lethal effect of the three OP insecticides on anuran tadpoles at the recommended doses of application in field, and their negative implications to the swimming and AChE activity at low concentrations show the negative impact of these agrochemicals on tropical amphibians. We suggest reevaluating the toxicological categories currently assigned by the Colombian government to these insecticides (monocrotophos > diazinon > chlorpyrifos), taking into consideration the aquatic fauna, as our results of lethality were contrary to this classification.