The ecology of the tree-hole breeding mosquitoes in the Northern Guinea Savanna of Nigeria.
Abstract
The following is based largely on the author's summary. As little information was available on mosquitos breeding in tree holes in West Africa, a study of their ecology was undertaken near Kaduna in the Northern Guinea Savannah zone of Nigeria from 1960 to 1963, special attention being paid to survival through the dry season, which lasts five months. Breeding in a Haura village and in an uninhabited forest reserve was compared. The forest reserve carried an open type of savannah vegetation and was traversed by a stream bordered by a narrow belt of riverine vegetation with evergreen elements. Because it was impossible to find enough tree holes and as bamboo pots proved unsatisfactory, gourds were used to supplement the observations on the natural tree holes. Mosquitos of the genera Aedes, Culex and Toxorhynchites were taken in both situations; Eretmapodites chrysogaster Grab, was taken in the forest reserve only. The same species were commonly found breeding in tree holes in different kinds of tree. In general, species of Aedes other than A, aegypti (L.) predominated' in the reserve and species of Culex in the village. In the reserve, the more humid riverine vegetation harboured species that were not found in the drier savannah region but are more usually associated with rain forests. New water-filled gourds put out in the middle of the dry season in the forest reserve were later found to contain larvae of several species representing both Culex and Aedes. As there were no suitable alternative breeding sites, it is concluded that the adults responsible for the breeding had been aestivating [cf. R.A.E., B 41 167-168]. When water was present in the gourds throughout the dry season, some species were able to extend their breeding season by a few months or even to continue to breed throughout the year, both in the reserve and in the village. When gourds were placed at heights of 6-35 ft. above ground level under the canopy of the riverine vegetation, there were more without larvae, or with young ones only, at 20, 28 and 35 ft. (31, 33 and 34 per cent., respectively) than at 6 and 12 ft. (10 and 12 per cent). A smaller proportion of the gourds at 28 and 35 ft. contained Aedes larvae than at the lower levels, but the proportion containing Culex larvae was similar at all levels. E. chrysogaster was found only above 20 ft. and was the commonest species at 35 ft. Gourds that were allowed to dry out under natural conditions and were then soaked yielded Aedes mosquitos that had survived the dry season as eggs in diapause. When such gourds were repeatedly dried and soaked, larvae continued to appear, usually only up to the third, but sometimes up to the sixth, soaking.