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Hostile invader: Ladybug species carries spores that kill competitors

Spores are tolerated by invaders but wipe out native species.

Hostile invader: Ladybug species carries spores that kill competitors

Invasive species have become a problem on nearly every continent. Native species that may have had millions of years to adapt to their environment are somehow trivially displaced by a species that originated somewhere else. How is it that the invaders can be so phenomenally successful against what should be well-entrenched competition?

A new study shows that in at least one case, some insect invaders engage in a bit of biological warfare, carrying a fungus that kills their competitors but which the host can tolerate. The fungus spreads because of a nasty habit the insects have—namely that they tend to eat each others' eggs. Somewhat ironically, all of this goes on in a species that tends to have a friendly reputation: the ladybug (or ladybird, for the anglophiles among us).

The invasive species in question is an Asian ladybug, the harlequin ladybird Harmonia axyridis. Because of its fondness for agricultural pests (fondness in the same sense that I have a fondness for lobster), Harmonia has been introduced to some countries where it wasn't native. When the invaders were introduced, the native ladybug species dropped like flies (pun intended) and was easily displaced by the new arrivals.

Originally, in addition to going after aphids, researched believed the ladybugs eat each others' eggs as a form of nutrition and as a way of cutting down on competition. They noted that when native ladybugs munched on harlequin eggs, the insects ended up dead within a couple of weeks. In an attempt to find out why, researchers injected the harlequin's hemolymph (the insect equivalent of blood) into the natives and found that it was lethal. Suspicion fell on a specific chemical compound in the hemolymph called harmonine (named after the genus name, Harmonia).

But the new paper does the obvious next step and rules this idea out. They synthesized harmonine, injected it into a native species, and found it had no effect. Going back to the drawing board, they looked more carefully at the invaders' hemolymph. When they did, they noticed it was full of spores from a fungus. These turned out to be from a microsporidia related to Nosomia, a fungus that has been implicated as one of the contributors to bee colony collapse.

This form of Nosomia obviously wasn't lethal to the invaders. But when the spores were cleaned up and then injected into native species, the bugs all died within two weeks. A control of heat-treated spores had minimal affect.

How does the harlequin species tolerate this perpetual Nosomia infection? The paper doesn't look into this, but one potential candidate is harmonine, the chemical that was originally the focus of this work. (It would be easy to inject harmonine along with the spores to see if this aids in survival, so this is a very testable hypothesis.) More generally, however, the harlequin ladybirds seem to go through most of their lives with a revved-up immune response. This suggests that they may expend a lot of energy to keep the Nosomia in check. If so, it ensures that they've got a big edge on any competition and may be worth the energetic cost.

Clearly, this sort of biological warfare won't explain every case of species invasion, but it may not be uncommon. As described in an accompanying perspective, North American crayfish have invaded Europe very successfully in part because they carried a disease that wiped out the locals. The perspective's author makes a direct comparison to a case of interspecies invasion, namely when Europeans arrived on North American shores with diseases that depopulated the continent ahead of their colonization.

Science, 2013. DOI: 10.1126/science.1234032  (About DOIs).

Channel Ars Technica