Ranger Blog: Lesser of two Weevils

Sarah Jones, Engagement Ranger Sarah Jones Engagement Ranger
The millpond at Dinefwr

Springtime is usually a season of new life and exploration for Dinefwr, but an invasive pond plant is threatening biodiversity of the waterways on our parkland National Nature Reserve.

A Water Fern (Azolla Filiculodies) originally found on the American continent has spread rapidly across the British Isles in recent years, Carmarthenshire seeming to be a new hot spot.


Spread mainly by spore dispersal and by wildfowl travelling between ponds, the Azolla is a definite cause for concern for the conservation team at Dinefwr.


The National Trust status is a conservation charity so our options to control the Water Fern, whilst protecting the existing habitat, are limited.


If the Azolla gets a good grip on a waterway or pond it will form floating mats of vegetation that can grow to up to 30cm thick. This can cut off the sunlight to the pond beneath and suffocate our native species, eventually killing off the wildlife that Dinefwr is so famous for. 


Ideally we wouldn’t want to use chemicals within the park and to control through mechanical means could result in the plant spreading further as the weed mat is broken up and dispersed. 


As a preferred, biological means of control we’ll be treating the Azolla using the novel approach of introducing a plant specific predator called the North American Weevil (Stenopelmus Refinasus).


This little insect is an invasive species in itself but only eats this particular type of Water Fern and will do so until it has disappeared - affectively eating itself to death!


We’d hope to see a big difference by the end of the year.