Bumble Bee Foraging: The Threshold Departure Rule

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Abstract
The response of queen bumble bees (Bombus appositus) to variations in the distribution and abundance of nectar in multiflowered Delphinium nelsonii inflorescences was used to identify a general rule for allocation of foraging time among plants. Bumble bees most often leave plants immediately after probing a flower that has a relatively small volume of nectar (short probe time) and most often stay to visit another unvisited flower on a plant after probing a flower that has a larger volume (longer probe time). The threshold departure rule was found to be resilient to changes in the number of open flowers on a plant, the arrangement of flowers on plants, the dispersion patterns of plants, and bee behavior of previously visited plants. The threshold departure rule was violated under two general circumstances: when the average energetic penalty for disobeying the rule was negligible, and when bees would have had to change the direction of flight, from up to down an inflorescence, to find an unvisited flower. This rule is distinct from a patch departure rule based on giving—up time that is prevalent in the behavioral ecology literature. When the threshold departure rule is property qualified it is similar to a “win—stay, lose—shift” departure rule prevalent in the psychology literature.