The harlequin ladybird Harmonia axyridis, one of the world’s most invasive insects, has become an invader of Europe and North America. Initially introduced for biological pest control, the harlequin ladybird, native to Asia, is now harming indigenous ladybird species, specifically the seven-spotted ladybird Coccinella septempunctata. Previously, some scientists believed the harlequin’s success was due to a harmonine, a toxic antibacterial chemical found in its blood. However, entomologist Andreas Vilcinskas of Justus-Liebig-University in Giessen, Germany believes the harlequin’s success is partly due to the presence of a single-celled parasite. 

The unicellular parasite exists in the larvae and eggs of the harlequin ladybird, but in a dormant and harmless state. However, when the parasite is injected into the seven-spotted ladybird, all of the beetles die within two weeks. Ladybirds typically prey off each other’s eggs and larvae. Therefore, by ingesting the invading harlequins, the native species are able to contract the fatal parasite.  

Other scientists believe this phenomenon might be widespread. Evolutionary biologist Nancy Morgan of Yale University believes unicellular parasites arriving with imported hosts are the cause of the decline in North American bumblebees.

However, some researchers believe there is a different explanation for the decline in the seven-spotted ladybug population. The harlequin’s success could be due its high resistance to a fungal disease that kills native species. Parasitic wasps, which lay their eggs in harlequins, could also be helping spread the unicellular parasite between species.

While the invasive harlequin ladybird may be a major contributor to the decline in the seven-spotted ladybird population, other biological aspects could play a role in this phenomenon.