Bees can be good for plants in more ways than one, scientists have found.

Researchers in Germany discovered that the flapping of bees' wings scared off caterpillars, reducing leaf damage. Many wasp species lay their eggs in caterpillars, and so caterpillars have evolved to avoid them. The sounds of bees' and wasps' wings are similar. Writing in the journal Current Biology, the scientists suggest this is an added bonus of having bees around, as well as the pollination they provide.

"Our findings indicate for the first time that visiting honeybees provide plants with a totally unexpected advantage," they write. "They not only transport pollen from flower to flower, but in addition also reduce plant destruction by herbivores."

Hairy moments

The ingredients for this experiment were bell pepper and soybean plants, beet armyworm caterpillars, and honeybees. Researchers set up experimental plots of the plants, added the caterpillars, and allowed the bees to enter some of the plots but not others. When the caterpillars had turned into pupae and buried away in the soil, the scientists went back into the cages and measured the extent of leaf damage - the amount of munching that the caterpillars had indulged in. In plants that had not fruited, the presence of bees reduced caterpillar damage by about 60%.

The researchers believe the caterpillars were sensing the bees' presence through the tiny hairs on their bodies, which enable them to detect vibrations in the air. "These sensory hairs are not fine-tuned," said lead researcher Jurgen Tautz from the Biozentrum at Wurzburg University. "Therefore, caterpillars cannot distinguish between hunting wasps and harmless bees." When plants had borne fruit, the caterpillars were able to hide in the fruit and the bees had much less effect.

In many countries, numbers of bees appear to be declining, for reasons that are not entirely clear. According to a recent Australian survey, this is not affecting crop yields on a global basis, but it is affecting pollination in some regions and is clearly of concern to entomologists trying to understand the causes. If this German research is confirmed, it will provide another reason for trying to keep bees alive and well.