POTENTIAL LARVICIDES IN NIGERIAN HERBAL RECIPES

POTENTIAL LARVICIDES IN NIGERIAN HERBAL RECIPES

Olawale H. Oladimeji*¹, Leonard Ani ¹ and Emmanuel Nyong ²

Department of Pharmaceutical and Medicinal Chemistry ¹, Department of Pharmacognosy and Natural Medicine ², Faculty of Pharmacy, University of Uyo, Uyo, Akwa Ibom State, Nigeria

ABSTRACT

 Investigations into the larvicidal potential of crude extracts of ten Nigerian plants were carried out against the fourth instar larvae of Anopheles gambiae mosquito. The phytochemical screening revealed that both anthraquinones and cyanogenic glycosides were absent in all the plants. However, alkaloids, saponins, tannins, cardiac glycosides, terpenes and flavonoids were either present or absent. The larvicidal activity expressed as % LA was concentration and incubation-time dependent. At 5%w/v (12 and 24h), only Carica papaya and Dacryodes edulis demonstrated remarkable larvicidal activity of 40% and 55% and 50% and 70% respectively while the rest were largely inactive. However, at 10%w/v (12 and 24h), seven of the ten plants namely; Antholeisia djalonensis (60% and 80%), Calotropis procera (50% and 70%), Carica papaya (70% and 80%), Cyathula prostrata (37% and 67%), Dacryodes edulis (90% and 100%), Pycanthus angolensis (45% and 50%) and Viscum album (33% and 73%) gave comparably stronger activities especially after 24h incubation time. This study indicates a potential use of these plants in the control of vector mosquitoes which cause malaria.

Anopheles gambiae mosquito

INTRODUCTION: Malaria is a very rampant and devastating disease in the tropical regions of the world.  Numerous efforts have been made in the past to control its morbidity and mortality ^{1, 2, 3}_.  Treatment of the vector at the various developmental stages with insecticides has received wide acceptability, though these synthetic products suffer from major disadvantages of resistance and environmental pollution ⁴.

The use of plants as alternative sources of potent chemicals for vector control has been extensively studied and documented ^{3, 5, 6, 7, 8}.In view of the continued search for potent mosquito larvicides of natural origin, this present investigation was carried out with some Nigerian plants against the fourth instar larvae of Anopheles gambiae mosquito.

MATERIALS AND METHODS: Collection of materials: Ten medicinal plants native to Nigeria were collected in the March, 2011 from various Local Government Areas of Akwa Ibom State, Nigeria. The plants were identified by Dr. (Mrs.) M. Bassey, of the Department of Botany and Ecological Studies, University of Uyo, Nigeria and voucher specimens labelled No H62 to No H71 were deposited in the herbarium of the Department of Pharmacognosy and Natural Medicine, University of Uyo, Nigeria. Anopheles gambiae larvae were bred in plastic buckets and appropriately identified.

Extraction and processing of Plant Materials: The plants were individually oven-dried (40^oC) and then separately ground into coarse powders. The resultant ground powders were then extracted with cold 96% aqueous ethanol at room temperature (27±2^oC) for 72h. The filtrates were also separately evaporated to dryness using a rotary evaporator (Buchi CH-920, Laboratorium Technic, Flawk/SG, Switzerland). The obtained residues were stored in amber bottles in a refrigerator (-4^oC) prior to the further tests.

Phytochemical screening: The dried crude ethanolic extract of each plant was separately investigated for secondary metabolites (alkaloids, saponins, tannins, anthraquinones, cardiac glycosides, terpenes, flavonoids and cyanogenic glycosides) according to the laid down rules ^{9, 10, 11, 12, 13, 14, 15, 16, 17}.

Bioassay for larvicidal activity:

The breeding of larvae of Anopheles gambiae: The larvae were bred by keeping outdoor basins of water under growing shrubs near houses for about two weeks. After this period, at least three groups of mosquito’s larvae were identified accurately in a container using classical methods ¹⁸.

Anopheles gambiae, Aedes aegypti and Culex piper-fatigans responsible for the transmission of malaria, yellow fever and filariasis respectively were so identified.  The fourth instar larvae of Anopheles gambiae were later selected, separated and the species authenticated at the Department of Entomology, Michael Okpara University of Agriculture, Umidike, Abia State, Nigeria before further work.  The method employed for the determination of larvicidal activity was adopted from that described by several authors ¹⁹ and WHO directives on such assay with modifications ²⁰. Thirty (30) Anopheles gambiae larvae in their fourth stage were put in recovery cups (250ml plastic jars) containing 10ml de-ionized water (pH 7.0) at room temperature (27± 2^oC). Three (3ml) volume each of the graded concentrations of the extracts (5 and 10 %w/v) were added to 90ml de-ionized water, mixed thoroughly and then poured into exposure cups (250ml plastic jars containing larvae food ). Each aqueous solution of the extract was set up in triplicates. Negative control (containing

90ml de-ionized water, larvae food and larvae) and as well as positive control (containing 3ml absolute alcohol, 90ml de-ionized water, larvae food and larvae) were also set up in triplicates. Both the test controls were set up, and maintained at room temperature (27±2^oC). The Anopheles larvae in each recovery cup were scooped and transferred by means of small nets into test exposure cups containing the sample solutions and or control, larvae food and de-ionized water ⁷. The larvae in the test and controls set-up were incubated for a period of 12 and 24h at room temperature (27±2^oC).

Therefore, the larvae were gently scooped into small nets, washed with de-ionized water, transferred into recovery cups containing 100ml of de-ionized water, maintained at pH 7.0 and allowed to settle. Prior to mortality determinations, the larvae in recovery cups were gently disturbed and made to go below the water surface by agitating the water with a sterile pipette. The dead and dying larvae which started to float on the surface were pushed down the recovery cups. The living larvae which were able to swim to the surface were allowed to do so within 5minutes following agitation. The larvae remaining and or staying at the bottom of the recovery cups unable to swim to the surface were regarded as dead.

RESULTS:

TABLE 1: PHYTOCHEMICAL SCREENING OF CRUDE ETHANOLIC EXTRACTS OF PLANTS

PLANT METABOLITES