The year 20222 was characterized by heat and drought in Germany. Apparently the field bumblebees also suffered from this. A study comes to the conclusion that the colonies of these wild bumblebees in Lower and Upper Franconia were significantly smaller in the drought year, lived shorter lives and produced fewer offspring, especially fewer new queens. Supplementary feeding with sugar water led to slightly larger bumblebee colonies, but did not result in more female offspring. Prolonged drought therefore also endangers future generations of these important pollinators.
In Germany, numerous species of wild bees and bumblebees ensure the pollination of plants in agriculture, the outdoors and in home gardens. But climate change is causing problems for insects. “Pollinator populations have to cope with rising average temperatures, changing precipitation patterns and extreme weather events,” explain biologists Hanno Korten and Ingolf Steffan-Dewenter from the University of Würzburg. “The frequency and severity of heatwaves and droughts have already increased, and future scenarios predict a further increase.”
Smaller colonies
To find out how these conditions affect wild bumblebees, Korten and Steffan-Dewenter established colonies of wild field bumblebees (Bombus pascuorum) at 25 locations in Upper and Lower Franconia in Bavaria in different years and monitored them throughout the season. The field bumblebee is usually considered a robust species, flying to numerous different flowers near its nest and even obtaining nectar through its long proboscis, which is inaccessible to species with shorter proboscis. The sweet nectar serves as a source of energy for the adult animals, while they need the protein-rich pollen primarily to raise their brood.
For their evaluation, the researchers compared the drought year 2022 with the climatically average year 2024. As a measure of the number of individuals, they recorded the total weight of the individual colonies. “Our study shows a clear difference between the two years,” reports Korten. “In the drought year, the colonies reached an average weight of only around 14 grams, while in the normal year they grew to around 140 grams.” The fewer individuals there are in a colony, the fewer workers are available to pollinate the flowers in the area.
Lack of food and lack of offspring
Since most plants produce fewer and smaller flowers with a smaller amount of nectar during drought, drought means a time of food shortage for bumblebees. The researchers provided some of the observed colonies with sugar water to compensate for the lack of nectar. While the supplementary feeding had hardly any effect in 2024, when there was enough natural nectar available, it caused the colony size to increase fivefold in the drought year – even if it was far from compensating for the negative effects of the drought. “The burden of drought is obviously so high that pure carbohydrate doses can only stabilize the vitality of the states to a limited extent,” says Korten. In 2022, most bumblebee colonies died in August, the unfed ones often died in July, while in 2024 more than half of the colonies were still alive at the end of September.
But according to the researchers, the smaller colony size and shorter lifespan are only the precursor to an even more serious problem: “The long-term stability of a population depends on the production of new queens, which are the only individuals to survive the winter and establish new colonies in the following year,” explains Steffan-Dewenter. But new queens, of all things, were hardly able to develop in the drought year. While the unfed colonies produced an average of 13.5 new queens in 2024, in 2022 more than half of the unfed colonies did not produce a single new queen. Even feeding him sugar water couldn’t change anything. Although more males emerged in the fed colonies, there was apparently a lack of protein-rich pollen for the development of queens. Without reproductive offspring, the bumblebee colonies die out locally and are no longer available as pollinators the following year.
Targeted nature conservation
“We conclude that increasingly frequent droughts pose a major threat to bumblebee populations and the pollination services they provide, even in flower-rich protected areas,” the authors conclude. “Our study supports the need for adaptive conservation management in near-natural, valuable meadow habitats.” In order to help the bumblebees, it is important, for example, to provide shady trees in open, flower-rich meadows and thus create cooler areas. Additionally, restoring wetlands could help improve the water-holding capacity of soils.
Source: Hanno Korten and Ingolf Steffan-Dewenter (University of Würzburg) et al., Proceedings of the Royal Society B, doi: 10.1098/rspb.2025.3056