Aggregated oviposition in Simulium ochraceum s.l. (Diptera: Simuliidae), an important Neotropical vector of Onchocerca volvulus

The Neotropical black ies of the Simulium position, the selection of the speciŽ c substrate within that stretch certainly involves visual ochraceum complex are major vectors of Onchocerca volvulus in the Americas, where and olfactory responses (McCall, 2002). In an Afrotropical species complex, S. damnosum >50% of all human cases of onchocerciasisare associated with their presence (WHO, 1995). s.l., oviposition aggregation is mediated by a volatile pheromone emitted from the In some parts of Mexico and Guatemala, where S. ochraceum s.l. is abundant and the freshly laid eggs (McCall, 1995). The pheromone is common to diVerent species in the primary vector, biting rates of up to half a million bites/person-year have been recorded complex (McCall et al., 1997), and indeed, in black ies generally, oviposition aggregation (Brandling-Bennett et al., 1981; Basañez et al., 1998). There have been surprisingly (Imhof and Smith, 1979; Hywel-Jones and Ladle, 1986) and chemically mediated few, thorough studies on the members of the S. ochraceum complex (Dalmat, 1955; attraction do not appear to be species-speciŽ c (McCall, 2002). Takaoka, 1981) and hence little is known of their natural history in general and of their In southern Mexico and Guatemala, members of the S. ochraceum complex oviposition behaviour in particular. Black ies lay their eggs within or close typically oviposit in tiny rivulets and streams, with minimal  ow rates, on the  oors of to running water. Many species are known to oviposit communally; many females may be forests on PaciŽ c-mountain slopes, at altitudes of 600–1500 m above sea level (Crosskey, attracted to a single site at the same time, depositing numerous eggs on a small area of 1990). Oviposition occurs during the day, between 11.00 and 15.00 hours (Dalmat, substrate (vegetation or stones) to create a large egg mass (Crosskey, 1990; McCall, 1955; Takaoka, 1981). Other than these observations, there is not much known about 2002). This behaviour, which appears to be common throughout the family Simuliidae, the oviposition behaviour of this vector complex. Laboratory studies have yielded little. may be an adaptation to the perilous nature of the environment in which the eggs are While studying the laboratory behaviour of Neotropical black ies, in an attempt to laid. Running waters can  uctuate markedly in depth within short periods of time, and establish a colony, Dalmat (1955) bloodfed tens of thousands of females but fewer than oviposition aggregation at single sites may ensure the creation of an egg mass that is 10% oviposited and none of the eggs developed (although >65,000 black ies suYciently heavy to remain submerged if the water level drops before the eggs hatch. If were used in some trials, the exact numbers of female S. ochraceum s.l., S. metallicum and left high and dry, the large gelatinous egg masses also retain water, protecting the inner S. callidum that were fed are not speciŽ ed). The main aim of the present, laboratory (earliest-laid) layers of eggs from desiccation (McCall, 1995; McCall and Cameron, 1995). investigation was to determine if the oviposition behaviour of S. ochraceum s.l. Although it is unknown how black ies locate a suitable stretch of water for oviinvolved olfactory attractants. Oviposition