Endothermic warm-up in two species of sphecid wasp and its relation to behaviour

Abstract. The ability of two sphecid wasps, Bembix rostrata and B.zonata (Hymenoptera: Sphecidae), to warm up endothermically is demonstrated under laboratory conditions. Mean warm-up rates of B.rostrata are comparable to bees of similar weight. Despite endothermic ability, field observations reveal that B.rostrata are not active below 22C. From observations at the nectar-foraging site (clumps of Thymus vulgaris flowers) it is calculated that the energy resources available to wasps are sufficient to power endothermic warm-up at low temperatures. Alternative explanations for the absence of wasp activity at low temperatures, such as the risk of parasitism, are suggested. Endothermy may be used periodically to increase flight efficiency in response to added load, such as prey carried by females, and mates by males.