Colorado River Bugs Spark Two Unprecedented Experiments—With Opposite Goals

Nov 6, 2020

In autumn swarms of flying insects cloud the skies on the lower Colorado River near Bullhead City. Caddisflies are a nuisance to recreationists who want to boat, swim or fish on the river. So city officials have started an unprecedented experiment to get rid of them. They recently tweaked the operation of Davis Dam to lower the river’s level and try to kill off the bugs. But just three hundred miles upriver another first-of-its-kind experiment is happening at Glen Canyon Dam, with the goal of SAVING caddisflies, not killing them. KNAU’s Melissa Sevigny reports on how the Southwest’s largest river ended up in a tug-of-war over a tiny bug.

An insect trap floats on the river below Davis Dam in Bullhead City


If someone in Bullhead City wants to complain about a bug, Michael Cavallaro is the one who answers the phone. He’s the city’s pest abatement manager and he gets a lot of calls. “These caddisflies,” he says, “are number one on Bullhead’s most wanted list.”

Michael Cavallaro takes a boat out on the Colorado River to set up insect traps.
Credit Melissa Sevigny

Caddisflies don’t sting or bite. They’re considered a sign of a healthy river. But for recreationists on the lower Colorado, the bugs are annoying. They get into eyes and ears and divebomb into picnic dinners. That’s why Cavallaro is working on a project to get rid of them. He drives a boat downriver, stopping to set up bug traps on the shore. They’re containers of pink antifreeze beneath a glowing blacklight. In an hour they’ll be full of dead insects.

Adult caddisflies swarm at sunset in Bullhead City
Credit Melissa Sevigny

“The light trapping scheme was designed to start a biomonitoring program, just baseline data set,” he explains as he sets up a bug trap. “We’ll process them on the boat, I’ll take them back to the lab.”

Cavallaro is checking on the results of the Davis Dam experiment, or “the anti-bug flow.” For two days in August and two days in September, dam operators lowered the river’s level until you could walk across it. The idea is to dry up the insect larvae that live underwater. Cavallaro says if it works, it could be a creative way to control the bugs so people can recreate on the river in comfort again. “The caddisflies in this scenario are a symptom of the problem,” he says. “The problem is massive changes impacted by hydroelectric dams.”  

Michael Cavallaro sets up a bug trap
Credit Melissa Sevigny

Davis Dam changed the muddy Colorado into a clear, cold stream that caddisflies love. They’re also attracted to all the lights of the houses and casinos along the river. That’s why there’s a boom of bugs in Bullhead City.But three hundred miles upriver, the Colorado flows through a different dam, Glen Canyon. That part of the river has the opposite problem. The dam killed the caddisflies. Ecologist Ted Kennedy says that’s a problem for the ecosystem.

“These aquatic insects are really the lifeblood of the ecosystem, they provide food for fish, they provide food for birds and bats when they emerge,” Kennedy says.

Citizen scientists collect bugs on the Colorado River below Glen Canyon Dam
Credit Dave Heramimtsohuk/USGS/Freshwater Illustrated

Glen Canyon Dam
Credit USGS

Kennedy is working on an experiment called a “bug flow.” Here’s how it works. Because of Glen Canyon Dam, the river goes up and down dramatically in response to hydropower demands in faraway cities. When the river drops, insect eggs glued to rocks on the riverbanks dry out and die. During a bug flow, dam operators keep the river low and steady on weekends, when hydropower isn’t so much in demand. Kennedy says that gives the eggs a chance to hatch. “It’s a potentially really powerful way to improve the overall health of the ecosystem.”  

Worldwide, insects in general and caddisflies in particular are threatened by habitat loss, pollution and other human activities. Jeff Muehlbauer is an entomologist who works on the “bug flow.” He says what’s happening downriver at Davis Dam with too many bugs is unusual.

People swimming in the lower Colorado River at sunset
Credit Melissa Sevigny

“They’re a pest because of a problem of our own making,” Muehlbauer says. “We’ve created that ecosystem downstream of Davis Dam.” He says tweaking dam operations could be a more sustainable option to control caddisflies than just dousing the riverbanks in pesticides. He says, “Of course with dams there’s hydropower involved, and money and dollars and energy. As long as all those groups say we can afford this, we’re on board with it, it’s a really appealing tool.”

Muehlbauer explains the plague of bugs in Bullhead City, and the absence of them below Glen Canyon Dam, are both human-caused catastrophes. They reveal how profoundly the Colorado River is altered by dams. But he says that means we can use dams to undo some of the damage, too.