Abstract:

  Selective discriminating behaviour of the ovipositing female for an appropriate oviposition habitat selection and the substances involved in oviposition site choice by vector mosquitoes have recently become a focal point of interest in the concept of integrated vector control management. In the current study, we isolated and identified α-amyrin acetate from Catharanthus roseusLinn (Apocynaceae) and assessed the  skin repellency, oviposition deterrency, ovicidal, gravid mortality and deleterious delayed mortality against the malarial vector Anopheles stephensiListon (Diptera:Culicidae). Water treated with the α-amyrin acetate had a high deterrent activity in ovipositing females: oviposition activity index values for the test species were -0.22, -0.38, -0.42 and -0.52 for α-amyrin acetate at concentrations of 0.007, 0.015, 0.025 and 0.050 p.p.m., respectively. High degrees of mortality were observed with various concentrations of α-amyrin acetate: 1.12 (control) to 7.20 for gravid females, and 0.62 (control) to 9.05 for oviposited females. The highest mortality in both gravid and oviposited females was observed soon after they came in contact with oviposition medium treated with the α-amyrin acetate, and this was found to be significant at doses higher than 0.015 p.p.m., suggesting possible contact toxicity of the α-amyrin acetate. The α-amyrin acetate had the most effective skin repellency with ED50 and ED90 values of 0.6659 and 1.7720µg/cm2 with biting protection time of 3.5 h. The α-amyrin acetate caused moderate ovicidal activity against various age groups of A. stephensi but it inflicted delayed effects such as high larval, pupal and adult mortality. The age of the eggs and the duration of the α-amyrin acetate treatment influenced the ovicidal activity observed. It is clear that α-amyrin acetate of C. roseus can affect the oviposition cycle of A. stephensiListon, thereby suppressing the vector population and adversely influencing transmission of the disease pathogen.  

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