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Buzzing bees protect plant leaves

Pest caterpillars don't eat as much if distracting pollinators zoom by

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2:43pm, December 22, 2008
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Traffic noise, honeybee traffic that is, can be a bane for caterpillars and a boon for plants.

The buzz buzz buzz of honeybees zipping overhead as they forage can keep caterpillars below from getting enough peace and quiet for a full meal, says Jürgen Tautz of the University of Würzburg in Germany.

In tests, bee flight-path distractions took such a toll on dining that caterpillars ate only about a third of the leaf area they consume in a bee-free zone, Tautz and Würzburg colleague Michael Rostás report online December 22 in Current Biology.

Bee traffic-noise as a pest deterrent is “a very cool and novel idea,” says Jeff Conner of Michigan State University’s Kellogg Biological Station in Hickory Corners. Earlier work, including his, showed that pests eating plants makes the plants less attractive to pollinators, but “this new study turns that idea on its head,” he says. The pollinators are making plants less attractive to pests.

So far Tautz has just tested the idea in a strictly controlled setup. He and his colleagues put up a pair of tents housing arrays of plants. In the various runs of the test, researchers used bell pepper plants, once with and once without fruits, as well as soybean plants. A beehive opened into one tent and some 50 bees at a time buzzed over the plants on the way to collecting sugar water from feeders in the corners.

As a sample pest, Tautz chose the beet armyworm (Spodoptera exigua), a caterpillar that feeds on some 50 plant species. Armyworms are relentless munching machines, but they stop moving, and sometimes drop off their perches, if a wasp flies by. Plenty of wasps eat caterpillars, and Tautz says that he’s found sensory hairs on caterpillars, including beet armyworms, that detect the wasps’ wing beats.  

Honeybees don't hunt caterpillars, but Tautz says he was out walking his dog one day, listening to the buzz of bees, when it occurred to him that, to a caterpillar, bees’ wing beats might sound similar to wasps’.

That seemed to be the case in his experiments. In tents with bees flying over plants without fruit, caterpillars did less damage to leaves than in quiet tents, the researchers report.

In the tent without bee traffic and with peppers already forming on plants, the quiet didn’t make as much of a difference in leaf damage. But that’s because caterpillars took advantage of the bee-free peace to move off the leaves and start eating the peppers themselves, Tautz says.

Conner says that protection from traffic noise doesn’t necessarily indicate any plant evolution that promotes this phenomenon. It could just be a happy side effect of pollination.

Whatever the history, it makes a good example of hard-to-spot indirect relationships between species in ecosystems, Tautz says.

Also, he says he can imagine that gardeners might someday take advantage of this effect. “Alternating rows of vegetables and flowers not only look beautiful, they may reduce the use of pesticides,” he says.

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