Bees food quality and body temperature

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In bees, it appears that food quality modulates thermal behaviour. For example, workers of the Asian honeybees A. cerana and A. dorsata landing at highly congested feeders tend to have higher Tth, and congestion depends on food quality (Dyer and Seeley 1987). Similarly, the Himalayan honeybee A. laboriosa maintains high temperature excesses when arriving at feeders to collect concentrated sugar solution (Underwood 1991). Thermal imaging has proved a valuable technique in studying this variability in thermoregulation of honeybee foragers under field conditions. Honeybees regulate Tth at higher levels and more accurately for fast exploitation of profitable food sources (Schmaranzer and Stabentheiner 1988). Honeybees foraging on different plant species also have variable Tth, with dandelion foragers being 10oC warmer than those visiting sunflowers (Kovac and Schmaranzer 1996). Perhaps the observation that bumblebees allow their Tth to drop below the minimum for flight while foraging on dense inflorescences, especially if they contain low reward flowers (Heinrich and Heinrich 1983), is a result of motivational state, not simply an energy conservation mechanism. The Tth of vespine wasps also depends on food concentration (Kovac and Stabentheiner 1999), and the Tth of water-collecting honeybees measured by thermography resembles that of bees feeding on 0.5 M sucrose, indicating similar motivation (Schmaranzer 2000). Similar results, showing Tth to be 3oC higher in bees feeding on high sucrose concentrations, have been obtained using thermocouples (Waddington 1990).

Not surprisingly, the metabolic rate of honeybees likewise varies with the reward rate at the food source and the motivational state of the bees. Direct effects of nectar load on metabolic rate (Wolf et al. 1989) can be eliminated by training bees to collect food in a respirometer so that they need not transport it (Moffat and Nunez 1997). The metabolic rate of free-flying bees collecting food in a much larger respirometer is also inversely proportional to Ta at constant sucrose flow rate (Moffatt 2001), supporting previous studies showing variation of heat production during flight (Roberts and Harrison 1999).

The beauty of infrared thermography is that it does not disturb social interactions such as the dancing behaviour of honeybees or the unloading of nectar by trophallaxis. Graduated thermal behaviour occurs during food unloading in the hive as well as at the feeding site. The temperature of dancing bees recruiting their nestmates increases with food quality and the number of brood cells, and decreases with distance of the food source from the hive and the amount of stored honey (Stabentheiner 1991, 2001). Foraging bees returning from feeders with high flow rates (8.2 mlmin—1 of 50 per cent w/w sucrose solution) have high Tth during trophallaxis and transfer the food quickly. The receiver bees also raise their Tth during trophallaxis, and Farina and Wainselboim (2001) use a thermogram to show a 3.7oC increase in Tth of a receiver bee during 18 s of food transfer, equivalent to a heating rate of 12.3oCmin—1 During this time, Tth of the donor bee was relatively unchanged. Figure 6.16 illustrates linear increases in Tth of nine receiver bees during contact with donors that had fed at different flow rates (with the exception of one interaction at the lowest flow rate). Calculations show that the temperature of the exchanged solution can not account for the increase in Tth of receivers, and the linear increase also suggests active warm-up rather than passive heat transfer.

Increasing Body Temperature Bees

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Contact time (s)

Figure 6.16 Thoracic surface temperatures of receiver honeybees during trophallaxis. Donor bees have returned from three feeders supplying 50% w/w sucrose solution at different flow rates, (a) 1.0 ml mm-1, (b) 2.4 mlmm~\ and (c) 8.2 mlmin-1. Source: Farina and Wainselboim (2001).

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Contact time (s)

Figure 6.16 Thoracic surface temperatures of receiver honeybees during trophallaxis. Donor bees have returned from three feeders supplying 50% w/w sucrose solution at different flow rates, (a) 1.0 ml mm-1, (b) 2.4 mlmm~\ and (c) 8.2 mlmin-1. Source: Farina and Wainselboim (2001).

It is not known whether receivers monitor the head temperature of the donor or the quality of the donation. From the colony point of view, increased Tth of the receiver bees ensures that their activity level is intensified and nectar from more profitable sources will be processed faster (Farina and Wainselboim 2001).

While the endothermic abilities of insects are determined by body size and shape, phylogeny, and environment (Stone and Willmer 1989b), sociality creates an entirely new dimension. Solitary bees show sophisticated thermoregulation compared to other insects, but they lack the thermal benefit and refuge of a stable nest microclimate (Willmer 1991b). Body temperatures of individual honeybees are much more than a consequence of flight metabolism and are finely regulated according to expected and actual foraging gains. As suggested by Waddington (1990), the thermal strategies of honeybees ensure that when profits are low the bees cool down and save energy, which has long-term benefits for the colony, but when profits are high the bees warm up and maximize short-term rates of energy gain. While a great deal might be known about the mechanisms of insect thermoregulation, the extent to which thermoregulation influences all facets of insect ecology is only beginning to be explored.

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