Biological control of red water fern, Azolla filiculoides Lamarck (Pteridophyta : Azollaceae), in South Africa

Azolla filiculoides Lamarck (red water fern) 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 u tilization. Mechanical and chemical control methods are either impractical or undesirable and biological control was thus envisaged as a solution. The weevil Stenopelmus rufinasus Gyllenhal was imported in late 1995 and, foll owing host-specificity screening, was released in late 1997. The weevil has been released at 46 sites throughout South Africa and establishment has so far been confirmed at 31 sites. Infestations of A. filiculoides were compl etely controlled at 20 sites and at a further five sites there were signifi cant reductions in the weed mats. Complete control, indicating situations w here not a single plant remains, was recorded in less than one year. The fl ea beetle Pseudolampsis guttata (LeConte) was screened as an additional nat ural enemy, but was rejected as it developed equally well on A. filiculoide s and all native southern African species of Azolla that were tested. Initi al indications are that S. rufinasus will contribute substantially to the b iological control of A. filiculoides in South Africa and that additional ag ents are not required at this stage.