Black flies are a perennial pest problem in New Jersey, and paramount among them are the S. jenningsi group species. Pollution tolerance values are reported for the 39 species of black flies known in New Jersey. Morphologically similar species were identified chromosomally to ensure accurate identification. Species of the same genus varied markedly in tolerance values, questioning the accuracy of generic-level tolerance values for the Simuliidae. An index for predicting the colonization and spread in New Jersey of the Simulium jenningsi group, which includes the major pest species of black flies in the eastern United States, is proposed, with implications for control programs worldwide.