The influence of constant and fluctuating temperatures on soybean aphid, Aphis glycines Matsumura, demographics have been well characterized in the laboratory. However, the effects of temperature and host phenology on soybean aphid population dynamics under field conditions are not well understood. Therefore, our objective was to evaluate the impact of temperature, soybean phenology, and leaf age on soybean aphid fecundity and survivorship in the field. In 2005 and 2006, we used a randomized complete block design with main effects of planting date, leaflet-cage type (i.e., painted versus unpainted), and leaflet age. Painted (black) and unpainted (transparent) leaflet-cages were placed in each plot (6 x 18 m) at two locations within the canopy (i.e., newly-expanding versus mature leaflets). At the individual level, exposure to higher temperatures did not significantly alter aphid reproduction or survivorship compared to aphids reared at ambient temperatures. However, total fecundity and survivorship were significantly affected by soybean phenology and leaf age; soybean aphids reared on late-planted soybean produced significantly more nymphs than aphids reared on early-planted soybean. In addition, individual apterous adults reared on mature soybean leaflets produced 24.9% fewer nymphs compared to aphids reared on newly-expanding leaflets. Results from the population-level experiment and its implications on soybean aphid population dynamics will be discussed.