EFFICACY OF NEW PYRETHRUM SYNERGISTS FROM OCIMUM KILIMANDSCHARICUM GUERKE AND TAGETES MINUTA L. AGAINST MALARIA VECTOR THE ANOPHELES GAMBIAE

Abstract

Mosquitoes are vectors that carry disease-causing viruses and parasites from person to person. Some of these diseases such as dengue, malaria, rift valley fever, yellow fever among others can be life threatening. Vector control is by far one of the most successful methods for reducing the incidences of such diseases. However, the emergence of widespread insecticide resistance and the potential environmental concern associated with some synthetic insecticides has indicated that additional approaches to control the proliferation of mosquito population are a priority research area. Concern on quality and safety of life in managing mosquitoes, has shifted steadily from the use of conventional chemicals towards alternative botanical insecticides that are target-specific, biodegradable and environmentally safe. Pyrethrins are natural plant compounds used in commercial vector control. They are usually formulated with synergists to improve quality, increase efficacy, mitigate resistance and make them cost effective. It was thus, necessary to explore new natural synergists to sustain the use of pyrethrins in vector control. In this study, essential oils from Ocimum kilimandscharicum leaves and Tagetes minuta flowers were extracted by steam distillation. Crude extracts were used with pyrethrins to conduct bioassays for larvicidal and adulticidal activity against 4th instar larvae and 4 day old adult Anopheles gambiae. For the bioassay, four concentrations (10ppm, 20ppm, 30ppm and 40ppm) each with 3 replicates, with a final total number of 80 larvae contained in each concentration. For each concentration of synergized mixture, twenty 4th instar larvae were used. Mortalities were recorded after 1, 3, 6, 9, 12 and 24 hour’s exposure, during which no food was offered to the larvae. Bioassay data was evaluated by regression and probit analysis and used to determine the lethal doses (LC50 and LC90) for the synergist mixtures. The results showed that LC50 and LC90 values for O. kilimandscharicum with pyrethrins were 0.00167 and 0.0076mg/ml while for T. minuta with pyrethrins it was 0.00361and 0.01644mg/ml respectively. Ocimum was a better synergist than T. minuta against larvae and adult stage. The components of the essential oils of O. kilimandscharicum was separated and identified by GC-MS. GC-MS resulted to nineteen compounds. The findings of this research could enable investigation of the active compound against An. gambiae and further exploration for large scale production of the synergists for commercial application.