Body temperature and energetics of foraging bees and wasps

In foraging honeybees and wasps there exist two thermal strategies. The honeybees and Vespine wasps are heterothermic insects. They are ectothermic during many duties inside their nest but heat up their thorax considerably (become endothermic) before they start foraging. This ensures proper function of their flight muscles and a high motility. In the endothermic state their energy turnover increases with decreasing ambient temperature. By contrast, Polistine wasps are poikilothermic, i.e. they do not show pre-flight warm-up regulation of the body temperature above the ambient level. Their energy turnover is supposed to increase with increasing ambient temperature. All these insects forage in the shade and in the sun. In the bees and Vespine wasps, however, it has remained unknown what they really optimize, their body temperature or their energy investment. An higher body temperature would help them to exploit profitable food sources faster. External heat gain via solar radiation could allow them to reduce their own investment of energy for thermoregulation, this way reducing the costs of foraging considerably. We will make the first simultaneous measurement of body temperature and energy turnover in freely moving bees and wasps. They will be trained to gather sugar water from artificial flowers inside a respirometric chamber where we can measure their respiration (CO2 production). Through an infrared transmissive window of this chamber we will simultaneously measure their body temperature in the sun and in the shade by infrared thermography, without disturbing their natural behavior. These measurements will be compared with those in Polistine wasps, which are supposed to have a passively increased energy turnover in the sun according to a higher body temperature. Highly motivated honeybees which forage from profitable food sources invest more energy to increase their body temperature. This way, they can harvest the food faster. It has remained an open question, however, whether colony need modulates body temperature. We will test this in two ways. We will heat the colony (and this way increase the need for water for hive cooling) and measure the body temperature of the bees which gather the water outside at a pond. In another experiment, we will manipulate a colony`s stock of honey or amount of brood, and measure the body temperature of bees gathering sugar water from a feeding station. We expect to measure higher body temperatures in highly motivated foragers (heated nest or low food reserves).

In foraging honeybees and wasps there exist two thermal strategies. The honeybees and Vespine wasps are heterothermic insects. They are ectothermic during many duties inside their nest but heat up their thorax considerably (become endothermic) before they start foraging. This ensures proper function of their flight muscles and a high motility. In the endothermic state their energy turnover increases with decreasing ambient temperature. By contrast, Polistine wasps are poikilothermic, i.e. they do not show pre-flight warm-up regulation of the body temperature above the ambient level. Their energy turnover is supposed to increase with increasing ambient temperature. All these insects forage in the shade and in the sun. In the bees and Vespine wasps, however, it has remained unknown what they really optimize, their body temperature or their energy investment. An higher body temperature would help them to exploit profitable food sources faster. External heat gain via solar radiation could allow them to reduce their own investment of energy for thermoregulation, this way reducing the costs of foraging considerably. We will make the first simultaneous measurement of body temperature and energy turnover in freely moving bees and wasps. They will be trained to gather sugar water from artificial flowers inside a respirometric chamber where we can measure their respiration (CO2 production). Through an infrared transmissive window of this chamber we will simultaneously measure their body temperature in the sun and in the shade by infrared thermography, without disturbing their natural behavior. These measurements will be compared with those in Polistine wasps, which are supposed to have a passively increased energy turnover in the sun according to a higher body temperature. Highly motivated honeybees which forage from profitable food sources invest more energy to increase their body temperature. This way, they can harvest the food faster. It has remained an open question, however, whether colony need modulates body temperature. We will test this in two ways. We will heat the colony (and this way increase the need for water for hive cooling) and measure the body temperature of the bees which gather the water outside at a pond. In another experiment, we will manipulate a colony`s stock of honey or amount of brood, and measure the body temperature of bees gathering sugar water from a feeding station. We expect to measure higher body temperatures in highly motivated foragers (heated nest or low food reserves).