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Table 3 Proportion of A. filiculoides targeted for control in each water body type used in cost calculations

From: A century of Azolla filiculoides biocontrol: the economic value of Stenopelmus rufinasus to Great Britain

Scenario

Region

Proportion of Azolla targeted for control

Assumptions

Rivers and streams (%)

Canals and feeders (%)

Drainage channels (%)

Lakes and ponds (%)

Ditches (%)

Scenario 1—Azolla, no weevils

England & Wales

10.0

90.0

50.0

70.0

5.0

Highest rates of management in major public access and recreation focused systems e.g., canals, angling and boating ponds and lakes. Canals with particular requirement to ensure public safety and manage invasive species. Drainage channels with some risks of blockage and responsibility to prevent spread. Ditches mainly agricultural and low priority except e.g., on nature reserves. River management only in slow sections and side channels where public have access and raise concerns

Scotland

10.0

90.0

50.0

70.0

5.0

Scenario 2—Azolla, naturalised weevils

England & Wales

5.0

70.0

20.0

40.0

2.0

Infestations less intense in all systems, but public pressure still significant particularly for canals and amenity lakes and ponds. Aesthetics important and substantial A. filiculoides outbreaks targeted. Drainage channels at reduced risk of major infestation and blockages. Infestations typically more transient and less damaging, regularly brought under control before management applied. Ditches of lesser concern and most river or stream infestations temporary and requiring limited intervention

Scotland

5.0

70.0

20.0

40.0

2.0

Scenario 3—Azolla, naturalised and augmented weevils

England & Wales

3.0

50.0

15.0

30.0

1.5

Most infestations observed are new outbreaks that A. filiculoides weevil has yet to locate. Infestations rarely reach large scale and density except in waters high in nutrients. Most sites actively managed are for aesthetics, leisure and safety under public pressure (e.g., canals, some public lakes and ponds), regulatory demands, or due to high sensitivity such as designated sites, reserves and private ponds. Significant outbreaks on some drainage channels targeted to prevent blockages and limit spread of INNS. Significant proportion of reported outbreaks are brought under control by naturalised weevil populations before augmentative release (or traditional management) is carried out

Scotland

3.0

50.0

15.0

30.0

1.5