The parametric decay of the fast magnetosonic wave to an ion Bernstein wave and a quasimode is analysed to estimate the amount of power converted nonlinearly at the plasma edge during ion cyclotron heating. Low decay thresholds for the pump wave amplitude are obtained when the quasimode frequency is at the hydrogen cyclotron frequency and the fast wave frequency is near appropriate cyclotron harmonics (or their sums) of the edge plasma. To obtain an upper limit for the growth of the decay modes nonlinear Landau damping of the Bernstein wave as a secondary process, in addition to the usual convective and linear damping processes of the decay waves, is studied. It is shown that due to the secondary process and finite interaction geometry in the decay a negligible power conversion from the fast wave to Bernstein modes may follow even if the pump wave amplitude clearly exceeds the threshold. At larger pump wave fields (>200V/cm) a non-negligible power conversion could be obtained in spite of saturation of the instability. Because the wave fields of the fast wave at the plasma edge of JET tokamak are estimated to be less than 200V/cm no serious power losses for the heating wave due to the Bernstein wave decay at the edge are expected. This seems to be in accordance with the recent observations in JET. These dependences are described and applications to H minority heating of deuterium and deuterium-tritium plasma by ICRF waves in JET are presented.