Melanism is one of the most marked phenotypic variations that naturally occur in a wide range of organisms. In this study, we established a homozygous melanism mutant strain with black pupae spontaneously occurring within a wild-type population of Spodoptera exigua. The S.exigua pupal melanic strain showed several viability advantages. The melanism is associated with faster development, heavier pupa weight and higher fecundity after eating seven different host plants. Female adults of the two strains both tend to attract xenogeneic male adults for mating, and the fecundity of the melanic strain is significantly higher than the wild-type strain. However, the melanism is associated with slower mean flight seed, shorter mean flight duration and distance. The melanic strain adults have weaker flight capacity in different ages. The viability advantages above will contribute in more generations per year, increasing population and more serious damage. Meanwhile, based on the mating competition results, the melanic strain will be able to interfere with the reproduction of wide-type strain and replace it. However, decreased flight capacity will influence the long-distance migration ability of the melanic strain and limit its range of damage