Didymo Control


Didymo (Didymosphenia geminata), commonly referred to as “rock snot,” is an alga native to the northern parts of Europe and North America. Didymo has adapted to a wide range of water quality conditions and habitats, and has been spreading since the 1980s. It creates extensive white, beige, or brown (not green) underwater carpets in moderately flowing freshwater streams that are clean and cool with stable flows.

Although didymo is not in Missouri at the time of this writing, procedures are being put in place to minimize the risk of this invasive being introduced to Missouri’s waterways. Missouri’s trout streams provide ideal conditions for didymo to flourish.

Newly established didymo colonies are characterized by small, thick, brown bubbles on rocks. It can appear slimy, but actually feels like wet cotton or scratchy wool. The longer the colony persists, the longer the filaments become. These filaments detach and float downstream like a flotilla of tissue paper, thereby spreading the invasive organism.

A closeup of a clump of didymo that has been picked up from the water in someone's hand
Didymo looks slimy but feels coarse.
MAF Biosecurity NZ

How Didymo Spreads

Felt-soled waders and wading boots, worn by many trout anglers, appear to be a likely pathway for the spread of didymo. Felt soles are porous and hold moisture for days. A single cell of didymo can survive in the sole of the boot. Didymo can then be introduced unknowingly to the next stream the angler visits. In addition to waders and angling equipment, canoes, kayaks and other watercraft are also considered pathways for didymo to spread.

Effects on Natural Communities

Didymo can alter the entire stream ecosystem. Didymo can create extensive blooms by attaching its stalks to rocks or plants and covering the stream bottom. Its cells secrete a mucuslike substance to create a firm hold on substrates and to help keep plant moist.

Changes in the stream can be extensive and can include:

  • An increase in suspended organic material
  • Fluctuating pH levels
  • Change in macroinvertebrate (large, aquatic insects) populations (especially the pollution-sensitive taxa — caddisfly, stonefly and mayfly) as their habitat is covered by didymo colonies
  • Effects on fish populations that feed on the macroinvertebrates
  • A shift to pollution-tolerant macroinvertebrates like midges, leeches, etc.

Once didymo is established in an area, wading is hazardous due to slippery, algae-covered rocks. The spread of didymo can also affect the fishing industry by clogging water intakes of boat motors and interfering with fishing gear and lines. Excessive blooms of didymo can render fishing impossible, with devastating economic consequences.


Once established, didymo is difficult to control or eradicate. If you will be using your waders and other gear in another body of water without sufficient time to completely dry them, please take the following precautions to prevent transporting didymo to new waters.

  • Check all gear and equipment after use and remove any visible algae. If you notice algae on your equipment at a later time, do not dispose of the algae by putting it down a drain. Dispose of it in the trash.
  • Clean all equipment with a 2 percent household bleach solution, 5 percent saltwater solution, or dishwashing detergent. Allow all equipment to stay in contact with the solution for at least three minutes. Soak all soft items, such as felt-soled waders and life jackets, in the solution for at least 20 minutes.
  • Dry all equipment in sunlight for at least 48 hours.
  • Consider replacing felt-soled waders with a new, environmentally sensitive alternative.

It is important that Missourians work together to prevent the spread of didymo. Please share this information with others.

Additional Resources