The Asian ladybird is found mainly in spring and autumn in masses. (Photo: Schmidtberg, Vilcinskas, University of Gießen)
Read aloud He looks cute and harmless: The Asian ladybug is like his native relative round and adorned with dark spots on a red background. But this beetle fights with hard bandages. Since he was introduced to Europe a good ten years ago, he is spreading rapidly, the domestic ladybug has no chance against him. The search for the reason for this superior power was like a thriller for a long time. But now German researchers have finally found out which weapons make this invasive species so unrivaled success. Originally from Japan and China, the Asian ladybug (Harmonia axyridis) is a ravenous pest killer: a single animal can kill more than 200 aphids in one day. That's why at the end of the 20th century he was first introduced to the US, then to Europe. It should help to decimate the plant pests in a gentle, because biological way. At the same time, his presence leads to serious problems. For where the newcomer spreads, disappears the native ladybug Coccinella septempunctata.

Initial observations had shown that more than just food competition is at stake here. Because when the beetles of both species encounter the eggs or larvae of a conspecific or the other species, they are thankfully consumed as a snack. But this is exactly where the Asian ladybird plays a trump card: If a native beetle eats its eggs and larvae, it dies shortly thereafter. For the novice himself, however, his own eggs and larvae as well as those of his native relatives are completely harmless. Is there a refined poison at work here? To clarify this, Andreas Vilcinskas from the University of Gießen and his colleagues first examined the beetle hemolymph - his blood, so to speak.

Poison in the bug's blood?

In fact, they found it: "Unlike native species, Harmonia's hemolymph contains a substance that has a strong antibacterial effect, " the researchers explain. Your assumption: This chemical baptized by them Harmonin could protect the beetle particularly well from attacks by pathogens that are still unknown to his immune system because of the new environment. At the same time, however, this substance could act as a chemical weapon against domestic competitors - simply by poisoning them. display

To test this, Vilcinskas and his colleagues artificially rebuilt the Harmonin and injected either female ladybugs with this substance or a dose of Harmonia hemolymph. Contrary to expectations, the beetles remained healthy even after high doses of pure harmonin. On the other hand, those who had received the hemolymph soon died. "It is therefore obvious that mortality in Coccinella beetles is not caused by harmonin, but by another component in the hemolymph, " the researchers conclude.

Parasite as a tamed "pet"

The search for the "secret weapon" of the invasive beetle continued. Next, Vilcinskas and his colleagues examined samples of Harmonia's hemolymph under a microscope, hoping to get an indication of the mysterious toxicity of this fluid. And they actually saw something that actually does not belong there: small spores of a parasitic protozoa. These so-called microsporidia penetrate into the cells of their host, multiply there and trigger or even kill serious illnesses. Interestingly, however, this parasite appeared inactive in the Harmonia beetle and did not harm it, even though the researchers found it in all stages and even in its eggs. Why the beetle is immune to the deadly parasite is not yet clear. "Whether the Harmonin protects him or the numerous antimicrobial peptides in his hemolymph, still needs to be clarified, " explain the researchers.

According to this finding, it has long been suggested that the domestic ladybug does not have this immunity and therefore dies as soon as it eats the parasitic Harmonia eggs and larvae. To test this, researchers isolated the microsporidia from the hemolymph of the Asian beetle, cleaned it, mixed it with saline, and injected it into some Coccinella septempunctata beetles. The sad result: all the beetles were dead within two weeks. "This confirms our hypothesis that these parasites contribute to the dominance of the Asian ladybug over the native species, " Vilcinskas and his colleagues note. This is a good example of the important role that the immune system can play in bioinvasions.

Andreas Vilcinskas (University of Giessen) et al., Science, doi: 10.1126 / science.1234032 © wissenschaft.de - === Nadja Podbregar

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