The adaptive value of inactive foragers and the scout-recruit system in honey bee (Apis mellifera) colonies

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Abstract
In honey bee (Apis mellifera) colonies, scouts search for productive forage sites and then recruit other workers to those locations using a waggle dance. A simple and tractable mathematical model of the honey bee scout-recruit system was developed to study the relationship between nectar availability, the efficiency of the honey bee's recruitment system, and the optimal proportion of scouts that maximizes net gain (benefit - cost), or, energetic efficiency (benefit/cost - 1). The models consider both the energetic costs and benefits of active scouts and recruits as well as the cost of an inactive forager reserve. They predict conditions when individual foraging is favored over the honey bee's recruitment system, when the colony should abandon foraging altogether, and the optimal proportion of scouts (when the scout-recruit system is favored). The models' predictions qualitatively match empirical data. Surprisingly, previous empirical data from the honey bee suggest that recruits' costs are greater than scouts'—recruits spend significantly longer searching for a forage patch than do scouts—thereby causing researchers to rethink how the scout-recruit system might be adaptive. Using average returns, the models demonstrate how the scout-recruit system is adaptive despite these apparent higher recruit costs relative to the scouts'. A sensitivity analysis demonstrates that the results are robust to a broad range of relative costs of active workers, inactive workers, and the energetic benefits of the forage. Consequently, the model is demonstrated to be relevant to many insect societies that employ a scout-recruit system.