When Europeans arrived in the New World, they brought devastating diseases like smallpox, which killed more native Americans than guns and other weapons. Infections go the other way too: When grey squirrels from North America arrived in the UK, they brought a squirrel pox virus that decimated the local red squirrels. Time and again, animals have invaded new regions and killed the locals by inadvertently bringing biological weapons with them.

Now, Andreas Vilcinskas from Justus-Liebig-University of Giessen has found that one the world’s most invasive insects—the harlequin ladybird—also belongs in the biological weapons club.

It hails from central Asia, but was willingly introduced to Europe, North America, and ...