Acute toxicity of essential oil compounds (thymol and 1,8-cineole) to insectivorous guppy, Poecilia reticulata Peters, 1859

Thymol and 1,8-cineole are now known bioinsecticidal monoterpenes occurring in many essential oils. The present study determined their toxicity on insectivorous guppy fish, Poecilia reticulata Peters, 1859 known to feed on mosquito larvae. The toxicity was recorded post 24 h treatment in experimental aquaria. The estimated median lethal concentration (LC50) values of thymol and 1,8-cineole for female fish were 12.51 and 3997.07 mg/L, respectively, and for males the concentration required was 10.99 and 1701.93 mg/L, respectively. There was significant inhibition of acetylcholinesterase (AChE) and carboxylesterase (CarE) in treated male fish after 1,8-cineole treatment, whereas thymol induced CarE in both sexes but induced only AChE in females. Overall, thymol and 1,8-cineole were moderately toxic to guppies compared to synthetic pesticides.