Effects of temperature on aquatic insect development, survival, and infectious disease

Climate change is increasing the incidence and severity of extreme events such as heatwaves, which is a possible cause for documented increases in emergent diseases. Freshwater habitats are particularly susceptible to disease and climate change compared to terrestrial and aquatic habitats. One emergent disease in freshwater communities is caused by the oomycete, Saprolegnia spp. Saprolegnia infection has been observed in fish, frogs, crustaceans, and aquatic insects such as caddisflies. Caddisflies occupy freshwater streams during their egg, larval, and pupal life stages. Larvae play important functional roles in nutrient processing and are a prey source for higher predators. Saprolegnia infection has been observed to cause high mortality in caddisfly eggs, most prominently in the species Ulmerochorema rubiconum. I showed that temperature treatments, in the form of short-term (spikes) or long-term increases, affected Saprolegnia infection and caddisfly hatching in ways that may interact to amplify the consequences of disease under climate change. Infection probability increased with both spikes and long-term increases in temperature. Hatching times of caddisfly eggs were shortened with long-term temperature increases but not with spikes. The quantity of Saprolegnia in the water was not determined by the infection present within nearby egg masses, but low temperature treatment samples did have significantly more DNA present than those at higher temperatures. Temperature had no effect on the development of hatchlings at the time of hatching, but there was an increase in mortality seen at higher temperatures. My results suggest that, while prolonged increased temperatures do increase caddisfly mortality, a greater threat to caddisfly populations may be posed by temperature spikes, as they have the potential to occur more often and result in increased total infection due to longer hatching times. With climate change forecasts predicting an increase in the frequency and magnitude of heatwaves, the resulting changes in aquatic insect survival and infectious disease are likely to affect caddisfly populations, as well as ecosystem functioning.