Astrophysical Constraints on Primordial Black Hole Formation from   Collapsing Cosmic Strings
  Primordial Black Holes (PBH) may have formed from the collapse of cosmic string loops. The spectral shape of the PBH mass spectrum can be determined by the scaling argument for string networks. Limits on the spectral amplitude derived from extragalactic gamma-ray and galactic gamma-ray and cosmic ray flux observations as well as constraints from the possible formation of stable black holes remnants are reanalyzed. The new constraints are remarkably close to those derived from the normalization of the cosmic string model to the cosmic microwave background anisotropies. 
U. F. Wichoski, Jane H. MacGibbon and Robert H. Brandenberger