Patterns of covert infection by invertebrate pathogens iridescent viruses of blackflies

Recently, it has been recognized that blackfly populations mar host two forms of infection by iridescent viruses (IVs)¡ a covert (inapparent, nonlethal) form which was common in springtime populations in the River Ystwyth, Wales, and a patent (obvious, lethal) form which was rafe. This study aimed to investigate the changes in frequency of the two types of infection in blackfly populations ayer the reproductive period of the flies, April-September 1992. Blackfly larvae sampled from three different sites along the river were bioassayed for the presence of covert IV infection. Of 870 larvae assayed, 17 were found to be infected. AII the infected larvae appeared to be Simulium variegatum, the dominant species during the sampling periodo IV infections were common in the spring (17-37% depending on site) but appeared absent in the S. variegatum population for most of the summer months, reappearing again in the autumn (0-20% infected).