Coping with the tropical summer heat
It is well known that bees have an extraordinary capacity to keep their hives warm in cold climates during winter.They do this by means of “heater bees” which occupy vacant cells or sit on top of brood cells radiating warmth to nearby cells. Heat transfer with the bees inside the cells is estimated to be 2-3 times more efficient than with the bees outside the cell (Kleinhenz et al 2003). Since heating is an energetically expensive operation for bees, beekeepers have spent a good deal of effort finding ways to help the bees along during cold winters, such as indoor hives, passive solar-heated hives, artificial heating, various means of insulation etc.
In the tropics, bees have the opposite problem: keeping cool in summer. As humans, we know that this can be harder than keeping warm in winter. However, once again we can marvel at the mechanisms employed by individual bees to keep themselves and their colony cool. Their primary method is regurgitating water from their honeycrop. This can be fanned around the hive to bring about evaporative cooling. In the last few decades, it has also been discovered that the bees anatomy is optimised for heat loss through the head, enabling it to fly at temperatures of up to 46°C when almost every other insect would die within minutes (Heinrich 1979). More recently, the phenomenon of “shielding bees” was discovered. During periods of intense sun, bees will aggregate en-mass on hot interior walls of the hive to shield the brood and other parts of the colony from excess warming (Starks and Gilley, 1999). Despite these mechanisms, it seems temperatures can easily become too hot for bees if due care is not taken. Even in winter time in the Townsville area bees are known to swarm in dark coloured hives because hive temperatures can creep above 43°C. It was shown that simply by replacing a rusty old lid with a clean white lid temperatures dropped by up to 15°C!
Methods of helping bees along in warm summers has received much less attention from beeskeepers and researchers. Lensky and Siefers (1979) experimented with lids held up on corner posts 5cm above the super and compared hive temperatures, swarming impulse and honey production in these hives with those that had closed, unventilated lids. They found that ventilated hives were cooler, had less swarming cells and swarms and produced 8.4% more honey than unventilated hives. Differences in honey production increased to 33% when hives swarmed. Most beekeepers in the tropics use migratory lids. These are ventilated and typically contain four 25mm round holes, two at each of the short ends. Together with shading your hives (natural or artificial) this is by far the easiest way to minimise heat buildup inside the hives during summer.
Maximum ventilation can be obtained by using a lid constructed with say 50% of the area covered with shade cloth, the rest corflute or some other insolating material. More area of shade cloth may help a little with keeping temperatures down, but will also upset your bees by giving them too much light. They do prefer it fairly dim inside the hive! Despite these arrangements however, hive temperatures can easily get up to 40°C in the Townsville summer if they are exposed to midday or afternoon sun (Fig 1).
If you want to get really fancy, you could fit a lid or a baseboard with a CPU fan which is thermostatically controlled by an electronic temperature controller. In one such experiment where a bottom-board mounted CPU fan was set to kick in at 36 °C, temperatures were up to 4°C cooler than the naturally ventilated hive (Fig 2). As soon as the fan kicked in, temperatures in the fan-forced hive stabilized rapidly and approximated ambient temperatures from around 1pm to 5pm (the danger period!) after which temperatures dropped rapidly.
While these results clearly show that fan-forced hives are considerably cooler than naturally ventilated hives, they are not very practical for the average beekeeper. These experiments do however highlight the need to do everything you can to keep hive temperatures under control in summer. This includes providing your hives with maximum shading and ventilation.
Kleinhenz M, Bujok B, Fuchs F and Tautz, J. 2003. Hot bees in empty broodnest cells: heating from within. The Journal of Experimental Biology 206, 4217-4231
Lensky Y and Siefers H. (1979). The effect of volume, ventilation and overheating of bee colonies on the construction of swarming queen cups and cells. Comparitive Biochemical Physiology 67:97-101
Heinrich, B. 1979. Keeping a Cool Head: Honeybee Thermoregulation. Science 205 (4412): 1269-1271
Starks PT and Gilley DC. 1999. Heat Shielding: A novel method of colonial thermoregulation in honey bees. Naturwissenschaften 86:438–440