Temperate insect species often enter diapause in preparation for overwintering. One such species is the invasive vinegar fly, Drosophila suzukii (Matsumura), which has seasonal polymorphisms, considered winter and summer morphs. To date, the morphs have been differentiated by color and size with winter morphs typically being darker and larger compared to summer morphs. 'Dark' and 'large' are subjective, however, and standardizing an identification process can ensure that the morph of interest is being accurately characterized. The goal of our research was to investigate a quantitative method to distinguish between D. suzukii morphs based on body and wing size. We reared winter and summer morph D. suzukii in the laboratory using standard procedures, and measured wing length, wing width, and hind tibia length. Additionally, we collected field D. suzukii to document the seasonal phenology of the morphs in Minnesota based on our model's cutoff criteria. A classification and regression tree analysis were used to determine which metrics would be best for predicting field-caught D. suzukii morphs. Using laboratory-reared flies as our known morphs for the training data in the classification model we developed classification trees based on wing length and the ratio of wing length to hind tibia length. The frequency of winter and summer morphs present in the field varied based on which classification tree was used. Nevertheless, we suggest ratio of wing length to hind tibia length as the most robust criteria for differentiating D. suzukii morphs because the ratio accounts for the size variability between laboratory-reared and field-caught flies and the error rate of misclassification is reduced to 0.01 for males. The results from this work can aid in future D. suzukii research by allowing scientists to objectively differentiate the morphs, and thereby improve our understanding of the biology and phenology of seasonal morph dynamics.