Introduced to Europe and North America for the purpose of controlling pesky greenhouse aphids, the Asian lady beetle escaped into the ecosystem and quickly dominated the local ladybug population.

According to a new report in the journal Science, the aggressive ladybug’s dominance is being assisted by parasites being carried within its own bloodstream that are highly lethal to other species.

“They keep them inactive in their own blood, we don’t understand how they do it yet,” co-author Heiko Vogel from the Max Planck Institute (MPI CE) for Chemical Ecology told BBC News. “But when the other [ladybugs] start to attack the invader’s eggs and larvae, they become active and kill the native ones.”

When the German biologists looked at the Asian lady beetle’s haemolymph, or blood, under a microscope, they were able to identify the tiny parasites: fungi called microsporidia. While the fungi are present in the eggs and larvae of the invasive species, they appear to exist in a dormant and harmless state.