Improved survival and oviposition of Simulium damnosum (Diptera: Simuliidae) in the laboratory

2000

English

Simulium damnosum s.l. Theobald (Diptera: Simuliidae), the main vector of onchocerciasis in Africa, has not yet been colonized in the laboratory. All Simuliidae are difficult to maintain through successive generations, and though varying degrees of achievement can be claimed for certain temperate species (Simmons & Edman, 1978, 1981; Brenner & Cupp, 1980; Ham & Bianco, 1984), and to a limited extent for S. damnosum s.l. itself (Cupp et al., 1981; Simmons & Edman, 1982), all species except one, Simulium decorum Walker (Simuliidae) (Brenner & Cupp, 1980) have evaded true colonization. Mass rearing of aquatic stages with a high yield, inducing successful mating, insemination, fertilization and oviposition, bloodfeeding mated adult females and maintaining adults alive for long periods all present problems. Progress has been made in each of these areas and though some valuable methods are now available, little advancement has been made in recent years. Production of infective larvae of Onchocerca volvulus Leuckart (Nematoda: Filarioidea) for molecular and immunological studies still relies upon the use of surrogate temperate species or baitcollected vectors. Genetic studies on the S. damnosum species complex are hampered by the difficulties in sustaining laboratory lines of populations or species, thus limiting crossing experiments and progress in understanding interspecific genetic transfer at various taxonomic levels within the complex. The latter has additional importance in monitoring the spread of insecticide resistance throughout West Africa, where blackfly control is routinely carried out for the prevention of onchocerciasis. Here we report on a series of experiments attempting to improve S. damnosum s.l. maintenance in the laboratory, with a longer-term view towards the establishment of a permanent laboratory colony. Our objective was to seek improved methods for obtaining increased survival of adult females and higher rates of fertilization and oviposition. In the laboratory, adult S. damnosum s.l. flies are usually maintained either singly in small tubes (Lewis, 1960; Duke, 1962; Raybould et al., 1982; Boakye & Raybould, 1985) or in groups within cups (Raybould & Mhiddin, 1974; Ham & Bianco, 1983; McCall et al., 1994). The standard method maintains single females within polypropylene tubes (4.5 cm ! 1.5 cm) with a strip of filter paper (as a dry resting substrate and for absorption of excreta) (Boakye & Raybould, 1985), but mortality rates can be high. Our initial observations suggested that simply increasing the length of the tube could greatly increase survival rates. Captive female S. damnosum s.l. are reluctant to lay eggs and often have to be induced. Immersion in water (Lewis et al., 1961; Raybould et al., 1979), carbon dioxide anaesthesia (Marr, 1962), decapitation (Lewis et al., 1961; Raybould et al., 1979) and brief (20 sec) shock-freezing at "20°C (P.J. McCall, unpublished observation) have all been employed to induce gravid S. damnosum s.l. to begin oviposition, but these extreme methods are often fatal. Simulium damnosum s.l. will freely oviposit in water under low light conditions (Cupp et al., 1981; Simmons & Edman, 1981, 1982; Boakye & Raybould, 1985; McCall et al., 1994). Although this has been successful with all S. damnosum species tested, few eggs develop suggesting that fertilization rates are low (Wenk & Raybould, 1972). We report on a simple method (McCall et al., 1994; McCall, 1995) to improve oviposition and egg development rates and we also investigated whether the S. damnosum oviposition aggregation pheromone (McCall, 1995; McCall et al., 1997a) might promote oviposition under laboratory conditions.

Wilson, M. D.; Osei-Atweneboana, M. Y.; Boakye, D. A.; McCall, P. J.