  Loops formed from a cosmic string network at early times would act as seeds
for early formation of halos, which would form galaxies and lead to early
reionization. With reasonable guesses about astrophysical and string
parameters, the cosmic string scale $G\mu$ must be no more than about $3\times
10^{-8}$ to avoid conflict with the reionization redshift found by WMAP. The
bound is much stronger for superstring models with a small string reconnection
probability. For values near the bound, cosmic string loops may explain the
discrepancy between the WMAP value and theoretical expectations.
