This study was conducted to investigate the effects of dietary protein levels on honey bee colonies, specifically the population growth, physiology, and longevity of honey bee workers during early spring. Diets containing four different levels of crude protein (25.0, 29.5, 34.0, or 38.5%) and pure pollen (control) were evaluated. Twenty-five colonies of honey bees with sister queens were used in the study. We compared the effects of the different bee diets by measuring population growth, emergent worker weight, midgut proteolytic enzyme activity, hypopharyngeal gland development, and survival. After 48 d, the cumulative number of workers produced by the colonies ranged from 22,420 to 29,519, providing a significant fit to a quadratic equation that predicts the maximum population growth when the diet contains 31.7% crude protein. Significantly greater emergent worker weight, midgut proteolytic enzyme activity, hypopharyngeal gland acini, and survival were observed in the colonies that were fed diets containing 34.0% crude protein compared with the other crude protein levels. Although higher emergent worker weight and survival were observed in the colonies that were fed the control diet, there were no significant differences between the control colonies and the colonies that were fed 34.0% crude protein. Based on these results, we concluded that a dietary crude protein content of 29.5–34.0% is recommended to maximize the reproduction rate of honey bee colonies in early spring.