The soybean aphid, Aphis glycines Matsumura, is a serious pest in Midwestern soybean, Glycine max L. Merrill, and has the potential to colonize a large geographic range throughout a single growing season. Our objectives were to describe colonization patterns on a statewide spatial and temporal scale, examine the changing proportion of winged forms throughout a season, and assess photoperiod as a potential trigger for alate production. In Minnesota, we define initial colonization as the period of time during the early vegetative growth when alates were present and alatoid nymphs were absent on soybean. Initial colonization during 2002 and 2003 was ∼2 wk. On average across Minnesota, summer migrants were first produced when most fields were at the initial flowering stages in 2002. In 2003, an outbreak year, initial detection of summer migrants occurred earlier during vegetative stages before flowering. The significant increase in the proportion of all potential migratory forms (i.e., alatoid nymphs and adults) occurred during the beginning of seed set for both years. During seed set, the mean proportion of alate A. glycines was 0.15 ± 0.04 (SE) in 2002 and 0.16 ± 0.06 in 2003. The mean proportion of alatoid nymph A. glycines was 0.14 ± 0.04 in 2002 and 0.29 ± 0.04 in 2003 during seed set. The total mean proportion of migratory forms was higher when the critical L:D photoperiod was 14.5:9.5 h/d. A regression analysis also indicated the proportion of winged A. glycines increased with decreasing photoperiod.