Aphids, abandon ship

Mammal breath drives insects to jump off plants

2:01pm, August 10, 2010
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Any mammal breath, be it goats’ or gazelles’ or grad students’, is bad breath to an aphid.

Exhalations send masses of pea aphids overboard, plummeting off the plant where they have been feeding, says Moshe Inbar of the University of Haifa in Israel. It’s combined warmth and humidity that gives breath its aphid-scaring power, Inbar and his colleagues report in the Aug. 10 Current Biology.

Abandoning food supplies and plunging to the ground counts as a desperate move for a tiny, wingless insect, Inbar says. But ground-level risks could be better than getting inadvertently chewed up as part of a grazer’s salad.

Graduate student Moshe Gish, a coauthor of the new paper, set the project in motion by reporting that he had found a way to gather plenty of pea aphids in a hurry for the lab’s various projects. Inbar remembers Gish explaining, “I simply breathe on them.”

Inbar says biologists have occasionally documented some aphids dropping off a plant when a predatory insect such as a ladybug swoops by. Gish reported a more dramatic rain of insects. When he let a goat feast on alfalfa, almost two-thirds of the resident aphids dropped off the plants and survived.

“It’s more than jumping,” Inbar says. For aphids to leave the plant, “they have to unplug.” Aphids feed by driving a needlelike mouthpart into the plant tissue until it reaches vessels carrying sap.

To figure out what prompted all the unplugging, the researchers tried scaring aphids in various ways. But a sudden shadow cast on plants didn’t bother the aphids, and a leaf-picking device that grasped and shook broad bean plants inspired only about a quarter of resident aphids to let go.

Chemical changes didn’t do much either. Gish, wearing a snorkel to divert his own breath from the experiments, found that aphids didn’t panic at air flow enriched with carbon dioxide, found in animal breath. Nor did they dive off the leaves when they caught whiffs of several volatile organic compounds that mammals exude, such as octenol. Gish even tested bovine nasal secretions, “which was quite disgusting, I must admit,” Inbar remembers.

In contrast, aphids reacted strongly when researchers made the air flow more like real mammal breath. Substantial numbers of aphids jumped when exposed to air with at least 90 percent humidity and temperatures around 35 degrees Celsius.

Mammal breath can also disturb other insects, says biologist Scott Camazine, now a physician in central Pennsylvania. As a graduate student at Cornell University, he and his colleagues found that bursts of warm, humid air prompted Bolitotherus forked fungus beetles to release acrid compounds that mice didn’t much like.

The new paper inspired Camazine to test reactions from backyard creatures using a turkey baster. Puffs of plain air from the baster did not evoke much reaction from a crab spider, for example, but at a light sigh from Camazine, the spider plummeted off its perch.

“Just go in your backyard and try it,” Camazine says.

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