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Azolla filiculoides Lamarck (Azollaceae) (water fern), is an invasive floating macrophyte capable of rapid growth leading to the complete coverage of water surfaces. The North American weevil Stenopelmus rufinasus, Gyllenhal (Coleoptera: Curculionidae) is a biological control agent that has become naturalised in the UK. To determine the effects of the weevil on A. filiculoides in ‘semi-controlled’ conditions, weevils were introduced at four stocking densities into outdoor floating tanks containing A. filiculoides. The introduction of weevils at all densities resulted in a significant decline in percentage cover of A. filiculoides compared to the control. Increasing weevil stocking density resulted in a more rapid reduction in the area of green A. filiculoides. The highest stocking density of 400 weevils per m2 caused complete kill within a month of introduction. The UK summer provides conditions that are suitable for weevil reproduction and herbivory, and even the lowest stocking density of weevils tested (100 weevils per m2) can have a substantial impact on the surface cover of A. filiculoides. Although the weevil is effective in summer, augmentation is required annually for reliable control.