Bees Are Good Indicators That It's Getting Way Too Hot

In 1988 the temperature in Wake County, NC hit a record high of 105ºF. Combined with a sultry humidity, the countryside was most uncomfortable.

It wasn't easy on bees either.

In a city like Raleigh, which is a heat island, that summer day was even hotter than in the surrounding forests. Add a few degrees when the radiant heat bounces off the tall walls and concrete sidewalks and the day became unbearable.

The heat islands of North Carolina's Wake County can be hard on people in the summer but they can also be hard on other organisms too. With global temperatures on the rise, could certain animals be warning signs of ecological danger?

It turns out that certain species of bees may be indicators for dangerous temperatures.

A team of researchers from North Carolina State University has just published a novel study in Biology Letter of how well some common bees endure heat. Cleverly, they combined laboratory studies and field studies to figure out the thermal limits for bees--how hot is too hot.

April Hamblin and her team first subjected bees to heat in a laboratory setting. They collected and tested fifteen species of bees common to central North Carolina. They put bees in containers and subjected them to heat until they were non-functional. This temperature was the critical thermal maxima, or CT_max.  

Individuals of the fifteen species had a CT_max between 112.3 F to 124.3 F. One would expect native species to have thermal tolerances higher than record highs in an area -- and all fifteen species survived beyond the 1988 record.

Some bees do worse than others in the heat. Bumblebees and the giant sweat bee Agapostemon virescens had the least ability to withstand thermal stress. In an era of rising temperature, death occurring near 113 degrees F could easily happen, especially in a heat island, with a shock to any local ecosystem.

Other bees showed significant ability to withstand more dramatic heat. Xylocopa virginica and Ceratina strenua are carpenter bees that survive at 120 F. But even a hot day, in certain areas of heat-islands, it is not inconceivable that even these bees could face danger.

Once the laboratory data for the bees was recorded, the team surveyed eighteen study sites in the heat island of Raleigh. Eleven samples and observations were taken at each site over two years and showed that the laboratory findings were strongly predictive of in vivo behaviors.

In completing the study, the group has established a workable model to test ecological alarm during this period of climate warming. The powerful correlation between laboratory studies and field observations should allow other researchers to look at other species of bees both in open and urban settings.