Variability in the life‐history patterns of four species of Hydropsyche (Trichoptera: Hydropsychidae) in southern Ontario streams

The life‐history patterns of four Hydropsyche species (Trichoptera: Hydropsychidae) found in the Credit and Humber Rivers of southern Ontario were investigated through rearing studies, larval and adult collections, and analyses of densities and mortality of field populations of pupae. Hydropsyche slossonae was univoltine but was probably capable of producing a second generation under favourable conditions at the warmest station at which it occurred. Hydropsyche sparna was partially bivoltine at upstream stations and bivoltine at downstream stations. At downstream stations, the common species were Hydropsyche bronta . which was trivoltine. and Hydropsyche morosa , which was bivoltine. Hydropsyche sparna, H. bronta and H. morosa all exhibited a split‐cohort development, in which not all of the summer‐produced larvae completed development, pupated and emerged before the end of the growing season. Overwintering populations consisted of young larvae that had hatched from eggs laid by adults of the previous generation. Analysis of the temporal changes in the densities of living and dead pupae provided a reliable way to separate overwintering and spring‐ and summer‐produced generations.