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Drought significantly reduces the reproductive success of bumblebee colonies. This is shown by a new study conducted by a research team at the University of Würzburg. This also has consequences for plant pollination.
How did the drought year of 2022 in Lower and Upper Franconia affect a specific bumblebee species? What differences are evident compared to the climatically average year of 2024? A research team from the Department of Animal Ecology and Tropical Biology (Zoology 3) at Julius-Maximilians-Universität Würzburg (JMU) investigated these questions. Their findings show that drought years significantly impair colony development: both lifespan and colony weight, as well as the production of queens and males, are then greatly reduced.
“Our work is the first study to demonstrate a negative effect of droughts on bumblebee reproduction,” says lead author Hanno Korten, describing the central finding. Traditionally, studies of bumblebee colonies have focused on the buff-tailed bumblebee (Bombus terrestris); in this case, however, the team examined the effects of drought years on the common carder bumblebee (Bombus pascuorum).
This long-tongued species belongs to the so-called “pocket-makers.” They store pollen in special pockets adjacent to the brood, from which the larvae feed themselves. This biology makes them more vulnerable than species whose larvae are fed directly by adult bees. In Bavaria, 82 percent of endangered bumblebee species belong to this group of long-tongued bumblebees, suggesting that the common carder bumblebee can serve as a valuable indicator of biodiversity.
In the course of the study, Hanno Korten and Professor Ingolf Steffan-Dewenter assessed the condition of bumblebee colonies at a total of 25 sites in Upper and Lower Franconia, comparing the drought year of 2022 with the year 2024. They have now published their findings in the journal “Proceedings of the Royal Society B: Biological Sciences.”
Bumblebee colonies lose weight dramatically
In ecology, the maximum weight of a colony is considered as a key indicator of fitness and the number of available workers. A low colony weight means fewer individuals for foraging, which directly reduces pollination success in the surrounding area, as fewer flowers can be visited in comparable time periods.
“Our study shows a clear difference between the two years: Unfed colonies reached an average weight of only about 14 grams in the drought year, while they grew to around 140 grams in the normal year,” says Hanno Korten. This corresponds to a tenfold increase, or growth of over 900 percent, under favorable climatic conditions.
Supplemental feeding with sugar water did increase the weight in the drought year to five times that of unfed colonies, but it was far from sufficient to compensate for the deficits compared to a normal year. “The stress caused by drought is evidently so severe that pure carbohydrate supplements can only partially stabilize the colonies’ vitality,” says Korten. When colonies are weakened to this extent, their daily work output declines, which directly impairs the pollination of wild and cultivated plants.
Number of queens drops significantly
However, this weight loss is merely a precursor to a far more serious problem: the near-total failure of the next generation. “The long-term stability of a population depends on the production of new queens, who are the only individuals to survive the winter and establish new colonies the following year,” explains Ingolf Steffan-Dewenter. If no reproductives are produced for the next season, “the colony’s survival through the summer may contribute to pollination but not to the long-term persistence of the population.”
The study documents a significant reduction in reproduction rates: While only 45 percent of colonies produced offspring during the drought year, the figure was 91 percent in a normal year. This is particularly evident in the number of new queens. In unfed colonies, their number rose from an average of just 0.4 in the drought year to 13.5 in the normal year—an increase of more than 30-fold.
The researchers identified the lack of pollen as the decisive bottleneck. While the provision of sugar water as a nectar substitute favored the production of males, it had no significant influence on the number of queens produced. Since pollen is the necessary protein source for larval development, its absence during a drought leads to an almost complete failure of female offspring. This poses a significant risk to the population: without the production of queens, the risk of local extinction increases in the following year. This threatens pollination reliability and, consequently, agricultural yields as well as the diversity of wild plants.
Targeted measures in landscape planning are needed
The results make it clear that even ecologically high-quality habitats such as calcareous grasslands do not provide sufficient refuge during droughts if active nature conservation management does not intervene. To increase pollinators’ resilience to extreme weather events, targeted measures in landscape planning are necessary, according to the researchers.
A key strategy is to promote shade-providing trees in otherwise open habitats to create cooler areas. Equally important is the restoration of wetlands and the implementation of measures that improve the soil’s water retention capacity on a large scale. In agricultural landscapes, greater emphasis should be placed on planting drought-resistant, summer-flowering plants to ensure a continuous food supply even during dry periods.
Hanno Korten, Department of Animal Ecology and Tropical Biology (Zoology 3), hanno.korten@uni-wuerzburg.de
Drought events reduce reproductive success of a long-tongued bumblebee species. Korten H, Steffan-Dewenter I. 2026. Proc. R. Soc. B 293: 20253056. https://doi.org/10.1098/rspb.2025.3056
Bombus pascuorum, the common carder bumblebee, visiting a flower.
Quelle: Hanno Korten
Copyright: University of Würzburg
A large nest of common carder bumblebees in a nesting box; the queen can be seen at the bottom left.
Quelle: Hanno Korten
Copyright: University of Würzburg
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