Disease can have large effects on population and community processes, particularly during periods of outbreak. Capturing those effects is often difficult in ecological studies, due to the haphazard nature of outbreak timing and limited information on the infective agent and epidemiology. Here, we investigated an outbreak of an apparent infection in caddisfly communities in upland streams in Victoria. Infection of the egg masses of Hydrobiosidae has long been noted at low levels, but greatly increased in the summer of 2018-2019. We identified the infective agent as a water mould (oomycete) in the genus Saprolegnia. Saprolegnia is a worldwide pathogen having important impacts in aquaculture and on amphibian populations. While known to infect insects, little is known about mode of infection or progression from infection through disease and mortality.
We characterised the extent of the outbreak with the use of prevalence surveys across space and time, establishing that infection rates of egg masses were much higher than previously seen in four rivers. Infection rates varied through space at multiple scales and among caddisfly genera. Infection declined into winter, but remained elevated compared to previous summers. In laboratory experiments, we demonstrated increases in egg mortality with increasing duration and extent of Saprolegnia infection within an egg mass. With early infection, mortality within an egg mass is nearly 100%, and at the scale of a single egg visible infections are apparently always fatal.
Although field mortality rates likely differ from those in the laboratory, the extent of visible infections in the field, combined with the high mortality associated with infection in the lab, suggest that the Saprolegnia outbreak greatly reduced egg survival. The consequences of this mortality for population and community dynamics depend on feedbacks between egg survival, caddisfly life history, and mechanisms of infection.