The interloper is the harlequin ladybird (Harmonia axyridis), one of the world’s most invasive insects. From its homelands in central Asia, H. axyridis was introduced to Europe and North America to control aphids. Since then, however, it has become a serious pest that has put native ladybird species under threat by outcompeting or even eating them.

Entomologist Andreas Vilcinskas of Justus-Liebig-University in Giessen, Germany, suggests that the harlequin’s success partly hinges on the presence of a single-celled parasite that he and his colleagues found living inside the ladybirds. The parasite does no harm to the harlequin, but if it infects other ladybird species, such as the native seven-spot ladybird (Coccinella septempunctata), it invariably kills them.

Ed Yong for Nature.