The harlequin ladybird was once a stalwart ally of greenhouse owners around the world.
Native to Japan, Korea and other parts of eastern Asia, the bright red beetles were prized for their aphid-eating abilities - until they caused serious declines in other ladybird populations.
Now, researchers have discovered the harlequin's secret weapon: a deadly parasite that lives harmlessly in its body but kills other species with abandon.
The findings, published in the journal Science, demonstrate how things can go awry when a foreign creature is introduced into an ecosystem, even when done with the best intentions.
Ladybirds are beloved by humans and are valuable to gardeners, who deploy the spotted beetles to eat plant-munching aphids rather than spray their shrubs, flowers and crops with harsh chemical pesticides.
It is a prime example of an environmentally friendly agricultural practice known as biological control. But one particular ladybird - the harlequin, or Harmonia axyridis - has proved to be a two-faced friend.
The bugs gobble up aphids at jaw-straining speeds, but spread like wildfire once they escape the greenhouse, quickly taking over native ladybirds' turf in large parts of Europe, North America and other regions.
In Europe, swarms of the pests have started taking wintertime shelter in houses, said study co-author Heiko Vogel, a biologist at the Max Planck Institute for Chemical Ecology in Germany.
The residence of Vogel's own mother has been plagued by the beetles, which can fly through windows, slip under carpets and burrow into any crevice a human home has to offer. "Thousands of beetles - even if they are nice-looking - crawling into your house is not fun," Vogel said.
German researchers analysed the harlequin ladybird's haemolymph - its blood, essentially - under a microscope and found extremely high spore counts of a single-celled fungus related to the parasite Nosema thompsoni.
With so many spores in their haemolymph, "you should be dead", Vogel said. The researchers soon realised that somehow, the beetle's immune systems had managed to tame the spores. The findings offer a cautionary tale for biological control. Vogel said: "You should think twice before you introduce an alien species into a new , even if it's for a good reason."