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Climate change means early flight start - risk of fewer bumblebees and reduced pollination

Bumble bee and yellow flowers. Photo.
A queen of the species Bombus terrestris. The species belongs to the group of bumblebees that have advanced their activity flight, which is now about 14 days earlier than a century ago. Photo: Maria Blasi Romero

With the arrival of spring, bumblebee queens take their first wing beat of the season and set out to find new nesting sites. But they are flying earlier in the year as a result of warmer climate and changing agricultural landscape, new research shows.
– The risk is that we will lose additional bumblebee species and have reduced pollination of crops and wild plants, says researcher Maria Blasi Romero at the Center for Environmental and Climate Science.

When spring comes and the ground warms up, bumblebee queens wake up from hibernation. Contrary to workers and males, queens are the only bumblebees that survive the winter and they spend a couple of weeks to find a place to nest, where they can lay eggs and start a colony.

But rising temperatures mean that they wake up earlier in the year. New research shows that, in Sweden, the first flight occurs on average five days earlier than twenty years ago.

– Across Sweden, we see that the increased temperatures due to climate change clearly affect when the queens wake up and fly to find a new nest, says researcher Maria Blasi Romero.

14 days earlier than a century ago

And it is not just the temperature that has an effect. The researchers have used the Lund Biological  Museum's collection to examine bumblebee queens as far back as 117 years in different areas of Skåne (Scania). This data shows that the first bumblebee flight in intensively farmed landscapes in Scania now takes place about fourteen days earlier than over a century ago.

The major change in Scanian landscapes during the past century is the loss of grassland habitats, such as meadows and permanently grazed pastures. Today, large agricultural fields dominate and often only a few different crops are grown. This has led to a general decline of farmland biodiversity. 

The fact that bumblebee queens leave their hibernation much earlier nowadays is therefore likely due to both a warmer climate, a lack of food during the flight period, and that the microclimatic conditions vary more in today's agricultural landscape than in the more varied landscapes of older times.

Bumblebees among flowers. Photo.
The garden bumblebee, Bombus hortorum, is one of the later-emerging bumblebees which does not seem to be able to shift their emergence time like the early-flying bumblebees. There is a risk that species such as this will be poorly matched to the growing season and therefore not find enough food. Photo: Maria Blasi Romero

The researchers have focused on ten bumblebee species and found that the species that already used to fly earliest in the season have become even earlier flyers, while the species that emerge a later in the season have not changed their flight season. There is a risk that this leads to a poor match between the activity periods of flowering plants and bumblebees, and that bumblebees do not get enough food.

– We see a clear risk that more bumblebee species are at risk of extinction locally, especially the species that usually emerge later in the summer. This could also lead to a decline in the number of bumblebees overall and that would have consequences for the pollination of crops and the functioning of ecosystems. Bumblebees are important pollinators, especially in northern latitudes such as in Scandinavia, says researcher Anna S Persson.

Actions that slow down the effects

The study highlights several measures that could reduce the effects of climate warming on pollinators and increase their access to flowering plants. A few examples are:

  • Preservation of natural grasslands, such as natural pastures.
  • Late season mowing at roadsides, after the flowering period.
  • Flower strips and hedges designed in a way that favors pollinators.
  • Increased sowing of clover-rich leys, that are partly allowed to flower.

The research group sees the findings as interesting in a larger perspective:
– Climate change and changing land use are two of the biggest threats to biological diversity. Different species respond differently to these changes, so it is important to know more about how and why that is. There are winners and there are losers among species, says researcher Romain Carrié.

The researchers also point out that it is crucial that biological collections, like the insect collection at Lund University, can be preserved and maintained to enable future research.
Link to the article Historical and citizen-reported data show shifts in bumblebee phenology over the last century in Sweden -Biodiversity and Conservation

Facts on Bumblebee queens

The first bumblebees to be seen of the year are always the queens, they are the only ones that survive the winter. At the start of spring, the ground warms up and the queens wake up from their hibernation. They usually live in burrows underground or in old mouse and vole nests in grasslands and fly for about a two-week period to find a new nest site to lay eggs and form a colony. The queens search for flowers with a lot of pollen and nectar that become food for themselves and their larvae. The larvae then develop into pupae and then into worker bumblebees that help gather food and build the colony. New queens develop in late summer, mate with males and they then overwinter until the following spring.

Bumblebees from the museum. Photo.

About the research

Article: Historical and citizen-reported data show shifts in bumblebee phenology over the last century in Sweden, published in the journal Biodiversity and Conservation.
Authors: Maria Blasi, Romain Carrié, Christoffer Fägerström, Emma Svensson, Anna S. Persson
Departments: Center for Environmental and Climate Science, the Biological Museum at the Department of Biology.
Method: The researchers used specimens of bumblebee queens collected in Skåne and stored at the Biological Museum at Lund University for 117 years, and citizen-reported observations from Artportalen over the past 20 years from all over Sweden.
Uncertainty in the research: As there is no structured monitoring data in the museum's collections, sampling bias has been used to correct statistically. This applies, for example, to where the collectors of the queens lived. For Artportalen's observations, it may be the case that the early, most common species may be especially reported at the first nice spring weather, while the later and rarer species are not reported as often.

Photo: Christoffer Fägerström/Lund University