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. 2014 Jul 9:7:315.
doi: 10.1186/1756-3305-7-315.

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

Ali Afify  1 , C Giovanni Galizia
Affiliations

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

Ali Afify et al. Parasit Vectors. .

Abstract

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 (10-12-103 ppm) and m-cresol (10-1-103 ppm), the 1:1 mixture of the two compounds at 102 ppm, and the two individual compounds at 102 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 (10-8-103 ppm), while m-cresol was behaviorally ineffective, except for a deterrent effect at the highest concentration (103 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 (102 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.

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Figures

Figure 1
Figure 1
A summary of literature data shows the published effects of p-cresol on different mosquito species. p-cresol has different effects (positive, negative and no effect) on various mosquito species. Data are contradictory for the effect of p-cresol on Ae. aegypti at 10-5-10-3 ppm concentration range. The effect of Bermuda grass infusion on Ae. aegypti is also plotted (at 1.99 ppm p-cresol). Data from: [17-27].
Figure 2
Figure 2
Oviposition cup and the filter paper. a) Oviposition cup with a filter paper immersed in the test solution, the bottom edge of the filter paper is cut and closed in a pointed shape. b) Opened filter paper with Ae. aegypti eggs ready for scanning. c) A piece of the filter paper with high magnification showing the individual eggs.
Figure 3
Figure 3
Chemical structure of p -cresol and m -cresol.
Figure 4
Figure 4
Oviposition activity indices (OAI) of p-cresol at a broad range of concentrations. p-cresol shows a dose dependent oviposition effect on Ae. aegypti gravid females, in which a low concentration (10-10 ppm) is stimulant while higher concentrations (10-8-103 ppm) are deterrent. Each data point represents the mean OAI and standard error of five oviposition cages (n = 5) except for 102 ppm (n = 13).
Figure 5
Figure 5
Oviposition response of Ae. aegypti to different concentrations (individually tested) of m-cresol. At the concentrations 10-1 ppm (a) to 103 ppm (e) m-cresol shows a deterrent effect only at high concentrations; only 103 ppm m-cresol received a statistically significant lower number of eggs than water (P = 0.003, paired t-test). (n = 5 for a, b, c, e; n = 13 for d). Asterisk indicates P < 0.05.
Figure 6
Figure 6
Oviposition response of Ae. aegypti to different concentrations (concentration choice test) of p-cresol and m-cresol. a) Response to water and 10-1 ppm, 1 ppm and 10 ppm of p-cresol were not statistically different (n = 5). b) Response to water and 10 ppm, 102 ppm and 103 ppm of p-cresol showed that increasing concentrations were increasingly deterrent (n = 5). c) Response to water and 10-1 ppm, 1 ppm and 10 ppm of m-cresol (n = 5) shows no deterrent effect of m-cresol. d) Response to water and 10 ppm, 102 ppm and 103 ppm of m-cresol (n = 5) shows a deterrent effect at high concentrations. Different letters indicate statistically significant differences. Pairwise t-test with Holm correction.
Figure 7
Figure 7
Oviposition response of Ae. aegypti to single odors and their mixture tested against water. a) p-cresol at 102 ppm was deterrent (P = 0.009, paired t-test, same data as Figure 4, replotted for comparison, n = 13). b)m-cresol at 102 ppm was not deterrent (P = 0.766, paired t-test, same data as Figure 5d, replotted for comparison, n = 13). c) The mixture was deterrent (P = 0.008, paired t-test, n = 11). Asterisk indicates P < 0.05.
Figure 8
Figure 8
Oviposition response of Ae. aegypti to m-cresol, p-cresol and water within the same cage. m-cresol at 102 ppm is deterrent (P = 0.001, Pairwise t-test with Holm correction) in the presence of p-cresol (n = 9). Different letters indicate statistically significant differences of mean.
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