Manager Bullhead City Pest Abatement District Bullhead City, Arizona
Since 2013, the Colorado River below Davis Dam has experienced a nuisance-level increase in net-spinning caddisflies, specifically Smicridea fasciatella. Immense swarms of adult caddisflies reduce the quality of life for tourists and riverfront residents, jeopardize river-based businesses, and pose public health concerns (e.g., asthmatic response or skin reaction) to some individuals.Integrated caddisfly management will require the use of biological, physical, cultural, and chemical control methods. Among the cultural control methods, water flow management is a workable tool, and the life history strategy of S. fasciatella indicate reduced river flow conditions will suppress their population. To better understand the efficacy of reduced river flows for nuisance net-spinning caddisfly control, we partnered with the Bureau of Reclamation’s Lower Colorado Basin Region to run a series of low-flow releases from Davis Dam. We used that opportunity to pair laboratory-based critical thermal maxima (CTmax) experiments with semi-field assessments, where S. fasciatella larvae were exposed to drying riverbed conditions. Mean (± S.E.) CTmax of late-instar larvae was 39.4 ± 0.2˚C with a range of 38 to 42˚C. Larvae were increasingly more hyperactive and stressed at 33˚C and started to lose motor function at 35 to 37˚C. Ultimately, stressed induced behaviors prompted by temperature increase are likely to negatively impact survivorship, which was evident in our riverbed in situ chambers. Mortality of late-instar larvae exposed to riverbed drying conditions varied with vertical migration, riverbed substrate, and access to aquatic macrophytes impacting survivorship. Future optimization of this novel management tool will further inform caddisfly abatement efforts.
