Bioevaluation of insecticidal and repellent plants from central region of Kenya and chemical identification of bioactive derivatives
Simiyu, Silas Khamala
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Despite considerable control efforts, malaria still remains the most prevalent and devastating disease in the tropics. With more than 40% of the world population at risk, malaria undermines the welfare of several South American, Asian and African states, endangering the survival of children (killing one child every 30 seconds) and straining scarce resources. It is estimated that US $ 2 billion is spent on malaria control and treatment programmes in Africa annually. The problem is becoming increasingly difficult to manage because of the continuous intensification and spread of resistance to anti-malarial drugs by the parasites. This poses a serious threat in increased severity of disease and health. Vector resistance to insecticides is a recurring theme and a major problem in malaria control programmes. The safety and efficacy of N N-diethyl-mtoluamide (DEET), the most potent of the modern synthetic repellent, is questionable. Other problems associated with repellents include vector resistance, avoidance and frequent repetitive application. The recent discovery of trans-cis-nepetalactone and p-mentane-3,8-diol that are more effective and environmentally friendly than DEET justifies bioprospecting from plants. In our search for new repellents we have continued with bioprospecting activities of the Kenyan flora. The essential oils from leaves of Artemisia afra, Senecio moorei, Cineraria grandifolia (Asteraceae), Nepeta azurea, Satureja pseudomensis (Labitae), Clausena anisata (Rutaceae) and Pseudocarum eminii (Umbelliferae) from Mt. Kenya region were evaluated for their repellency and insecticidal activity against adult female An. gambiae mosquitoes. The essential oil of Nepeta azurea was the most effective repellent (RD50 6.5x10-7mg/cm2). The essential oils from other species studied also showed significant repellency effect. The order of repellency was: P. eminii > S. moorei > C. anisata > S. pseudomensis > C. grandifolia > A. afra with RD50 = 4.39x104 , 1.27x10-3 , 7.86x10-3, 1.15x10-2, 2.2x10-2, 3.07x10-2 mg/cm2, respectively. The essential oil from A. afra was insecticidal at 1% w/v (LD50= 2.26x10-2 mg/cm2). The constituent compounds were identified through GC, GC-MS and GC co-injection and bio-assayed for repellent and insecticidal properties.