British bumblebees could be rescued - or threatened - by a foreign invader from continental Europe that resists parasites, say scientists.
The tree bumblebee, Bombus hypnorum, has spread rapidly since its arrival in the UK 13 years ago.
Each year it expands its range by an estimated 4,500 square miles, an area half the size of Wales.
A new study of tree bumblebee queens collected from the wild has shown they carry high numbers of a worm parasite that renders other bee species infertile.
Yet a quarter of the queens were able to produce offspring, indicating they were immune to the infection.
Experts cannot yet make up their minds whether this is a good or bad thing. While the foreign bees may divert parasites from vulnerable native species, they might also compete for food and nesting sites.
Researcher Catherine Jones, from Royal Holloway, University of London, is in the "good" camp. She said: " Bees are essential to our food chain and the populations of our native bumblebees have declined in recent decades.
"The arrival of tree bumblebees could be hugely beneficial to us by absorbing parasite pressure from our native species, as well as helping to pollinate wild plants and crops."
But colleague Professor Mark Brown, also from Royal Holloway, warned: "While these findings show promising signs for bee populations in the UK, we still don't know whether there could be any negative impacts if the bumblebees compete for food or nesting sites.
"Further research should focus on how our native bees are affected and the pollination services that this new species provides."
The research is published in the Journal of Animal Ecology.