Gravid females of the mosquito Aedes aegypti avoid oviposition on m-cresol in the presence of the deterrent isomer p-cresol

Background: p-cresol (4-methylphenol) and its isomer m-cresol (3-methylphenol) have been shown to activate the same sensilla in Aedes aegypti (Linnaeus) mosquitoes. Whereas p-cresol has been suggested to play a role in oviposition site choice, the behavioral significance of m-cresol is unknown. Methods: Here, we assayed the oviposition behavior of Aedes aegypti towards p-cresol and m-cresol using cage assay. Specifically we tested different concentrations of p-cresol (10(-12)-10(3) ppm) and m-cresol (10(-1)-10(3) ppm), the 1:1 mixture of the two compounds at 10(2) ppm, and the two individual compounds at 10(2) ppm together in the same cage. Results: We show that (1) p-cresol is a stimulant at a low concentration and deterrent over a broad range of higher concentrations (10(-8)-10(3) ppm), while m-cresol was behaviorally ineffective, except for a deterrent effect at the highest concentration (10(3) ppm) (2) in concentration choice tests (different concentrations tested against each other), both compounds were deterrent only at the highest concentration (3) a 1:1 mixture of both compounds exhibited a deterrent effect on oviposition (4) when presented in separate cups but together in the same cage, p-cresol and m-cresol (10(2) ppm) both received significantly less eggs than water alone. Conclusions: Our results suggest that p-cresol is a strong oviposition deterrent with a stimulant effect at only a very low concentration, while m-cresol is not a deterrent per se. However, in the presence of p-cresol in the vicinity, m-cresol acts as a deterrent. This finding adds a new twist to the possible interactions of different odors in oviposition site choice: not only the source itself, but nearby odors also influence a mosquito's choice.