The rate nu at which negative ions nucleate vortex rings in He II has been measured at 25 bar for electric fields E up to 12kV/cm and temperatures T down to 0.4 K. A strong temperature dependence of nu observed for T < 0.6 K and E < 5 kV/cm is attributed to the influence of isotopic impurities. Although the temperature-independent behavior found for T < 0.6 K and 5 < E < 12 kV/cm is consistent with a theoretical prediction by Bowley, the relatively very much larger values of nu measured near 1K indicate a serious deficiency in the model on which the theory is based.