Larvicidal activity of picralima nitida: An approach in malarial vector control

Abstract

Mosquitoes constitute a serious Public Health menace, resulting in millions of death worldwide each year. Emergence of insecticide resistant strains of the mosquitoes poses a serious threat and hence calls for alternative control measures. This study assessed the larvicidal efficacy of the methanolic and aqueous extracts of different parts of Picralima nitida against the 4th instar larvae of the malaria vector Anopheles gambiae. Larvicidal activities of the leaf, seed and pulp of the plant were therefore studied on laboratory reared larvae of A. gambiae at concentration ranges of 0.5 mg/ml to 5.0 mg/ml. The LC50 and LC95 values were obtained from probit analysis using SPSS version 16.0, at 95% confidence limit (CL). The Median Relative Potency of the extracts was also obtained using probit analysis at (P ≤0.05). Results of the study indicated that the LC50 and LC95 values of the aqueous leaf extracts were 3.14 mg/ml and 42.15 mg/ml at 24 h, 0.35 mg/ml and 4.73 mg/ml at 48 h and 0.16 mg/ml and 2.20 mg/ml at 72 h. A lower larvicidal activity was however observed with the methanolic leaf extract: LC50 value of 48.38 mg/ml, 15.82 mg/ml and 0.33 mg/ml at 24 h, 48 h and 72 h respectively. Methanolic seed extract on the other hand, exhibited a higher degree of potency compared with the aqueous seed extract with a low LC50 value of 0.87 mg/ml, 0.21 mg/ml and 0.15 mg/ml at 24, 48 and 72 h respectively; and an LC95 values of 0.74 mg/ml, 0.18 mg/ml and 0.12 mg/ml at same time interval. A higher efficacy of activity was exhibited by the aqueous pulp extract than the methanolic pulp extract; with lowest LC50 of 2.79 mg/ml at 72 h, and LC95 value of 10.40 mg/ml. The relative potency estimate of the aqueous extract at 24 h gave the following result: aqueous leaf extract was 5.48 times more potent than the aqueous seed; the aqueous pulp 2.20 times greater than the aqueous seed whereas the aqueous leaf was 2.50 times more potent than the aqueous pulp. Similar trend was also observed at 48 and 72 h. Comparatively, at 24 h the methanolic seed extract was 269.76 times more potent than the methanolic leaf extracts; the methanolic pulp extract 2.40 times more potent than the methanolic leaf while the methanolic seed gave a potency of 112.49 times more than the methanolic pulp. However at 48 and 72 h, there was a reversal in trend with the methanolic leaf extract showing a relative potency of 2.73 and 11.51 times that of the methanolic pulp. The LT50 (Median Lethal Time) of the extracts, evaluated at concentration of 1.0mg/ml with P = 0.05, was the following LT50; 28h, 3.6 h and 57 h for aqueous leaf extract, methanolic seed extract and methanolic leaf extract respectively. Similarly, comparative evaluation of the overall efficacy of the various extracts showed that the methanolic seed extract exhibited the highest degree of activity (P<0.05), followed by the aqueous leaf extract and methanolic leaf extract. Conclusively, aqueous seed, methanolic pulp and aqueous pulp extract showed a relatively lower activity and an LT50 value of 300 hrs and above at a concentration of 1.0 mg/ml. The results of this research therefore underscores the efficacy of the plant and further suggest the use of methanolic seed, aqueous leaf and methanolic leaf extracts of P. nitida as an eco-friendly alternative in malaria vector larviciding.