Seasonal abundance and parity of stock-associated Culicoides species (Diptera: Ceratopogonidae) in different climatic regions in southern Africa in relation to their viral vector potential

Abstract

Seasonal abundance and parity in Culicoides populations, in the vicinity of livestock, were determined at seven sites in five different climatic regions with 220 V down-draught blacklight-traps. In 418 collections made between October 1983 and December 1986, a total of 2 134 171 Culicoides, of which 342 571 were identified to species level and sexed, were collected; 267 of these collections (182 321 Culicoides) were graded for parity. In the frost-free summer rainfall area, Culicoides were collected in large numbers in light-traps throughout the year; this implies breeding and possible virus transmission throughout the winter in certain parts of South Africa. However, where frost occurred, Culicoides numbers usually peaked in late summer and dropped sharply after the first frost. In the latter areas, small Culicoides collections during winter may be due to low winter temperatures and rainfall; low temperatures negatively affect adult activity and reduce the rate of development of larvae and pupae; low rainfall would lead to a reduction of available larval habitats. Relatively large numbers of Culicoides were collected in winter in the temperate frost-free winter rainfall area. In each of the four summer rainfall areas, one Culicoides species remained dominant throughout the year: at two of these areas this species was C. imicola. Other abundant species in some of these summer rainfall areas were C. schultzei s.I. and C. zuluensis. In the winter rainfall area, C. zuluensis, C. magnus, C. gulbenkiani and C. imicola shared abundance. It was established that abdominal pigmentation is an indicator of parity in C. imicola in South Africa. With the increase in Culicoides numbers towards the end of summer, there was also a rise in the proportion of parous (pigmented) females in most Culicoides species, which signifies a higher vector potential for African horsesickness and bluetongue towards the end of summer. This coincides with the seasonal occurrence of viral diseases transmitted by Culicoides species. Nulliparous (unpigmented) females of all Culicoides species were present throughout the year at all sites where Culicoides were continuously collected, confirming uninterrupted breeding in these areas.