Aggregated oviposition in the Simulium damnosum complex is mediated by eggs in a laboratory bioassay

The phenomenon of aggregated oviposition in blackflies (Diptera: Simuliidae) was investigated under controlled laboratory conditions, using wild-caught females of the Simulium damnosum complex in Sierra Leone. A method was developed for inducing Simulium females to lay eggs, and used as a bioassay to measure the responses of gravid S. damnosum s.l. to freshly laid eggs of the same species complex. In a series of two-choice tests, significantly more ovipositing flies chose substrates already containing eggs over control substrates (P = 0.004). The time from introduction of flies into the oviposition system to the onset of egg-laying was significantly less when eggs were already present (P = 0.049). Flies responded more quickly when more eggs were present and the relationship between egg-batch number and the time of this response was curvilinear (P = 0.012). Ecological advantages and disadvantages of such aggregation behaviour and the possible role of semiochemicals in its mediation are discussed.