Microplastics for Microorganisms: How Caddisfly Larvae May Use Microfibers for their Homes

Text and images by Joseph Barnes, except as noted.

I was about a month into the ARMN Spring 2026 Basic Training course, juggling work and classes, while knowing I would have to miss a field trip focused on stream macroinvertebrates (animals without a backbone that can be seen with the naked eye). Fortunately, a few weeks earlier I had attended the Virginia Association for Environmental Education Conference, where I had the opportunity to take a stream monitoring training that covered many of the same techniques. I hoped it would fill the gap left by the missed field trip.

It did, and it also gave me the perfect story to share with fellow ARMN trainees as part of our required coursework.

I have a background in plastic pollution research, so, naturally, I gravitated toward a presentation on one of my principal interests—microplastics. One of the stream monitoring trainers had brought microscopes to better see the macroinvertebrates we were counting and identifying, and when it was my turn to look, I noticed something only a person who had spent years counting microplastics under a microscope would notice—a blue synthetic microfiber nearly tangled around a caddisfly larva.

I had photographed microplastics incorporated into a caddisfly larvae case when I conducted microplastics research, but the cases were abandoned and this was my first time actually observing a caddisfly in real life. Seeing a microfiber beside a living caddisfly made that research suddenly feel all the more real. While I shared the story about how the natural world interacts with microplastics during my ARMN training in early spring, I waited until July to share this story in the ARMN blog to highlight Plastic Free July, a global movement that encourages people to reduce single-use plastics and think more intentionally about the products they use every day. Going completely plastic-free for a month is highly unrealistic for most of us, but even small changes matter, so I hope this story encourages readers to learn more about the plastic pollution problem.

Now, envision a dark and stormy night from your home. Imagine you’re curled up on the couch under a warm blanket with a cup of tea while wind and rain batter the windows outside. You’re comfortable because your home protects you from the storm.

I think this image is the closest to what casemaker caddisfly larvae experience in clean streams. Caddisflies (order Trichoptera) are tiny moth-like insects, but their larvae—their beginning life stage—start underwater in freshwater streams. They graze on leaves, grasses, and algae on the rocks underwater. But, as in our dark and stormy night, the caddisflies also have to weather intense conditions, like fast-moving water or fish trying to eat them. To protect themselves from these conditions, many caddisflies carry their homes with them.

The caddisfly species that make these homes are known as “casemakers,” and they produce silk that binds together tiny pebbles, sticks, leaves, or other materials into a portable protective tunnel-shaped case. The weight of their home helps them stay on the rocks and not be carried away by the running water and provides camouflage from predators by looking like a bunch of pebbles.

Let’s revisit our dark and stormy night vision. While I equated the safety of our four walls to a caddisfly larvae’s case, I specifically mentioned a warm blanket. Oftentimes, the blankets and clothes we use and wear are made of polyester or other types of petroleum-based synthetic fabrics that are woven together. If you look closely at your shirt sleeve right now, you may be able to see the woven fibers.

Unlike the caddisfly’s home, the materials we wrap ourselves in are not as tightly woven, and little plastic threads are constantly shedding from our clothes. These little plastic strands, called microfibers, easily end up in our waterways where caddisflies could incorporate the plastic microfibers into their cases, into their “homes.” In Northern Virginia, everything that washes down neighborhood storm drains eventually makes its way into local streams such as Four Mile Run, Long Branch, Holmes Run, or Donaldson Run before flowing into the Potomac River and ultimately the Chesapeake Bay. ARMN members and community volunteers support Arlington County’s and the City of Alexandria’s stream monitoring initiatives by routinely surveying these waterways, tracking the health of aquatic life through the presence of sensitive macroinvertebrates like caddisflies. Caddisflies are more than interesting insects—they’re also important indicators of stream health. Because many species require cool, clean, well-oxygenated water, their presence helps stream monitors assess the ecological condition of our local waterways.

Scientists are still studying how microplastics affect freshwater insects. Researchers have documented caddisfly larvae incorporating microfibers into their cases, but the long-term ecological consequences remain largely unknown. Plastic fibers may offer little structural benefit, could increase visibility to predators if brightly colored, or may be ingested (serving no nutritional value) while the larvae build or maintain their cases.

We all have the same basic needs for food and shelter, but the products that we use to meet those needs often don’t come from nature, and they can have cascading effects all the way down to our smallest organisms. While none of us can eliminate plastics entirely, we can reduce these unintentional impacts by keeping clothing longer, repairing garments when possible, washing synthetic fabrics less frequently and all laundry in full loads, and choosing natural fibers such as cotton, hemp, or wool when practical. Every piece of plastic that stays out of our streams helps protect the countless small organisms that form the foundation of healthy aquatic ecosystems.

The next time you walk beside one of Northern Virginia’s streams, remember that beneath the surface is an entire community of tiny engineers quietly building homes from whatever materials we leave behind. By making a few thoughtful choices, picking up trash, and continuing to care for the streams in our own communities, we can help ensure those homes are built from sticks and pebbles rather than plastic.

If you are in the area and want to learn more about local initiatives, come by the Arlington Central Library on Tuesday, July 21 from 5:30 – 7:30 for a free movie screening of “We’re All Plastic People Now,” hosted by Clean Virginia Waterways and Oceana! ARMN will also have a table set up outside the auditorium. The movie will be followed by expert panelists for audience Q&A. Register for your free ticket at https://www.cleanvirginiawaterways.org/film-screening.