Bumblebee application in hot climates
 
Bumblebees keep their nest temperature between 28-33 °C. For this purpose they either ventilate in case of warmer temperatures or incubate the brood in case of colder temperatures. The best pollination activity is achieved when the majority of the bumblebees is not involved in regulating the internal climate, but free to pollinate.
 
Below, the tables can be found which roughly
 
 
Red range: Pollination activity; no. Brood; mortality. Adults; mortality.
Orange range: Pollination activity; reduced. Brood; Risk of damage. Adults; reduced activity.
Green range: Optimal temperature range.  
 
  • <8ºC: Workers stay inside the hive for incubating activities in order to heat up the brood.
  • 8 - 28ºC:  there is little need to occupy themselves with the brood temperature so most workers are free to do their pollination work. At this temperature range, only some of the workers are incubating the brood and closing the protective layer to ensure insulation.
  • 29-32°C: Bumblebees will start to stay inside for ventilation.
  • 33°C: All bumblebees will remain inside/around the hive in order to ventilate.
  • 37°C: Bumblebees will start to spray sugar water on the brood in order to cool it, and in some cases bumblebees will create large openings in the cotton.
  • 38°C: Mortality of brood.
  • 40°C: Adults start to die.  
The effect of climate on pollination and colony quality is also related to time: the longer unfavorable conditions occur, the higher the impact. Details can be found in the following paragraphs.
 
Hot conditions
In tropical and subtropical regions, temperature may not rise up to 38 till 40 ºC, but still result in dead larvae: Temperatures above 32-35 ºC inside the hive can result in workers fighting the heat all day long and eventually the hive will starve.
 
In case the temperature rises above 32 °C for longer than 4 hours per day:
• Place/reopen colonies in the late afternoon or early morning (also when the hives have been removed during spraying).
• Place the hives at a low height between 0.40 and 1 m.
• Place the hives near the isles in the shade.
• Place the hives out of direct sunlight by adding additional shading, preferably using Styrofoam board. Keep some ventilation space between the hives and cover.
• Place the hives in the coolest place of the tunnel/greenhouse.
 
If the temperature is between 32-37 °C for more than 6 hours, do not introduce bumblebees or temporary remove them until the temperature drops.
 
Extreme conditions
If the temperature exceeds 35 °C for longer than 4 hours, place the hives below ground level in the suggested manner showed below (see diagram).
 
 
Hives can stand temperatures between 37-40 ºC for no more than 1-2 hours temperatures. In that case do not introduce bumblebees or temporary remove them until the temperature drops.
 
Crop development
It should be emphasized though that during hot periods or in hot climates both pollination and fruit setting can be problematic at certain times, even when bumblebee activity and visitation (bite marks) have been good. 
From temperatures of 30 °C on pollen starts to become sterile and will not release well anymore (especially when exposed to high temperatures in 5 to 9 days before flowering), pollen tube growth might be slowed down, seed buds start to die and fruit growth can even stop. It is of utmost importance to keep temperatures below 30 °C in the greenhouses/tunnels as much as possible in order to avoid any harmful effects of high temperature on plant, flower and fruit development.
 
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