Caddisflies are generally harmless but can swarm in huge numbers near bodies of water, especially human‑altered waterways. In Arizona near the Colorado River, a new study finds residents consider caddisfly swarms to be a significant nuisance, but tolerance has improved as residents learn more about the insects and ongoing management efforts. (Photo by dontfencemein via iNaturalist, CC BY-NC 4.0)

By Grant Bolton, Ph.D.

Grant Bolton, Ph.D.

On summer days, around sunrise and sunset, along the Colorado River in Bullhead City, Arizona, tourists and residents often look out and see a flurry that almost looks like snow. Stepping outside quickly reveals the source. Hundreds to thousands of adult caddisflies swarm through the air, landing on clothing, buildings, sidewalks, and occasionally finding their way into unwelcome places.

Although caddisflies are usually regarded as harmless aquatic insects, a new study published in November in the Journal of Medical Entomology suggests that, under the right environmental and urban conditions, their synchronized emergence can create significant nuisance events.

Scientists and environmental managers commonly use caddisflies as indicators of good water quality. Along with mayflies and stoneflies, their aquatic larvae are part of an index used to assess clean, oxygen-rich freshwater systems. In this way, caddisflies are often seen as a biological success story rather than a problem.

That success, however, can become highly visible. When caddisflies emerge from the water at the same time, adult populations can reach extraordinary densities. Similar mass emergence events have been documented along major river systems worldwide, including the St. Lawrence, Mississippi, Winnipeg, Danube, Shinano, and Uji rivers. In areas where large rivers intersect with human development, these natural events can quickly become disruptive.

In Bullhead City, the swarming is not considered a public health threat. Caddisflies do not bite or sting, are not known to transmit disease, and, unlike cicadas, do not produce sound. Still, their sheer numbers can interfere with outdoor activities, affect tourism, and degrade the quality of life for residents near the river.

Caddisflies are generally harmless but can swarm in huge numbers near bodies of water, especially human‑altered waterways. In Arizona near the Colorado River, a new study finds residents consider caddisfly swarms to be a significant nuisance, but tolerance has improved as residents learn more about the insects and ongoing management efforts. (Photo by kneubaue via iNaturalist, CC BY-NC 4.0)

The study points to human-altered waterways as a key factor in amplifying these swarms. The construction of dikes, hydroelectric dams, and reservoirs can unintentionally create ideal conditions for caddisfly larvae. Impounded waters tend to collect large amounts of organic matter, an important food source for larvae. Sudden changes in water flow from dams can also reshape downstream riverbeds, creating suitable habitat for these bottom-dwelling insects.

In Bullhead City, the primary species responsible for nuisance swarming is Smicridea fasciatella. The Bullhead City Pest Abatement District, originally established in 1984 to manage black fly populations, has since expanded its focus to include this net-spinning caddisfly.

The Pest Abatement District has developed several strategies to reduce nuisance impacts and raise public awareness of peak swarming periods. Rather than attempting to eliminate caddisflies, abatement efforts have focused on limiting their effects. These strategies have included stocking trout in areas with high caddisfly densities and adjusting river flow to reduce larval habitat and breeding sites.

To evaluate whether these efforts influenced public perception, entomologist Michael Cavallaro conducted surveys of Bullhead City residents as part of a study examining the nuisance status of S. fasciatella.

In his study, Cavallaro evaluated how residents of Bullhead City rated the caddisfly nuisance in 2021 and then followed up to compare their responses in 2024. In 2021, 68 percent of respondents described the caddisfly problem as intolerable. By 2024, perceptions had shifted, with 61 percent of respondents reporting that the swarms were tolerable at certain times of the year.

Survey results also showed that perceptions varied widely. Residents living closer to the river were more likely to describe the swarms as severe, and awareness of the Pest Abatement District’s efforts differed among respondents. Together, the findings suggest that public concern is shaped not only by insect abundance but also by how often people encounter swarms and what they expect to experience.

Caddisflies are generally harmless but can swarm in huge numbers near bodies of water, especially human‑altered waterways. In Arizona near the Colorado River, a new study finds residents consider caddisfly swarms to be a significant nuisance, but tolerance has improved as residents learn more about the insects and ongoing management efforts. Clear, science-based guidance can help residents understand what caddisflies are, when swarming is most likely to occur, and which control options are practical at the household level. Shown here are the interior panels of an educational pamphlet titled “Your Neighbor Guide to Caddisflies,” distributed by the Bullhead City (Arizona) Pest Abatement District. (Image originally published in Cavallaro 2025, Journal of Medical Entomology)

Caddisfly swarming most often disrupted leisure and recreational activities, including sitting outdoors, sunbathing, and eating outside. Some respondents also reported respiratory discomfort during heavy swarming events. The study notes that fine hairs shed by adult caddisflies may irritate the airways and could worsen breathing problems or allergies in sensitive individuals.

The study also shows the importance of education in managing nuisance insects. In many successful abatement programs, public outreach has proven just as important as physical management efforts. Clear, science-based guidance can help residents understand what caddisflies are, when swarming is most likely to occur, and which control options are practical at the household level.

Cavallaro’s findings suggest that continued public engagement, including surveys and educational materials, can help refine messaging and improve community response. In tourism-dependent communities like Bullhead City, expanding outreach to include visitors can help balance ecological realities with economic and social needs.

Together, the study shows that nuisance swarming is shaped as much by human experience as by insect biology. While environmental conditions and management strategies influence caddisfly abundance, clear communication and public education play a central role in how communities understand and tolerate these events. In places like Bullhead City, managing expectations may be just as important as managing insects.

Grant Bolton, Ph.D., is a freelance writer and voice actor with a Ph.D. in entomology based in western Missouri. Email: [email protected].