Afrotropical culicoides : biosystematics of the Imicola group, subgenus Avaritia (Diptera: ceratopogonidae) : with special reference to the epidemiology of African horsesickness

Abstract

A biosystematic study of seven Afrotropical and two Oriental species of the Imicola species-group was undertaken; this group of Culicoides of the subgenus Avaritia includes C. imicola the most important vector of the viruses of bluetongue (BT) and African horsesickness (AHS) known in the Old World. Five African species are redescribed i.e. C. imicola, C. pseudopallidipennis, C. bolitinos, C. miombo and C. loxodontis. Two new species are described, and the extralimital C. brevitarsis and C. nudipalpis are discussed where relevant. These nine species comprise the Imicola group, one of 10 groups constituting the subgenus worldwide. Due to confusion in the literature, the Imicola group is redefined and distinguished from the Orientalis group (also redefined); 21 species of approximately 70 world species of Avaritia are reassigned to either of the two groups. A key to all nine known species of the Imicola group is given; shortcomings in the taxonomy of the Orientalis group are discussed. The adult morphology of both sexes of the nine Imicola group species was studied; this revealed deficiencies in the descriptive format currently used in taxonomic studies of world Culicoides. Accordingly, greater detail has been introduced into descriptions and includes the use of new character states. Methods for mounting Culicoides on glass slides are also improved to ensure more accurate observation and measurement of diagnostic taxonomic features; furthermore, the descriptions are based on long series of each sex. Illustrations were made from specimens mounted symmetrically, and no feature omitted from any bodypart illustrated. Certain aspects of the life-cycle of most species were investigated but especially those of C. bolitinos, C. loxodontis and C. kwagga. The immatures of these develop exclusively in the dung of the elephant, the buffalo, the blue wildebeest, both species of rhinoceros and Burchell' s zebra. Two of these species, C. bolitinos and C. kwagga, have broadened their resource range as they can invade and mature in the dung of cattle and horses. This has obvious implications for the transmission of viruses, especially where indigenous herbivores are run with domesticated livestock. In an intensive two-year survey a comparison was made between the Culicoides fauna of a natural area, the Kruger National Park (KNP), and that found in areas changed by Man, namely livestock farms adjoining the KNP. This part of the eastern Transvaal lowveld is a main focus of AHS in South Africa. Light-trapping, rearing from dung, and pootering off live hosts, revealed that some species of the Imicola group are exclusively associated with certain herbivores; these include the elephant and the zebra which are suspected or proven reservoir hosts for AHS. The results thus throw further light on the epidemiology of this disease, and also show that Man plays a decisive role in determining the numbers, and distribution, of particular Imicola group species under certain conditions. In the case of C. imicola, the commonest and most widespread of all species, this range expansion, or establishment of foci, is due to man's maintenance of domesticated livestock in confined species, and especially where these are kept on irrigated pastures. However, the serendipitous discovery of a large imicola-free zone in South Africa indicates that edaphic conditions likely play an even more important role than Man and climate in determining the prevalence and abundance of C. imicola. This area is the sandy dune field west of Port Elizabeth and holds promise as a natural quarantine zone for the import and export of livestock.