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We’ve all had an unwanted guest that we can’t seem to get rid of. The mother-in-law who stays too long or the neighbor who won’t stop talking when you’re trying to walk into the house.

Aerial View of the Colorado River

And we know the tactics to get rid of them. But what do you do when your unwanted guest is a pest? Literally.

Some Arizona residents whose homes back up to the CAP canal face these unwanted guests twice a year when caddisflies hatch. Caddisflies are a small, moth-like insect that live in water as larvae and on land as an adult.  They are harmless to humans and pets; however, they can be quite a nuisance when they emerge from the water in spring and fall.

Because of the impact to neighbors, CAP has invested significant time, labor, and financial resources into researching control mechanisms that will help to reduce the caddisflies. Unfortunately, our findings are no different from what scientists have concluded for the past 100 years; there is no effective, responsible method to significantly reduce or eliminate caddisfly populations. 

But CAP hasn’t given up. We know that catfish will eat caddisfly larvae in the canal. The fish won't eliminate the caddisflies, but they will reduce the population and provide some relief. Since 2011, CAP has stocked over 55,000 channel catfish in various canal locations to eat caddisflies. The fish are purchased from a supplier in Arkansas and transported to Arizona during spring (when temperatures are cool).  Stocking rates for each section of the canal are based on published literature that outlines the quantity of fish that a volume of water can sustain.  

“We know that this isn’t the ultimate solution, but the fish provide some level of control, although it varies from year to year,” said Scott Bryan, CAP Senior Biologist.

Additionally, CAP is collaborating with experts on the Colorado River and various researchers at Universities to find new solutions that may provide relief, including utilizing pheromone traps to attract and kill males, gene modification to reduce caddisfly populations and altering water temperatures to reduce/disrupt the reproductive cycle of the insect. Results from some of those studies are expected next year.

The bottom line is that although the CAP aqueduct is man-made canal, it contains untreated Colorado River water, which means that anything that can be found in the Colorado River, including caddisflies, will also be in the CAP canal. And although they are unwanted guests for our canal neighbors, in a vast majority of the world, caddisflies are highly coveted because they are a food resource for prized trout and they are indicators of good water quality.

For our “river-adjacent” neighbors who have caddisflies, we will continue to stock catfish and actively work with researchers to help provide some relief from these insects. As a homeowner, you can also take a few steps that might help to reduce the impact of these unwanted guests:

  • Keep backyard lights at a low level and limit yard floodlights
  • Replace white light bulbs with yellow “bug” lights 
  • Request that white streetlights in your neighborhood be replaced with yellow 
  • Reduce the amount of vegetation in your yard and surrounding your property
  • Bug zappers may attract caddisflies away from "sitting areas," but may also attract more caddisflies to your yard 
  • An insect fogger may provide temporary relief (caddisflies will likely return in a day or less)

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