Plants can't flee voracious insects. But a new study says plants under attack can call in their own version of an airstrike: Trees waft special odors into the air, which attracts birds eager to perform pest control.
The scientists behind the new research say perhaps crops could be specially bred to attract more birds, replacing pesticides. The results also show that contrary to conventional wisdom, birds rely on smell, not just sight.
"When we started on all this, I was a little bit skeptical … because the traditional view is that birds don't use their sense of smell very much," says ecologist Marcel Visser of the Netherlands Institute of Ecology, who took part in the research, published online last week in the scientific journal Ecology Letters, but "in the end, they can do it."
Scientists have long known that when insects nibble on leaves, the damaged leaf tissue releases a flood of telltale odors. Some give a freshly mowed lawn its characteristic grassy smell, while others are used in the perfume industry, says study co-author Marcel Dicke of Wageningen University in the Netherlands. The chemicals also draw carnivorous insects such as wasps to the besieged plants' rescue.
Plants being eaten alive would do better to draw birds, which can snap up many more pests than can pest-eating insects. So the researchers decided to find out whether birds, too, can sniff out a pest-ridden plant.
The scientists positioned apple trees in an aviary, then released chickadee-like birds called great tits inside. The birds flew more often to trees infested with caterpillars than to uninfested trees. The researchers also found that the preyed-upon apple trees released different levels of some airborne chemicals than unscathed trees. But were the birds getting the message?
To find out, the scientists picked the caterpillars and damaged leaves off a caterpillar-infested tree and put it in the aviary. They made sure the birds could smell it but not see it, by placing the tree behind a fabric door. Next to it they placed an uninfested tree the birds could see but not smell, thanks to a clear plastic door. The birds visited the infested tree more often, apparently drawn by the odors it wafted into the air.
Scientists who weren't connected to the study say the new findings help make the case that birds can sniff out pest-ridden trees. But these scientists say this single study is not totally definitive, because it's hard to know for sure whether the birds are truly following their noses to the caterpillar-infested trees.
"There are ways to do the experiments a little more conclusively," says Ian Baldwin of the Max Planck Institute for Chemical Ecology, who studies insects' response to plant chemicals. But the study "provides a very nice step forward."