We often think of bacteria as making animals sick, but a type of bacteria seems to turbo-charge the sweet potato whitefly, Bemisia tabaci, a serious insect pest for farmers worldwide.

The bacteria, a species of Rickettsia, causes infected whiteflies to lay more eggs, grow up faster and survive into adulthood in higher numbers than uninfected bugs. It also causes more of the insects to be born female.

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"It's instant evolution," said Molly Hunter, a professor of entomology at the University of Arizona and the principal investigator in a study examining the strange super-symbionts.

"Our lab studies suggest that these bacteria can transform an insect population over a very short time," said Hunter in a University of Arizona press release.

"Rickettsia-infected whiteflies lay more eggs, more of those eggs survive, and there is the reproductive manipulation toward producing more female than male offspring. These effects are not unheard of, but the strength that we found here is unusual," said Hunter.

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The bacteria has been observed in Arizona for six years. In 2000, only 1 percent of whiteflies were infected. That number shot up to 50 percent in 2003. Now, nearly all Arizona whiteflies carry the bacteria.

The bacteria seems to be a special gift from mother whitefly to her offspring. Hunter found that the bacteria didn't move between adult flies, but that infected mothers gave birth to infected baby bugs.

Since the bacteria is only passed on by the mother, it makes sense that the bacteria would evolve to increase the ratio of females being born. But that might not be in the insect's long-term best interest.

"In general, taking the sex ratio control away from the host is not a good thing for the host," she said. There is a reason why most living organisms have roughly equal proportions of sexes. If there were more females, then any individual producing more males would produce more progeny. This is one of the reasons a one-to-one sex ratio is really common in nature."

You may have seen whiteflies in your garden. The tiny white critters like to hang out on the undersides of leaves. When you shake the plant, they fly up, but eventually settle back to their shady spots under the leaves. There, they make a living by sucking the juices out of plants, like little insect vampires.

The bacteria are making a bad problem even worse. The type of sweet potato whitefly they infect, called B biotype, was already a major problem for farmers, on every continent where crops are grown.

Don't let the name fool you — the B biotype whitefly doesn't just dine on sweet potatoes. And that's just the problem. It eats more than 600 types of plant, so it never goes hungry.

"Here in Arizona, it probably starts out on weeds in the spring, and then moves on to melons, and when melons are done, it moves in big numbers onto cotton and feeds on that all summer long," Hunter said. "In the fall, it moves on to vegetables, and so it just keeps going."

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"In the late 1980s and early 1990s, when this new biotype arrived in the Southwest, the population just exploded," Hunter said. "Sometimes you could see clouds of whiteflies in the air, gumming up windshields."

"With integrated pest management practices, many developed by colleagues here at the UA, their impact has decreased tremendously, but they still are the worst pest in Arizona's cotton industry. If it wasn't for whiteflies, farmers would be spraying cotton a lot less," Hunter said.

Now the tiny insects, which measure less than 1/16 of an inch, have been super-powered by the Rickettsia bacteria and pose an even bigger threat. But all hope is not lost; Hunter wonders if there might be a way to turn the tables.

"It would be interesting to see if by having a microbe that has this big effect in one direction, if you could make it so that it has an effect in the other direction, to help control the pest," Hunter said. "Could we use symbionts in a way to make things less of a problem, to manage pest populations in a more sustainable way?"

Knocking down whiteflies would be a huge help for farmers. The insects, not true flies, but actually members of the order Hemiptera, are more closely related to true bugs and aphids, another pest. And like their aphid cousins, they leave behind a sticky residue that can become home for fungi, which further damage plants' leaves. Plus, they transmit more than 100 viruses, such as the mosaic virus.


IMAGE 1: The sweet potato whitefly, (Bemisia tabaci) (Wikimedia Commons).

IMAGE 2: The sweet potato whitefly, (Bemisia tabaci) (Wikimedia Commons).

IMAGE 3: Mosaic virus on cabbage (Wikimedia Commons).