Economic value of Stenopelmus rufinasus to Great Britain
Azolla filiculoides (Azolloideae: EPPO Observation List) is an invasive aquatic fern species native to North America and widespread in the EPPO region. It can have negative effects on the aquatic system when it forms dense mats over the surface of waterbodies preventing sunlight from penetrating the water. Since its introduction to Great Britain, A. filiculoides has become naturalised and spread throughout lowland regions of southern and central Great Britain in ponds, lakes, canals, ditches and other slow-moving fresh water and it also infests some sites in more northerly locations. Management using traditional approaches, (manual/mechanical removal or chemical treatment) has limited efficacy and the effects are short-lived. Repeated measures are often required at significant cost. The natural enemy Stenopelmus rufinasus (Coleoptera: Curculionidae) was first recorded in Great Britain in 1921, probably entering the country as an accidental introduction on imported A. filiculoides plants. S. rufinasus has been used successfully as a classical biological control agent in South Africa, although in Great Britain, its development and dispersal ability are restricted due to climatic constraints. Thus, to overcome this, augmentative releases of mass-reared S. rufinasus are carried out. In the absence of biocontrol, the expected average annual costs of A. filiculoides management were estimated to range from 8.4 to 16.9 million GBP (approx. 9.4 to 19 million EUR). The impacts of naturalised S. rufinasus populations on A. filiculoides were expected to reduce management costs to 0.8 to 1.6 million GBP (approx. 0.9 to 2 million EUR) per year. With additional augmentative releases, A. filiculoides management costs were estimated to be lower still, ranging from 31.5 to 45.8 thousand GBP (approx. 35.5 to 51.6 thousand EUR) per year, giving an estimated benefit to cost ratio of augmentative S. rufinasus releases of 44:1 to 88:1.
Sources
Pratt CF, Constantine K, Wood SV (2022) A century of Azolla filiculoides biocontrol: the economic value of Stenopelmus rufinasus to Great Britain. CABI Agriculture and Bioscience 3, 70. https://doi.org/10.1186/s43170-022-00136-0