The drift velocity of negative ions in He II, moving under the influence of electric fields large enough to quench the production of charged vortex rings, has been measured for a wide range of pressures and temperatures. It is found that, although the intrinsic (temperature-independent) vortex nucleation mechanism provides a major source of momentum loss, particularly at low pressures, the corresponding effect for the thermally activated (temperature-dependent) mechanism is immeasurably small. The underlying physical reasons for this interesting distinction remain unknown but some possible explanations are discussed.