How efficient is the Langacker-Pi mechanism of monopole annihilation?

We investigate the dynamics of monopole annihilation by the Langacker-Pi mechanism. We find that considerations of causality, flux-tube energetics, and the friction from Aharonov-Bohm scattering suggest that the monopole annihilation is most efficient if electromagnetism is spontaneously broken at the lowest temperature (${\mathit{T}}_{\mathrm{em}}$≊${10}^7$ GeV) consistent with not having the monopoles dominate the energy density of the Universe.