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Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) control in South Africa: a 10-year review.


Azolla filiculoides Lamarck (red water fern) is a floating aquatic fern that was introduced to South Africa in 1948 and, by 1990, had infested a large number of water bodies and impacted water utilization and aquatic biodiversity. The frond-feeding weevil, Stenoplemus rufinasus Gyllenhal, was released against this weed in 1997. Pre-release studies showed this agent to be host-specific, damaging, capable of a high rate of population increase and had a wide thermal tolerance. The weevil was released at 112 sites throughout South Africa and rapidly dispersed to all sites of A. filiculoides. Quantitative post-release evaluations revealed that the weevil caused a dramatic reduction in the populations of the weed, with local extinctions occurring at the majority of the sites within the space of a year. In the last 10 years, the weed has reoccurred at a number of sites. These re-infestations did not reach the levels recorded before 1997 and were brought under control by the weevil. The weevil has shown the predicted wide thermal tolerance in the field and an ability to disperse unaided, up to 300 km. Despite local extinctions of the host plant, the weevil has been able to persist by moving between infestations of the weed. A. filiculoides no longer poses a threat to aquatic ecosystems in South Africa and is considered to be under complete control.