Under threat from a helpful bacterium? (Image: STR/AFP/Getty)

Trouble could be brewing in the sweet-potato fields of the south-western US. In just six years a bacterium has infected nearly the entire population of a pest that devastated crops in the 1980s and 90s – and the microbe messes with the pest’s sex life to help them both spread further.

Infected whiteflies – not true flies but aphid-like pests – produce nearly twice as many offspring as uninfected ones. That’s a big problem, because the pest costs millions of dollars in crop damage worldwide.

Martha Hunter of the University of Arizona in Tucson showed that the proportion of whiteflies infected with the bacterium Rickettsia bellii in Arizona, New Mexico and California soared from 1 per cent in 2000 to 97 per cent in 2006.


To explain the remarkably swift proliferation, Hunter’s colleague Anna Himler replicated the epidemic in the lab. She let a whitefly population, 14 per cent of which were infected with Rickettsia, suck the sugary sap from cowpea, melon and cotton plants in the lab. In just five generations the infection rate had soared to between 40 and 80 per cent, making it a good model for what had been observed out of doors.

Vertical rocket

Parasites typically spread in one of two ways: horizontally – in other words, within a generation – or vertically, from parent to offspring. In Himler’s set-up, whiteflies that were in close contact but not mating did not infect one another, ruling out horizontal transmission. But when she allowed a male to mate with an infected female, she found that almost all the offspring were infected with Rickettsia, suggesting the bacterium is passed from mother to offspring.

What’s more, infected females laid far more eggs than their uninfected peers, and more of their offspring survived to adulthood. Rickettsia is clearly a boon for the whitefly parents.

Then Himler noticed something else in her data: infected female whiteflies produced far more daughters than sons. That made sense from the bacterium’s point of view, since it depends on female whiteflies to increase its own population.

Tug of war

“On the one hand the bacterium benefits its host,” says Himler. “But there is also an evolutionary tug of war happening within the whitefly.

“Suddenly the whitefly has a whole new genome inside it and those genes are trying to promote what the bacteria wants,” she explains. “It’s mutual up to a point. If the sex ratio becomes too skewed, it could become detrimental to the whitefly.”

Sweet-potato whitefly was a huge problem for farmers in the south-western US two decades ago, when fields would be besieged by huge clouds of the pests. Today, it is under much stronger control, and if Rickettsia and whitefly are preparing a comeback, there are no signs of it yet.

“So far we haven’t gotten any reports from local farmers of clouds of whiteflies,” Himler says. “If Rickettsia is increasing the whitefly’s fitness in the wild, it’s possible that predators or environmental factors are dampening the effect.”

Journal reference: Science, DOI: 10.1126/science.1199410