  We revisit the problem of charged string pair creation in a constant external
electric field. The string states are massive and creation of pairs from the
vacuum is a tunnelling process, analogous to the Schwinger process where
charged particle-anti-particle pairs are created by an electric field. We find
the instantons in the worldsheet sigma model which are responsible for the
tunnelling events. We evaluate the sigma model partition function in the
multi-instanton sector in the WKB approximation which keeps the classical
action and integrates the quadratic fluctuations about the solution. We find
that the summation of the result over all multi-instanton sectors reproduces
the known amplitude. This suggests that corrections to the WKB limit must
cancel. To show that they indeed cancel, we identify a fermionic symmetry of
the sigma model which occurs in the instanton sectors and which is associated
with collective coordinates. We demonstrate that the action is symmetric and
that the interaction action is an exact form. These conditions are sufficient
for localization of the worldsheet functional integral onto its WKB limit.
