Photo by H. Hillewaert. Wikimedia Commons
(Click on the following link to listen to an audio version of this blog …. Bees in the changing climate
Bees have been, by far, the most frequent subject of my short essays on this blog site. Since 2006, I have written 31 blogs about all sorts of bees! We have explored some of the basic biology of honey bees, bumble bees, mason bees and a broad array of wild bee species.
Even people who claim to “hate” insects admit to liking bees. They recognize that both wild and domesticated bees are vital pollinators of the world’s agricultural crops and wild plants. They recognize that honey is a unique (and delicious) product of domesticated bees and may even have an inkling of the remarkable social complexity and communication biology in bee communities. They may also have an understanding that bee populations all around the world are in trouble, and their declining numbers and shrinking distributions are primarily due to human activity (habitat destruction, pesticide use and human-generated Climate Change)!
Climate Change is a global phenomenon driven by the accumulation of greenhouse gases in the atmosphere. These greenhouse gases originate primarily from our use of fossil fuels. The Environmental Protection Agency describes the impacts of Climate Change, in addition to the obvious increases in atmospheric temperatures, as,
“Increases in ocean temperatures, sea level, and acidity. Melting of glaciers and sea ice. Changes in the frequency, intensity, and duration of extreme weather events. Shifts in ecosystem characteristics, like the length of the growing season, timing of flower blooms, and migration of birds.”
There are so many aspects of Climate Change that could play havoc with bees!
Squash bee. Photo by USDA. Public Domain
Two recent papers explored the impact of rising temperatures on bee activity. The first of these was a paper published in Ecology and Evolution on February 15, 2024 that looked at the heat tolerance of hoary squash bees (Xenoglossa pruinose). Hoary squash bees are important pollinators of squashes and pumpkins, and their ability to tolerate rising temperatures may play an important role in the cultivation of these crops in a Climate Change altered world.
Bees are all small bodied, ectothermic (“cold blooded”) organisms that rely on their surrounding environment for their body heat and for temperature regulation. Female hoary squash bees are larger than males and spend the relatively cool morning hours of their summer seasonal activity period gathering pollen from flowers to feed to their offspring. As they fly from flower they pollinate the visited plants. Around mid-day they cease visiting flowers and return to their underground nests where they are insulated from the extreme heat of the day. The nature of the soil in their habitat area is important in the thermal properties of their burrows. Sandy soils have much poorer insulation properties than more loamy soils.
It is hypothesized than rising summer temperatures will reduce the food gathering activity period of these female hoary squash bees, and , thus, reduce the amount of food available to feed their young. The reduction in activity will reduce the extent of their pollination. The food reduction will also lower the reproductive rate of this important pollinator! The rising summer temperatures may also cause burrows dug into sandy soils to be ineffective refuges against the afternoon heat. These bees, then, will experience a significant reduction in their habitat areas because of the necessity of avoiding sites with sandy soils.
Squash bee. Photo by USGS. Public Domain
Male hoary squash bees are smaller than females and have very different activity patterns and ecological roles. The males do not gather food for their offspring, but instead spend the cool, morning hours visiting flowers to drink nectar. As they fly from flower to flower seeking nectar they also, inadvertently, transfer pollen from one flower to another. By mid-day, warmer air temperatures cause the male hoary squash bees to seek out thermal refuges in the deep folds of the very flowers they have been feeding on and pollinating. These flowers are not as thermally buffered as the soil burrows of the female hoary squash bee, so the males will experience both warmer afternoon temperatures and also colder night-time temperatures.
It is hypothesized that the male bees will be active in nectar gathering and pollination for shorter periods of time as day temperatures rise. This could cause both stress on the males because of lack of sufficient food and also reduce their efficiency of pollination. Further, the exposure of the males to extremely warm (possibly lethally warm) afternoon temperatures in their above ground flower refuges could significantly reduce the population of hoary squash bees!
The authors of this paper also examined the heat tolerance of individual hoary squash bees in the laboratory. They measured the bee’s “critical thermal maximum,” i.e. the maximum temperature at which an individual is still able to function.
Larger ectothermic organisms are expected to have a higher heat tolerances than smaller ectothermic organisms because of their reduced surface area to volume ratio, and in hoary squash bees, the larger females had a higher overall heat tolerances than the smaller males. Also, within cohorts of each gender, the larger the individual was, the greater its heat tolerance. However, when these heat tolerances were standardized to grams of body weight, it was noted that male hoary squash bees had significantly higher levels of heat tolerance than females. Possibly there was some physiological heat tolerance mechanism operating in the flower dwelling, high afternoon-heat exposed males!
Male spaghetti squash flower. Photo by AngeldLuff. Public Domain
Further, the authors of this paper noted that bees collected from warmer sites had less variation in their individual heat tolerances than bees collected from cooler sites. Possibly, poor heat tolerance abilities were selected against and eliminated from the bee populations living in these already warm habitats. Some bee species have evolved physiological and behavioral mechanisms to reduce the direct impact of environmental heat on their bodies. It is not known, though, if hoary squash bees have developed any of these strategies.
Bees also have significant levels of parasites in heir bodies. The hoary squash bees that were tested for heat tolerance were also evaluated for protozoan (trypanosomes), bacterial (Spiroplasm apis) and microsporidian (Varimorpha apis) parasites. Interestingly, only trypanosome parasites reduced the heat tolerance of the bees and it only affected the females! Female hoary squash bees are much less abundant than the males, so this influence of trypanosome parasites could further reduce female numbers in a warming world and have significant impacts on the overall numbers of these important bees!
Bumblebee. Public Domain
Another paper (published in the Proceedings of the Royal Society B (August 28, 2024)) examined the impact of heat waves on the ability of bumble bees (Bombus spp.) to use their sense of smell. Bumble bees are vital pollinators of many vegetables, fruits, nuts and legumes and are active in the pollination of about one-third of our crop plants. Bumble bees find flowers by vision but then select the specific flower they want to visit by the chemical sensations picked up by their antennae.
On very hot days, this ability to sense chemical clues from optimal flowers is lost, and may stay lost for several days. The consequence of this is that bumble bees can no longer tell which flowers have sufficient nectar to supply their energy needs, and they also cannot tell which flowers have been or have not been pollinated! The overall efficiency of pollination, then, decreases significantly.
Bombus pascuorum. Photo by R. Bartz. Wikimedia Commons
Interestingly, in this study wild-caught bumble bees (Bombus pascuorum) were more significantly affected by heat than commercially purchased bumble bees (B. terrestris). No explanation of this observation was presented.
And, finally, in a paper published in Environmental Research Letters (October 17, 2023) a diverse group of Penn State scientists analyzed 50 years of beekeeping records to determine the causes of declining honey production across the country. They found that herbicide use in fields near bee hives reduced honey production by both direct poisoning of the bee colonies and also by reducing the number of flowers available for the bees to visit. Factors that caused declines in soil productivity also reduced flower densities and reduced the production of honey. Extreme weather conditions (both drought and severe storms) also led to reduced honey production because of the negative impacts on flowers.
One of the hallmarks of Climate Change, as we listed above in the EPA list of Climate Change characteristics, is the increased incidence of extreme weather and weather fluctuations. These weather events will reduce the number of flowers and, consequently, continue to reduce the productivity of honeybees.