Genetically determined melanic color forms of the spittlebug Philaenus spumarius (L.) absorb more incident radiation, reach higher equilibrium temperatures, and are more frequent in cooler, higher latitude populations. Across five Northeastern Minnesota localities from 1974 to 2021 mean September temperature increased 2.70°C, while melanic dorsal color forms decreased significantly in frequency, from 22.1% to 14.2 % in females, and 12.7% to 7.3% in males. The frequency of dark ventral abdominal pleurites decreased in both males and females, and in males the frequency of TYP, the more pigmented of the two local non-melanic color forms, decreased relative to the alternative POP phenotype. In Tinley Park, Illinois, where temperature increased 1.85°C from 1971 to 2021, the frequency of MAR, the only common dark color form, did not change, but the relative frequency of TYP versus POP decreased in both males and females. These results suggest that: 1) northern melanic color forms are maintained by selection for thermal melanism, selection that has relaxed as a result of warming climate; 2) MAR frequencies are determined by other forms of selection, possibly selection for warning coloration; and 3) climate warming may shift the phenotypic balance between TYP and POP toward POP. If these changes are representative of broader geographical areas, the P. spumarius polymorphism may prove to be a useful barometer of climate change.