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Mosquito Fern

(Azolla Pinnata)

Photo of mosquito fern
Photo credit: David Nicholls,,

Mosquito fern is a free-floating, perennial aquatic fern. The aquatic plant is quite small, measuring only 3/8"-1" in diameter. Mosquito fern is generally found in quiet waterbodies, protected from wind action.


Other names for this plant include:
  • Common names: Azolla, feathered waterfern, ferny Azolla, water velvet
Ecological threat:
  • Native plant richness and abundance decrease in the presence of mosquito fern; zooplankton and phytoplankton decrease significantly. As a result, fish production decreases due to the lack of habitat and break in the food chain.
  • Although in its native range mosquito fern provides habitat for wildlife and fish, the habitat it provides is one of lesser quality than that of which it replaces.
  • This leads to decreases in dissolved oxygen concentration and light penetration, decreases in pH levels and nutrient richness and overall lower water quality.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for mosquito fern was based upon this literature review [PDF] developed by the department.


Leaves: Mosquito fern is olive green in spring and early summer. By mid-summer and extending into winter, the leaves add anthocyanins, thus causing the leaflets of mosquito fern to appear red.

Roots: The roots of the mosquito fern hang down into the water approximately 1" from the base of the plant. The roots extract nutrients from the water as the large mats remain free-floating in the water.

Similar species: Mosquito fern can often be mistaken for the similar-looking species, duckweed (Lemnoideae spp.)


Currently, there have been no reports of mosquito fern in Wisconsin. Have you seen it? Send us a report.


Mechanical: Manual removal of the plant is a control option. This can be done by skimming the plant from the surface with fine-mesh nets or a similar device. To yield a higher success rate, a thorough collection must be done to limit the number of plants present that are able to reproduce.

Chemical: It has been found that decreasing the amount of phosphorous in the affected waterbody will limit the growth potential of mosquito fern. An abundance of phosphorous, an essential mineral for growth of mosquito fern, can enter a waterbody in a number of ways, but typically is contributed via run-off from agricultural areas.

Biological: Mosquito fern cannot survive prolonged periods of freezing temperatures, so typically perennial growths of mosquito fern occur in regions that are protected from prolonged freezing.


View mosquito fern pictures in our photo gallery!


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