VLF nonlinear wave growth can only be initiated by signals above some threshold amplitude. Current theories do not predict this for a uniform or smoothly varying geomagnetic field. It is shown here that if near-resonant electrons see fluctuations in the total field of sufficient frequency and amplitude, cyclotron wave trapping and thus nonlinear growth cannot occur. MHD waves in the VLF interaction region have sufficient amplitude (>2&ggr;) on rare occasions to raise the threshold above VLF growth saturation level, while MHD background levels of around 0.05&ggr; would approach a base level threshold only a little below an estimate made from a VLF event and described here.