Seasonal polyphenism of spotted‐wing Drosophila is affected by variation in local abiotic conditions within its invaded range, likely influencing survival and regional population dynamics

Overwintering Drosophila often display adaptive phenotypic differences beneficial for survival at low temperatures. However, it is unclear which morphological traits are the best estimators of abiotic conditions, how those traits are correlated with functional outcomes in cold tolerance, and whether there are regional differences in trait expression. We used a combination of controlled laboratory assays, and collaborative field collections of invasive Drosophila suzukii in different areas of the United States, to study the factors affecting phenotype variability of this temperate fruit pest now found globally. Laboratory studies demonstrated that winter morph (WM) trait expression is continuous within the developmental temperature niche of this species (10–25°C) and that wing length and abdominal melanization are the best predictors of the larval abiotic environment. However, the duration and timing of cold exposure also produced significant variation in development time, morphology, and survival at cold temperatures. During a stress test assay conducted at −5°C, although cold tolerance was greater among WM flies, long‐term exposure to cold temperatures as adults significantly improved summer morph (SM) survival, indicating that these traits are not controlled by a single mechanism. Among wild D. suzukii populations, we found that regional variation in abiotic conditions differentially affects the expression of morphological traits, although further research is needed to determine whether these differences are genetic or environmental in origin and whether thermal susceptibility thresholds differ among populations within its invaded range. We explored how seasonal traits are correlated with functional outcomes in cold tolerance in D. suzukii, and whether there are regional differences in trait expression. While wing length and abdominal melanization were the best predictors of the larval abiotic environment, the duration and timing of cold exposure also produced significant physiological variation. Because we observed morphotype variation among wild D. suzukii populations, in different regions of the Eastern United States, a population‐centered research and modeling approach may be required to assign morphotype labels and predict overwintering survival outcomes.