We present a model for the behavior of communally nesting insects. Females may forage for food to provision offspring or may remain in the nest, with the option of eating and replacing nest mates' eggs. Orphaned brood are at risk of predation. The optimal behavior of solitary females is determined using stochastic dynamic programming; static and dynamic evolutionarily stable strategies (ESSs) are then calculated for colonies of various sizes. A solitary female should forage if her brood is smaller than a time-dependent threshold. Females in small colonies should forage. In colonies above some threshold size, the static ESS is for one female to forage and the rest to cheat. The dynamic ESS in large colonies is for no females to forage until some time close to the end of the foraging season and for all females to forage thereafter. Mixed dynamic ESSs, with some foragers and some cheats, may arise if individuals differ in their chances of surviving a foraging interval or if females with new offspring vary their guarding behavior, depending on the numbers of cheats and new cells in the nest. We discuss these predictions in the light of published observations and preliminary data on the halictine bee Lasioglossum (Chilalictus) hemichalceum.