Ladybirds around the world are in danger from an invader that inadvertently wipes out its competitors using a biological weapon.

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.

Some scientists had previously thought that the harlequin owed its success to haemolymph, a toxic antibacterial chemical found in its blood. Harmonine allows the harlequin to resist certain diseases and to poison native ladybirds that eat its eggs. However, Vilcinskas and his team found that high concentrations of harmonine do not kill seven- spot ladybirds -- but a dose of the harlequin's haemolymph does. When Vilcinskas and his colleagues looked at the harlequin's haemolymph under a microscope, they found the culprit: a microsporidian parasite. These exist in the eggs and larvae of all harlequin ladybirds, but in a dormant and apparently harmless state.

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, it invariably kills them.

Other ladybirds are not so lucky. Ladybirds often eat each other's eggs and larvae, so by consuming invading harlequins, native species could contract the fatal parasitic infection in the wild. And there could be other reasons for the harlequin's success. For example, it is highly resistant to a fungal disease that kills native species.