Biological control of red water fern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae), in South Africa.
A. filiculoides is a floating, South American fern that is invasive on still and slow-moving water bodies in South Africa, where dense mats of the weed degrade aquatic ecosystems and limit their utilization. Mechanical and chemical control methods are either impractical or undesirable and biological control was thus envisaged as a solution. The weevil Stenopelmus rufinasus was imported in late 1995 and, following host-specificity screening, was released in late 1997. S. rufinasus has been released at 46 sites throughout South Africa and establishment has been confirmed at 31 sites. Infestations of A. filiculoides were completely controlled at 20 sites and at a further 5 sites there were significant reductions in the weed mats. Complete control, indicating situations where not a single plant remains, was recorded in <1 year. Pseudolampsis guttata from Florida, USA, was screened as an additional control agent, but was rejected as it developed equally well on A. filiculoides and all native southern African species of Azolla that were tested. Initial indications are that S. rufinasus will contribute substantially to the biological control of A. filiculoides in South Africa and that additional agents are not required at this stage.