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 other parts of the world, by people who were seemingly undeterred by the outcomes of bringing cane toads to Australia or mongooses to Hawaii. Like those other invaders, the harlequin has brought ruin to local ladybirds, many of which have declined dramatically since its incursion.
There are probably many reasons for that. Perhaps it simply outcompete other species for food, or eats them directly. It carries a potent slew of antibacterial chemicals in its blood (or haemolymph) that makes it remarkably resistant to disease. For example, it can shrug off a deadly fungus that kills other ladybirds.
One of these antibacterials is a toxic chemical called harmonine. Many scientists suspected that this substance was poisoning other ladybirds that tried to eat the harlequin’s eggs. But Vilcinskas found that harmonine doesn’t affect native species at all. When he injected the seven-spot ladybird with high doses of the stuff, they were fine. But when he shot them up with the harlequin’s unfiltered haemolymph, they died. The invader clearly has something in its blood that’s deadly to other ladybirds, but it’s not harmonine.
Vilcinskas found the culprit by looking at harlequin haemolymph under a microscope. He found it swarming with microscporidians—a type of single-celled parasitic fungus. These parasites are found in every harlequin that the team examined, but don’t seem to do any harm. They stay in an inactive state and their genes are completely inactive. “I have worked on insect immunity for 20 years, and I had never [before] seen a haemolymph sample that was full of microsporidians that do not harm the carrier,” says Vilcinskas.
It’s possible that harmonine and other antibacterials allow the harlequin to tolerate its parasite. But the native seven-spot ladybird isn’t so well-defended. When Vilcinskas injected them with the microsporidians, they all died within two weeks.
This might be why so many native ladybirds die when the harlequin invades. Since all ladybirds eat each other’s eggs, those that chomp on the harlequin’s young could get a mouthful of lethal microsporidians.
Of course, they need to actually prove that. Helen Roy, who leads the UK Ladybird Survey, says that injecting seven-spots with microsporidians is a far cry from showing that they actually get infected in the field. For a start, she says that seven-spots very rarely eat harlequin eggs, so their chances of getting infected by microsporidians would be few and far between. Then again, seven-spots seem to be holding their own against the invaders, and are unusual among British ladybirds in showing no population declines. Perhaps other species are more wanton in their feeding habits and pay the price?
Either way, Vilcinskas’s team need to show that wild ladybirds do eat harlequin eggs, that they contract microsporidian infections, and that this contributes to their downfall. “The next steps would be to assess ecological relevance,” says Roy. “What does this mean in the real world?”
Lori Lawson Handley, who also works on the UK Ladybird Project, wonders if the microsporidians could be travelling between species through a more grisly route. Some parasitic wasps, like Dinocampus coccinellae, lay their eggs in ladybirds, and they could be spreading the parasites from the harlequin to other species. Their stings could be the equivalent of dirty needles.
A version of this piece also appears at Nature News.
Reference: Vilcinskas, Stoecker, Schmidtberg, Rohrich & Vogel. 2013. Invasive Harlequin Ladybird Carries Biological Weapons Against Native Competitors. Science http://dx.doi.org/10.1126/science.1234032