Agricultural pests: White death

THOUGH the example comes from China, the sort of problem that African agricultural researchers have to grapple with (see article) is well illustrated by a paper in this week's Science about the whitefly Bemisia tabaci—an insect that is not actually a fly, but is related to aphids and leafhoppers.

Whiteflies are pests in every continent that they are found in—and they are found in every continent except Antarctica. They cause damage directly, by consuming plant juices, and indirectly, by spreading viral diseases. But Liu Shusheng, of Zhejiang University, in Hangzhou, and his colleagues have found a strain of the species that delivers a double whammy. Not only does it spread diseases, but it is also vastly more successful when it lives on plants infected with the diseases in question than when it subsists on healthy plants.

Though classified as a single species, Bemisia is actually a complex of “biotypes”, and biotype B, which arrived in China in the late 1990s, is rapidly taking the place over from the wimpy natives. Among the pathogens spread by whiteflies in China are tobacco curly shoot virus and tomato yellow leaf curl virus. These predate the arrival of biotype B, but the new arrival does particularly well on plants infected by them.

Dr Liu found that type B insects lived six times longer on infected plants than uninfected ones, and their population per infected plant might rise as high as 13 times that on an uninfected one. That mutualism does not seem to apply to the aboriginal whiteflies of China.

The virus, then, helps the insects multiply and in doing so helps itself to spread. Why that should be true only for this particular biotype is unknown. Finding the answer would make a lot of farmers happier.