Cuckoo bumblebees are obligate parasites that must successfully invade and gain control of a host bumblebee colony to achieve reproductive success. As apex species, these cuckoos have been considered as potential indicator species of healthy bumblebee habitat for some time. However, our limited understanding of cuckoo bumblebee ecology makes it difficult to understand their full impact on host populations and the challenges they face as a group. In this talk, Sofia describes this fascinating host-parasite relationship and presents experiments she has conducted to unravel the complexities of the cuckoo bumblebee lifecycle.
🐝 Learn more about cuckoo bumblebees in our Bumblebees of the UK online course: https://courses.biologicalrecording.co.uk/courses/bumblebees
Q&A with Sofia Dartnell
Sofia Dartnell is a PhD candidate at the University of Cambridge. Before moving to the UK, Sofia completed her undergraduate degree in biology at Pomona College, California. There, she spent time researching the pollinator communities visiting Los Angeles native plants as well as the impacts of urbanisation and habitat loss on pollinator communities. Outside of her research, Sofia is passionate about teaching, with experience teaching undergraduate-level entomology, conservation science, and tropical biodiversity courses.
What ratios of males to females do you see in cuckoo bumblebee populations?
Bearing in mind that all female cuckoo bumblebees are queens, because there is no worker caste, the ratio of males to queens is about 3:1. This is a similar ratio of males to queens to that seen in the host bumblebee species. The reason the ratio is skewed in bumblebees generally (i.e. not 1:1) is because the energy investment required from a colony to produce a queen is much greater than that needed to produce a male.
Can cuckoo bumblebee queens and their hosts ever live alongside one another?
Interesting question – it actually depends on the species. With Bombus vestalis and its host Bombus terrestris, the two species I studied for my research, the invasion of the cuckoo bumblebee (B. vestalis) is typically fatal for the host (B. terrestris). In other species, however, the cuckoo bumblebee does sometimes not kill the host queen upon invasion and instead coexists in the colony with her. It’s believed this is done because the host queen has better control of the worker bees’ reproductive success than the cuckoo bumblebee would have, thus allowing for more efficient ‘control’ of the colony. It’s a risky approach as sometimes the workers wise up to the fact there is an intruder and will destroy some of the cuckoo bumblebee’s eggs and larvae.
Have you done any work on the chemical signals that cuckoo bumblebees use to detect host bumblebee colonies?
Personally, not so far. There has been some interesting work done by other scientists in this area, though. One interesting discovery I remember reading about is that cuckoo bumblebees which have multiple host species are specialised to detect the shared chemical profile that overlaps between their hosts, which is cool.
How do cuckoo bumblebees subdue and kill their host queens?
There was a scientist called Richards in the late 1900s who conducted detailed morphological study of cuckoo bumblebees and their hosts. He found that cuckoo bumblebees have a much thicker exoskeleton than their hosts – almost like they’re wearing armour – as well as a longer, more powerful stinger. From my own observations, the primary way that the cuckoo bumblebees go about taking down the host queen is by tackling them and trying to sting them. The worker bumblebees of the host colony will often join the fight and attack the cuckoo bumblebee, and even occasionally go so far as to rip the cuckoo bumblebee’s wings off, but the cuckoo bumblebee doesn’t need its wings once it’s subdued the queen, so these are relatively expendable. Only rarely do the workers successfully stop an invasion of a cuckoo bumblebee, though this depends on the size of the colony being invaded. Success is highest when the colony is small.
Literature References
The key paper by Sofia upon which much of this entoLIVE was based is below:
Dartnell and Dicks (2026) ‘Brood parasitism reduces but does not prevent Bombus terrestris reproductive success’: https://onlinelibrary.wiley.com/doi/10.1111/1744-7917.70235
Other literature references of potential relevance are:
- Björn (1983) ‘The role of trail pheromones in host selection by Psithyrus rupestris (Hymenoptera, Apidae)’: https://www.researchgate.net/publication/292894016
- Bunk et al. (2010) ‘The role of trail pheromones in host nest recognition of the social parasitic bumblebees Bombus bohemicus and Bombus rupestris (Hymenoptera: Apidae)’: https://link.springer.com/article/10.1007/s00049-010-0044-4
- Fisher (2011) ‘Evolution and host specificity: a study of the invasion success of a specialized bumblebee social parasite’: https://www.researchgate.net/publication/237999337
- Gardner et al. (2020) ‘Reliably predicting pollinator abundance: Challenges of calibrating process-based ecological models’: https://besjournals.onlinelibrary.wiley.com/doi/10.1111/2041-210X.13483
- Sramkova and Ayasse (2009) ‘Chemical ecology involved in invasion success of the cuckoo bumblebee Psithyrus vestalis and in survival of workers of its host Bombus terrestris’: https://link.springer.com/article/10.1007/s00049-009-0009-7
- Whitehorn et al. (2022) ‘The effects of climate and land use on British bumblebees: Findings from a decade of citizen-science observations’: https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2664.14191
Further Info
- Bumblebees of the UK online course: https://courses.biologicalrecording.co.uk/courses/bumblebees
- Bumblebee Conservation Trust’s BeeWalk Survey Scheme: https://www.bumblebeeconservation.org/what-we-do/our-projects/current-projects/trust-led-projects/beewalk/
- Understanding a Bee’s Buzz: Biology to Robotics (entoLIVE blog): https://biologicalrecording.co.uk/2026/01/13/understanding-a-bees-buzz/
- Bumblebees & Their Differing Habitats: How a Decade of Citizen Science Has Increased Our Knowledge (entoLIVE blog): https://biologicalrecording.co.uk/2023/05/25/bumblebee-declines/
- The Mind of a Bee: An Exploration of the Intelligence of Bees (entoLIVE blog): https://biologicalrecording.co.uk/2025/02/18/mind-of-a-bee/
- The Plight of the Bumblebee: A 12-Year Reintroduction Case Study (entoLIVE blog): https://biologicalrecording.co.uk/2024/03/25/bumblebee-reintroduction/
- Backyard Bees: Private Green Spaces and Pollinator Diversity in Cities (entoLIVE blog): https://biologicalrecording.co.uk/2026/04/22/backyard-bees/
- Plants for Pollinators: Using DNA to Explore Relationships in Gardens (entoLIVE blog): https://biologicalrecording.co.uk/2024/12/03/plants-for-pollinators/
- Bees and Beyond: Ecosystem Services at a Garden Scale (entoLIVE blog): https://biologicalrecording.co.uk/2024/09/05/bees-and-beyond/
- Bees and Heatwaves: Preparing For A Warming World (entoLIVE blog): https://biologicalrecording.co.uk/2024/06/05/bees-and-heatwaves/
- Beginner’s Guide to Planting For Pollinators: https://biologicalrecording.co.uk/2025/04/01/planting-for-pollinators/
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entoLIVE
entoLIVE webinars feature guest invertebrate researchers delving into their own invertebrate research. All events are free to attend and are suitable for adults of all abilities – a passion for invertebrates is all that’s required!
entoLIVE is delivered by the Biological Recording Company and currently sponsored by the British Entomological & Natural History Society, Royal Entomological Society, and Amateur Entomologists’ Society, with promotional support from Buglife.
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