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Dragonfly Migration Tracked With Tiny Radio Tags

John Roach
for National Geographic News
October 12, 2005
 
Dragonflies fitted with tiny radio transmitters may aid scientists'
efforts to track where the insects buzz off to on their southward
migrations. The results should shed light on this little-studied
behavior, according to the project leaders.

"We don't know where they go or, to be honest, why they do it," said Michael May, a dragonfly expert at Rutgers University in New Brunswick, New Jersey.

Only about a dozen of the approximately 400 known dragonfly species are believed to migrate, journeying from the northern U.S. and southern Canada to the southern U.S., the Caribbean, and Mexico each fall.

Scientists believe these migrants' offspring then return north. "My thought is they emerge fairly early in the spring down there and probably pretty quickly build up an energy store to migrate north," May said.

But until recently, individual dragonflies have not been followed. And since so many dragonflies stay put year-round, scientists say it's impossible to know for sure where the migrants go. Sometimes even individuals of a migratory species stay put.

To get the details, a field team has been fitting green darner dragonflies (Anax junius) with radio transmitters. The National Geographic Society funds the work, which is being led by Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University in New Jersey.

Tags Attached

Wikelski and colleagues attached the first tags in September. The field team is currently tracking the insects as they make their flight south along the Atlantic seaboard.

The transmitters—which weigh 0.01 ounce (0.3 gram) and are about 0.4 inch (1 centimeter) long—are glued to the insects' undersides. A single wire antenna runs the length of the abdomen, and a tiny battery powers the device for up to a week.

Wikelski has a receiver that picks up the transmissions within a range of about one mile (one and a half kilometers) on the ground and five miles (eight kilometers) in the air.

As a pilot, Wikelski is able to put the receiver in his plane and follow the insects.

"It's important to do it from the plane, because they move so much," he said. "From the ground it's almost impossible to follow them."

Last year the researchers conducted field tests on cicadas, which are easier to follow and recapture than dragonflies, because they fly slower and don't travel as far.

Based on these tests, Wikelski said, the tags have little impact on insect flight, except perhaps making them slightly heavier. The extra weight allows the bugs to move faster when flying with a tailwind.

Migration Speculation

The researchers plan to publish their preliminary results in a peer-reviewed scientific journal later this year. In the meantime, they offer up a few speculations on how dragonflies migrate.

May said the southbound dragonflies appear to take advantage of tailwinds generated by cold fronts blowing from the north. The northbound migrants likely ride the warm fronts blowing from the south.

Greater uncertainties exist on how dragonflies navigate. May noted they have huge eyes, which they use to detect their insect prey. Perhaps they also steer by visual landmarks, he said. Or, like some birds, they could use an internal compass.

And while the research certainly advances understanding of dragonfly migration, Wikelski said it also demonstrates the feasibility of a global tracking system for insects. Satellites, if funded, could easily detect the transmissions sent by these tags.

"We can now track small insects, like locusts in Africa, before they get to the stage where they are a problem," he said.

The work could also be applied to tracking birds with radio tags, which might have widespread implications for human health.

"There's a call out for a million people dead from avian influenza, but we have no clue where these birds are going," he added. "It's absolutely mind-boggling."

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