Flight metabolism and kinematics of pollen and nectar load carriage in honey bees

Although many inserts carry loads during flight, the energetic cost of load carriage remains poorly understood. Honey bee foragers provide a useful model system for evaluating the energetics of load carriage in flying insects. Honey bees coiled pollen and nectar which are carried in different manners (pollen is carried externally on the legs while nectar is carried internally in the crop) and this may generate different aerodynamic requirements. We addressed two questions: 1.) What is the effect of loading on the metabolic rate (Vco₂) and wing kinematics (wing beat frequency (WBF) and stroke amplitude (SA)) during hovering flight and 2.) is the effect of load on flight energetics determined by the type of resource carried (pollen vs. nectar). Although Vco₂ of pollen and nectar foragers increased significantly from the unloaded to the loaded state, surprisingly, the magnitude of the load (0-71 mg) did not affect flight Vco₂. As bees loaded themselves. WBF remained constant, but SA increased significantly. The increased power required to lift the load is generated by SA rather than WBF. These results suggest increased efficiency in converting metabolic energy to mechanical power output as load increases. Pollen foragers had significantly higher Vco₂ than nectar foragers, consistent with previous findings that pollen and nectar foragers differ genotypically and that pollen foragers have higher thoracic temperatures. The ability of honey bees to maintain constant Vco₂ while carrying loads equal to their own body mass is a remarkable phenomenon which necessitates a reevaluation of the current paradigms of the energetics of load carriage during flight.