We here report that the northern expansion of Z. maha was accompanied by an extraordinary phenomenon: the outbreak of color-pattern modifications. In contrast to most species, which exhibited normal physiological adaptation to new environmental conditions, Z. maha individuals at the recently expanded northern range margins, Fukaura, Japan, showed various color-pattern changes on their wings with no other obvious aberrations. In this paper, we document an observation of phenotypic diversification of Z. maha in the field. We reproduced the modified phenotypes physiologically as well as genetically in our laboratory. Based on these data, we discuss possible mechanisms of this outbreak and the importance of phenotypic plasticity in butterfly wing color-pattern evolution