Like tiny Russian dolls, the mealybugs that infest your houseplants carry bacteria inside their cells that are themselves infected with another type of bacteria. A new study by researchers at the University of California, Davis, shows that instead of spreading from bug to bug, the second set of bacteria infected the first several times in the past and are now being passed along and evolving with them.

The knowledge could be useful for working out how the insect species are related to each other, aiding pest control efforts.

Scientists had previously known that mealybugs carry two different types of bacteria. In 2001, researchers at Utah State University led by Carol von Dohlen showed that one type, called the secondary or S-endosymbiont, was actually inside the other, called the primary or P-endosymbiont. The P-endosymbionts were themselves inside specialized cells in the mealybug's body.

The P-endosymbionts seem to benefit mealybugs by making essential amino acids not found in their diet of plant sap, said UC Davis microbiology professor Paul Baumann.

"The insect has domesticated a bacterium for its own use," Baumann said. So far, no one knows what benefits flow to the insect or the P-endosymbiont from the S-endosymbionts, he said.

Baumann, with postgraduate researcher My Lo Thao and entomology professor Penny Gullan studied DNA sequences of P-and S-endosymbionts from several different species of mealybugs to see how they were related to each other.

The P-endosymbionts are all descended from an infection of an ancestor bug 150 to 250 million years ago, Baumann said. The S-endosymbionts had infected P-endosymbionts at least four times. Since then, the bacteria have been passed down through generations of bugs and split into new species at the same time as their hosts.

Because the evolutionary trees of the bugs, their bacteria and their bacteria's bacteria are so similar, the bacterial DNA sequences can be used to identify the insects and work out how the different species of mealybug are related to each other. Bacterial DNA is easier to work with than insect DNA, Baumann said.

Mealybugs belong to the same group of insects as aphids and psyllids. Many members of the group are significant pests on farms, gardens and houseplants.

The research is published in the July issue of Applied and Environmental Microbiology.