The dodder, strangleweed or witch's shoelaces (Cuscuta) is a parasitic plant that attacks tomatoes, carrots, onions, citrus trees, cranberries, alfalfa and even flowers. It is usually yellow or orange and can blanket and kill its host. Dodders sprout clusters of small white or pink flowers that produce their seeds.

It is an agricultural pest and poses a problem for farmers because chemicals that kill the weed also kill the crop host plant. Dodder ranks among the U.S. Department of Agriculture list of top ten weeds. The pest costs tomato growers in California alone about four million dollars (U.S.) a year, experts say. How dodder finds a host plant has puzzled scientists for long now.

They thought it simply grew in a random direction, with discovery of a plant to attack being a chance encounter.

Recently, scientists from Pennsylvania Statue University have discovered the dodder can sense chemicals released by host plants and then head for that direction.

It can even identify favorite plant hosts over less desirable plant hosts.

This study showed for the first time that plants can interact with one another, solving a decades-long debate about whether chemicals are involved in plant-to-plant interactions.

"This is a pretty cool example of plants behaving in a way that people think only animals behave," ecologist Richard Karban of the University of California, Davis, said. Dodder has no roots and lack chlorophyll, so they cannot produce food through photosynthesis.

When the shoot finds a victim, it begins growing in a coil around the host plant, injecting needles into stems and leaves to suck out water and nutrients. As dodder seedlings have only a limited amount of food in their seeds, they must quickly find a host plant or else they die. "If they take the wrong direction they are pretty much doomed," said Consuelo De Moraes co-researcher.

Scientists had thought the parasitic plant used a hit-or-miss tactic, and any hook-up was the result of a chance encounter.

Scientists found in a lab experiment that 80 % of the dodders grew toward a tomato plant.

When they tried to trick the dodder seedlings with artificial tomato plants, they didn't take the bait: 77 % grew towards the real plants. The shoots didn't grow in any particular direction.

The team replaced the real and fake tomato plants with aromatic chemicals, one containing the scent of real tomato plants, the other an unrelated solvent. The dodder seedlings overwhelmingly headed for the real-tomato smells.

Moreover, the dodders could distinguish between a favorite host plant, such as the tomato, and a poor one such as wheat. When placed in the middle of wheat and tomato plants, the dodders swayed aboveground in circular movements in the direction of the tomato plants. "I think the more interesting thing is that they could make choices based on the volatiles. They could use that information to distinguish between a good host and a poor host," De Moraes said.

Scientists suspect the dodder is equipped with sense receptors that can smell their hosts. They hope to figure out how this mechanism works. "So they just have the phloem flowing right out of the host into themselves, and bringing with it all the stuff that's in there, all the resources that are floating around," said co-researcher Mark Mescher.

"Dodder will infect wheat if there is no choice, he said, but they discovered that one of the volatile chemicals given off by wheat repels dodder, so it will choose the tomato if allowed to pick."

Curtis Swift, an agriculture agent at Colorado State University, said, "Previous to this research the movement of dodder toward a suitable host was thought to be independent of host plant influences. The identification of volatiles responsible for the directed growth of the parasite toward a host plant is very interesting."

"Even more interesting and possibly more important is the identification of a volatile that serves as a repellent," he added, urging further study to see if that chemical can be used as a spray to protect crops.

"So, finding one compound that tends to be repellant could lead to ways to either treat crops to resist dodder or even engineer them to produce the compound themselves," Mescher said.

"The plants don't have a nose, of course, so it's not clear how they sense the chemicals given off by potential host plants. When the seedlings start out, they tend to rotate in various directions, and they somehow sense the direction where the chemicals are strongest and then grow toward them," Mescher said.

Plants use a variety of chemicals as lures or repellents to encourage or discourage a variety of animal behaviors. "We showed a few years ago that plants produce different chemicals during the day and at night," study co-author Mescher said.

"The nighttime [chemicals] can actually be repellents to the moths that lay the eggs that become caterpillars," which in turn munch plant leaves.

Plant daytime chemicals lure beneficial insects. The bugs help rid the plant of unwanted pests, such as caterpillars. "There are a lot of complex interactions between insects and plants that are known to be mediated by these sorts of volatile, airborne chemical cues," Mescher said.

It is possible that plants can sense chemicals emitted by other plants that are attacked by insects, as some recent researches indicated. "The latest study, he adds, is the first to show that these aromatic signals also can govern interactions between plants and other plants."