The Asian lady beetle (Harmonia axyridis)—a relative of the ladybug—was originally imported to the U.S. in 1916 in an attempt to naturally control pests. After escaping greenhouses, they spread uncontrollably throughout the U.S. and Europe in a matter of decades. But no one understood why they were such successful conquerors. (Related pictures: “Invasive Species in the United States.”)
Now, a recent study published in Science finds that Asian lady beetles contain tiny fungus-like parasites that infect and kill native species, wiping out the invaders’ competition for food.
Asian lady beetles are aggressive, known to attack the larvae and eggs of other insects like butterflies and native lady beetles with which they compete. But that’s not what makes them so deadly. The invaders themselves contain high concentrations of a fungal parasite called microsporidia.
“If one of the native lady beetles eats larvae and eggs of Asian lady beetles, [the natives] die off,” said Heiko Vogel, a biologist at the Max Planck Institute for Chemical Ecology and co-author of the study. (Related: “How a Cat-Borne Parasite Infects Humans.”)
“We were wondering, ‘What was going on there?'” he added.
Vogel and his team identified Asian lady beetles from 12 different countries using physical examinations under a microscope and DNA analysis. After confirming the species, the researchers analyzed the beetles’ blood in order to figure out what was infecting and killing native bugs.
That’s when they found the fungal parasite lying dormant within the invaders. While Asian lady beetles are immune to the parasites, other insects are highly susceptible to its deadly effects. (Learn about a fungus that controls ant brains.)
“We didn’t believe our results at first,” Vogel said. “We wouldn’t have dreamed that this parasite, this fungus, would be a problem for the native beetles.”
When another insect eats an Asian lady beetle, the parasites activate. Microsporidia penetrate the new host’s cells, siphoning off its energy in order to multiply. The parasites continue to replicate until they’ve completely taken over and killed their host.
Then the fungus produces spores, which can survive harsh environmental conditions while waiting for the cycle to begin again. Asian lady beetles naturally contain a high concentration of these spores.
“We found these huge amount of spores in the blood of the Asian ladybird beetles, and we’re like, ‘What the hell?'” Vogel said. “When you see something like this, usually the insect is dead.” (See photos of how a parasitic fungus takes over ant brains.)
The invaders are incredibly hardy insects. Asian lady beetles have a hyperactive immune system, which could be what’s protecting them from succumbing to the spores in their blood.
And these conquerors aren’t just a problem for other insects. Every winter, Asian lady beetles gather by the thousands to weather the cold temperatures inside any cave-like dwelling they can find—including houses. Vogel’s own mother walked outside four years ago to find thousands of the pests attached to the side of her house, attempting to force their way in.
“They’re trying to sneak into crevices,” Vogel said. “You can’t open a window anymore or you’ll have thousands inside.”
The Asian lady beetles en mass have been known to cause health issues for people prone to asthma. They also give off a horrible smell when they sense danger as a deterrent against predators. One smelly beetle is nothing more than an annoyance, but if you have thousands of them, the stench can be a big problem, Vogel said.
Any efforts to eradicate the beetle, which also feasts on agricultural crops like grapes and apples, are likely a waste of resources, the biologist added. The bugs are just too resilient to control.
“I don’t think we can do a lot against [the Asian lady beetles],” Vogel said. “They’re so successful, and they’re spreading so fast. There’s no stopping them.” (Learn more about invasive species.)