Foraging behavior has been extensively studied in harvester ants (Hymenoptera: Formicidae); however, there is little information about the determinants of foraging success. We developed a path analysis model to quantify the functional relationships among the components of foraging at the colony level (onset, duration, number of foragers) and foraging success. Variation in the onset of foraging among colonies directly influences the amount of time a colony has available for foraging, and contributes substantially to the total biomass of food retrieved. This difference in foraging effort provides a causal explanation for previously observed differences in colony growth rates.