A Staff Report from the Straight Dope Science Advisory Board

Are toxic orange ladybugs the latest environmental threat?

August 21, 2000

Dear Straight Dope:

I was recently told that the U.S. is being overrun by toxic orange lady bugs. These lady bugs, according to my source, were imported from Asia to combat aphids and other garden pests. However, they have become a threat to human health because they give off a toxic vapor which causes asthma and they invade homes.  Since hearing this I have noticed that the only ladybugs I see are orange. Where have the red ladybugs gone? Are these big orange Asian bugs taking over? Who is responsible for importing them?

You're thinking of the Asian ladybug, Harmonia axyridis, intentionally released by the USDA in a well-intentioned but ultimately misguided attempt to add yet another "beneficial" insect to our fauna. Ironically, most of the evidence suggests that the USDA attempts at introduction only worked in the western U.S., and that the ones running rampant through most of the rest of the country were accidentally brought in, like most pests. As an aside, I appear to be the first entomologist to record this species in the western U.S., in Seattle in 1991.

Asian ladybugs have no natural enemies here, so they outcompete our native ladybugs in the wild and build up large populations rapidly. This is good insofar as it concerns their ability to control aphids, which is what they feed on. Unfortunately, it's not good that they're driving our native ladybugs to extinction. This particular species also has the annoying habit of choosing human homes as places to spend the winter. They don't give off a toxic vapor, at least not in amounts large enough to be detectable by a human nose, unless they are handled or crushed--and all ladybugs give off the exact same "toxic chemical," a foul-smelling yellow oil. The toxic effect on humans is negligible; you'd have to eat a few pounds (thousands) to get sick. But you'd notice it if you were a sparrow, fence lizard, etc. That's why ladybugs are brightly colored--predators associate the bright color with poison, so they stay away. The phenomenon of orange/black or yellow/black = toxic is common in nature and is called aposematism.

What's special about this species is the number that accumulate indoors over a large geographic area. People in parts of the western U.S. have had to put up with the native convergent ladybug (Hippodamia convergens) entering their homes in the winter for well over 200 years, but now that this new species is bothering easterners, the press gets into the act, and people start to panic. The truth is that we have been introducing new exotic species of ladybugs into the U.S., both intentionally and accidentally, for decades now, and in many parts of the country (like most of the eastern seaboard) it's almost impossible to find a native species any more. If the Asian ladybug didn't happen to invade people's homes, this new introduction would have been ignored just like every other case like it.

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