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Recognition of the importance of debilitating effects of insect virus diseases is currently growing. Commonly observed effects of sublethal infection at the individual level include extended development times, reduced pupal and adult weights, and lowered fecundity. However, for the most part, sublethal infections are assumed to be present in survivors of an inoculum challenge, rather than demonstrated to be present by microscopy or molecular techniques. Invertebrate iri- descent viruses are dsDNA viruses capable of causing disease with symptoms obvious to the naked eye, a "patent" infection, that is lethal. Furthermore, inap- parent "covert" infections may occur that are non-lethal and which can only be detected using bioassay or mo- lecular techniques. In this study, replication of Inverte- brate iridescent virus 6 in Aedes aegypti larvae was demonstrated in the absence of patent disease. A sensi- tive insect bioassay (using Galleria mellonella) allowed the detection of covert infections, which were more common than patent infections. A concentration- response relationship was detected for the incidence of patent infections. Covert infections were up to 2 orders of magnitude commoner than patent infections, but the prevalence of covert infections did not appear to be related to virus inoculum concentration. Exposure of larvae to virus inoculum resulted in extended juvenile development times. A reduction in the mean and an increase in the variability of fecundity and adult progeny production was observed in females exposed to an in- oculum challenge, although formal analysis was not possible. Males appeared capable of passing virus to uninfected females during the mating process. Covertly infected females were smaller and had shorter lifespans than control or virus-challenged females. A conservative estimate for the reduction in the net reproductive rate (R0) of such insects was calculated at slightly more than 20% relative to controls. Inglés