Invasive bumblebees – pollinators can be a threat to biodiversity
Seeing Bombus terrestris, the buff-tailed bumblebee, on my terrace, I usually don’t think it may do any harm. That it may become invasive. It’s one of the most common bumblebees in Europe and often one of the first bees in spring. In some years, the queen already begins to fly end of February. But now it’s used commercially, and that’s where problems begin. Buff-tailed bumblebees thrive in greenhouses and they build up large colonies. Several hundreds of workers may be small compared to honey bees. But many bumblebee species have colonies with only around 100 workers, so B. terrestris colonies are giants compared to that. In addition, another bumblebee trait makes them very attractive: they buzz-pollinate. This means that they shake the pollen out of the anthers by “buzzing”, i.e. vibrating with their flight muscles, on the flowers. Plants like tomatoes or aubergines need this type of pollination.
Tomatoes are a classical greenhouse crop. It was a break-through when bumblebees first became available as “pollination units” for greenhouses in 1988. They were introduced in several countries and are now the second most used managed pollinator species. The first, in case you’re wondering, of course, are honey bees (A. mellifera). The very first introduction in a country out of its natural range was New Zealand, though. This happened in 1875, long before the increasing awareness of the harm invasive species can do. I also remember an anecdote that Darwin said the wealth of Great Britain depended on bumblebees… I don’t remember the details and don’t find them now. But if you know the story, please share it in the comments!
What makes a bee invasive
On the CABI Invasive Species Compendium, buff-tailed bumblebees are classified as potentially highly invasive. The traits that influence this are the same that make B. terrestris so suitable for mass-rearing:
- Early beginning of their flight season
- A wide range of habitats
- They feed on a wide range of different flowers
- The species is very flexible in their life-cycle
- A single colony produces a large number of queens
Being so flexible and good in adapting to new conditions, they spread in new habitats quite quickly. They escaped from greenhouses. Field uses made it even easier for them to spread and establish where they arrived. This, unfortunately, didn’t add to the bee diversity in these places but created problems. Buff-tailed bumblebees compete with native bees for resources like flowers or nesting places. They alter the native flora as well: in Tasmania, this bumblebee helps the spread of invasive plants. They transmit parasites and pathogens. This is the reason why in Chile, they put the world’s largest bumblebee, Bombus dahlbomii, at risk. This is something I take personally: this species is closely linked to my personal history as a bee researcher. Only recently, a group of scientists claimed to stop the introduction of buff-tailed bumblebees to Chile and to begin to control them. You can find some information here (in Spanish).
Invasive bees are a threat to bee diversity
A recent paper analysed the potential impact the buff-tailed bumblebee may have on Chinese bumblebee diversity. The authors examined how much the habitat of 24 native species would overlap with the area suitable for B. terrestris. China is one of the hotspots for bumblebee diversity, 125 species live in this country, including also some species that occur nowhere else (endemic species). Only five of the analysed 24 species didn’t have any habitat overlap with the European species. Species like B. braccatus which are endemic and also already rare in China share a potential habitat of 1,063,179 km². Let that sink in for a moment. There are seven other endemic species that also have similar overlaps.
This analysis was done by modelling and didn’t consider all Chinese species. However, the pollination crisis already hits China severely in some regions. You may know pictures from hand pollination in this country. This should be a warning not to put the precious natural resources even further at risk. The authors of the study claim proper management strategies and regulations for the introduction of B. terrestris. But honestly, I wonder why we should still do this at all. Why still make the same mistakes over and over again? Perhaps Chinese bumblebee species and other wildlife are more resilient to invasive species than in other countries. But is it worth taking the risk? Instead of developing management strategies for the introduced species, why not develop these strategies for native pollinators?
Impact of managed bumblebees in their native range
In pollination, there’s no “one fits all“. It’s the pollinator diversity that keeps ecosystems stable. Therefore, managed pollinators are part of the problem. We know that honey bees transmit diseases to other bees. This is true also for the buff-tailed bumblebee, as mentioned above. In addition, this risk isn’t limited to areas, where this species isn’t native. It may be a danger also in its natural range. Some years ago, Whitehorn and his colleagues compared soft fruit farms in Scotland using commercial bumblebees with others that didn’t. They assumed that even if a pathogen is already present, managed bumblebees could have higher parasite loads. By this, parasites could spread to individuals of the “wild” population of B. terrestris and other bumblebees. Though the data didn’t clearly indicate a spillover of pathogens to wild species, they found that a gut parasite (Crithidia) increased more on farms with managed bumblebees.
Another study discusses the differences that exist between commercial and wild buff-tailed bumblebees. These colonies may differ from their wild relatives to better suit the human needs. And in fact, there were several differences:
- Commercial queens had a shorter diapause (i.e. “overwintering”) than their wild relatives.
- They began faster with egg-laying.
- The colonies from commercial queens got larger (about twice as much workers) than “wild colonies”.
- Commercial colonies produced about four times more young queens.
Problems of “domestication” of bees
These were lab experiments, so the comparison wasn’t with colonies in the field. But both “types” of bumblebee were reared in the same conditions so that their differences stuck out. The impact in the field may be different, smaller or larger, who knows. But, and this is important, it shows that manipulating a species changes it. Commercial bumblebees aren’t harmless only because they’re native in an area. I know that responsible breeders of B. terrestris check the colonies for parasites and diseases before selling them. So this risk may be manageable. However, this doesn’t consider other problems that come with the “domestication” of bees. Colonies from mass-rearing begin earlier, grow faster, reproduce at a higher rate. These are traits that suit us, our need for crop pollination. But it may not be suitable for the bees themselves.
In fact, this reminds me a lot of the most used managed pollinator, the European Honey Bee. Beekeepers want large colonies that don’t swarm, start early in the year and produce large amounts of honey. They keep hundreds of colonies at the same place and transport them over large distances. In my humble opinion, most of the problems in beekeeping are man-made, including parasites like Varroa. We may be about to make exactly the same mistakes with a second bee species. And I doubt that the pollination crisis will be solved by this strategy. Instead, we urgently need strategies for helping pollinators all over the world. There’s already lots of research indicating possible solutions.