Glenn O. Araka
Abstract: The long term use of many insecticides is constantly threatened by the ability of insects to evolve resistance mechanisms that render the channels ineffective. Such resistance poses a serious threat to insect pest control worldwide. Resistance may result from either an increase in the ability of the insect to detoxify the insecticide or by changes in the target protein with which the insecticide interacts and in which case metabolic or target-site resistance will arise. In resistance, those insecticides involved act on the voltage-gated sodium channel proteins found in insect nerve cell membranes. The correct functioning of these channels is essential for normal transmission of nerve impulses and this process is disrupted by binding of the insecticides, leading to paralysis and eventual death. Some insect pest populations have evolved modifications of the sodium channel protein which prevent the binding of the insecticide and result in the insect developing resistance. In this study Anopheles gambiae 3rd instar larvae both susceptible and field strains were tested against the plant extract from C. cinerariifolium (Cc), E. camaldulensis (Ec) and N. tabaccum (Nt) in the concentrations of 50, 100, 150, 200 and 300 ppm, on exposure for 24 hr to observe the out-coming resistance ratio (RR). The results indicated that all the laboratory susceptible strain showed complete larval mortality (100 %) when subjected to test concentrations. The larvae showed no resistance and the RR ranged from 0.885 lowest (Cc ethanol January 2010) to 1.250 highest (Cc aqueous April 2010). Considering the field strain RR varied from 1.003 lowest (Cc ethanol January 2010) to 4.989 highest (Nt aqueous in March 2010). A majority of the field strain was susceptible to low concentrations of the extracts and hence obsessed with high RR notably N. tabaccum aqueous extract (1.981), Ec aqueous extract (1.823) and Nt hexane extract (1.561). A weak resistance was portrayed by Cc ethanol extract (RR 1.003), Ec hexane (RR 1.005), Cc ethyl acetate (RR 1.066), Nt DCM (RR1.039), and Nt methanol extract (RR 1.081). The crude leaf extracts which exhibited resistance close to RR1 but, however, counted as no resistance were: Cc DCM (RR 0.996), Ec methanol (RR 0.998), Ec DCM (RR0.993) and Nt ethanol leaf extract (RR 0.999). It was observed that RR increased as the rains increased from the months of March to June 2010.
Keywords: Resistance ratio, susceptable strain, temephos, plant extracts, C cinerariifolium, E camaldulensis, N tabaccum, 3rd instar larvae, rain season, concentrations