European Frogbit | Hydrocharis morsus-ranae L.

California Pest Rating for
European Frogbit | Hydrocharis morsus-ranae L.
Pest Rating: A |  Proposed Seed Rating: P


Initiating Event:

This plant has been rated as “A” on the CDFA Plant Pest Rating list.

History & Status:

European frogbit is an annual herbaceous aquatic plant native to Europe. European frog-bit is generally free-floating but in situations where the vegetation is dense enough, the leaves may become emergent. It has been found in the Great Lakes Basin since the 1930s, but is now spreading into inland streams and lakes within the larger area surrounding the Great Lakes. It is considered invasive as it can displace native flora, possibly resulting in habitat impacts on native fauna by reducing oxygen content of bodies of water. European frogbit populations increase in size rapidly by vegetative reproduction and forms dense mats. These mats can infest irrigation canals.  There are no effective controls for this species at this time. European frogbit was planted in ponds in Ottawa in 1932. It apparently escaped from these ponds; by 1939 it was found in the Rideau Canal and by 1967 it had spread into Lake Ontario, Lake Erie, and several localities in New York.

Official Control: None in California.

California Distribution:  European frogbit does not occur in California at this time.

California Interceptions: None.

This threat European frogbit would pose to California is evaluated below.

Consequences of Introduction: 

1.  Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score: 2

—Low (1) Not likely to establish in California; or likely to establish in very limited areas.

—Medium (2) may be able to establish in a larger but limited part of California.

—High (3) likely to establish a widespread distribution in California.

Risk is medium (2), as the plant could occur in wetlands such as the Delta.

2.  Known Pest Host Range: Evaluate the host range of the pest.
      Score: 3

—Low (1) has a very limited host range.

—Medium (2) has a moderate host range.

—High (3) has a wide host range.

Risk is high (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

3.  Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score: 3

—Low (1) does not have high reproductive or dispersal potential.

—Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

Risk is High (3). The plant produces via numerous seeds and overwintering turions, as well as spreading rapidly in water via vegetative growth.

4  Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below. Score: 1

A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.

—Low (1) causes 0 or 1 of these impacts.

—Medium (2) causes 2 of these impacts.

—High (3) causes 3 or more of these impacts.

Risk is low (1) as the plant can impede irrigation.

5.  Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B.  The pest could directly affect threatened or endangered species.
C.  The pest could impact threatened or endangered species by disrupting critical habitats.
D.  The pest could trigger additional official or private treatment programs.
E.  Significantly impacting cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score: 3

—Low (1) causes none of the above to occur.

—Medium (2) causes one of the above to occur.

—High (3) causes two or more of the above to occur.

Risk is high (3) as the plant could invade the water systems of California, disrupt natural lake communities and potentially lower biodiversity by covering lake surfaces.  

Consequences of Introduction to California for European frogbit:

Add up the total score and include it here. (12)

—Low = 5-8 points

—Medium = 9-12 points

—High = 13-15 points

6.  Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.
Score: 0

—Not established (0) Pest never detected in California, or known only from incursions.

—Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

—Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

—High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

7.  The final score is the consequences of introduction score minus the post entry distribution and survey information score: (12)


It’s not here yet, but it is invasive in the Great Lakes Region.

Conclusion and Rating Justification:

A potentially weedy plant, especially in mountain lakes and cooler freshwater basins. Deserves an A rating, as it’s so invasive in other states.


Catling P.M., Dore W.G., 1982, Status and identification of Hydrocharis morsus-ranae  and Limnobium spongia  (Hydrocharitaceae ) in northeastern North America, Rhodora 84(840): 523-545.

Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  16+ vols.  New York and Oxford.

Global Invasive Species database:

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312;[@]

Comment Period:  CLOSED

The 45-day comment period opened on Tuesday,  May 26, 2015 and closed on July 10, 2015.

Pest Rating: A |  Proposed Seed Rating: P

Posted by ls