DECLINE OF MELANIC COLOR FORMS IN NORTHERN MINNESOTA POPULATIONS OF PHILAENUS SPUMARIUS (L.) (HEMIPTERA: APHROPHORIDAE)
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.Abstract
Philaenus spumarius dorsal color forms and their conventional three-letter abbreviations: POP (populi), TYP (typicus), TRI (trilineatus), MAR (marginellus), LAT (lateralis), FLA (flavicollis), LOP (leucophthalmus), LCE (leucocephalus), LOP-LCE (apparent male manifestation of LCE). Delineation of color forms follows Thompson and Halkka (1973) as amended by Thompson (1984a).
Collection localities in St. Louis, Lake and Cook counties in the Arrowhead Region of Northeastern Minnesota.
Polymorphism in abdominal pleurite color, with arrows indicating pleurites (solid arrows for dark pleurites, open arrows for white pleurites). A. Female LCE with dark pleurites. B. Female TRI with white pleurites. C. Male POP with dark pleurites. D. Male POP with white pleurites. The females are from Tofte, 1974, the males from Illgen City, 2021. The female example in A is toward the black end of the dark pleurite spectrum. The “white” pleurites of males in the Minnesota populations are typically tan brown, as in D. In more southerly populations the male and female “white” pleurites are typically light tan. In both sexes the pleurites vary in intensity of pigmentation within color categories.
Regional trends in mean September temperature, 1974–2021, St. Louis, Lake and Cook counties Minnesota. The jagged line shows year-to-year variation. The wavy line is a smoothed time series. The regression line shows the estimated 47-year temperature trend and provides estimates of beginning and end temperatures where it intersects the 1974 and 2021 boundary dates. Source: NOAA National Centers for Environmental information, Climate at a Glance: Divisional Time Series (NOAA, 2022).
