A tool for reducing vibrations in wind turbine blades at standstill includes an elongate sleeve formed of a net-like material for fitting over the blades, wherein the sleeve is formed with at least one protruding structure extending along at least a part of the length of the sleeve having an undulating form, and which is arranged so that when the sleeve is fitted on a blade the protruding structure or structures lie at the leading and/or trailing edge of the blade. A method for securing wind turbine blades against oscillations is also disclosed.