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Radar Used to Track Butterflies' Loop-de-Loops

By Rick Weiss
Washington Post Staff Writer
Monday, April 18, 2005; Page A07

As every child knows, it's tough to keep up with a butterfly.

With their erratic flight paths and ability to achieve speeds in excess of 10 miles per hour, these otherwise enchanting insects have for decades frustrated entomologists seeking to understand how they navigate, pollinate and make dates with potential mates in the wide open spaces they inhabit.

Now researchers in England have proved the potential of a new technology for butterfly watching -- one that may free scientists from having to prance after these insects in goofy fashion. All that's needed is a mobile harmonic radar device and someone dexterous enough to attach tiny antennas to the insects' backs.

The customized radar tracking approach, in which insects are fitted with lightweight, copper-and-steel electronic "transponders," has yielded the first real-time mapping of the flight paths of individual butterflies in an open, agricultural setting.

The work offers tantalizing clues that butterflies' seemingly carefree loops are in fact systematic searching behaviors they use to reorient themselves as they look for sources of nectar. The technique may also clarify the impact that land development is having on butterfly migrations and could tell conservationists whether butterflies would make use of corridors of open space as they struggle to survive in an increasingly fragmented habitat.

Insects have been showing up on radar screens since the 1940s, when British pilots tinkering with the still-new equipment reported "ghost" aircraft -- sometimes called "angels" -- in open skies. The angels turned out to be radar reflections of insects swept skyward by thermal updrafts.

But the challenge of tracking individual bugs has been much more difficult, especially near the ground, where most insects fly. Standard ground radar quickly loses track of such small targets in the clutter of reflected signals from trees, shrubs and structures.

One approach -- attaching small transmitters to the backs of insects -- has come close to fruition. But while transmitters are now small and light enough for an insect to carry, even the tiniest batteries to power them are still too heavy for a six-legged takeoff.

So, in recent years, scientists have been experimenting with an alternative approach, called harmonic radar. Like conventional radar (which stands for "RAdio Detection And Ranging"), the system sends radio signals of a particular wavelength from a rotating dish. But as it listens for echoes, it pays attention only to signals of exactly half the original wavelength.

That's where the transponders come in -- antenna-like wires that require no power source of their own but which specifically convert incoming radar signals of a given wavelength into outbound signals of half that wavelength.

The system has been used in the past decade in a handful of studies to track tsetse flies (which carry the parasites that cause sleeping sickness) in Africa; moths as they fly upwind following the scent of potential mates; and honeybees and bumblebees.

The latest study, described in the April 6 online version of Proceedings of the Royal Society B: Biological Sciences, is the first involving butterflies. It focuses on the small tortoiseshell, Algais urticae, and the peacock butterfly, Inachis io, both common in Britain.

The first challenge was attaching the transponders, which are more than a half-inch long and weigh 12 milligrams each, or about one-thirtieth the weight of a small, No. 1 paper clip.

Elizabeth Cant, a doctoral student at Rothamsted Research in Hertfordshire, and her colleagues started by performing wax jobs on the butterflies' backs, to remove the hair. They added a few drops of colored paint for identification, then used small disks of double-sided tape to attach the antennas.

The team conducted preliminary tests in large screened cages to see whether the devices altered the insects' natural behavior. Time spent walking, flying and foraging for nectar was not different from that of untagged butterflies. And in one instance, in which the team saw a pair of their subjects mating, it involved a male with an antenna.

In an interview, Cant declined to speculate as to whether antenna-bedecked males might be especially attractive to females. But the device certainly did not seem to have deleterious effects, she said.

Then came the field studies. In a 50-acre plot with open fields, tree-lined windbreaks and flowers, the team used the radar system to follow the precise flight paths of two dozen of the fluttering insects.

The results indicated that butterflies do not seem to have a preference for following tree lines; they fly through those breaks when necessary and track along them only when nectar-bearing flowers happen to be growing in line with those trees. The work also confirmed previous, less well-documented suggestions that butterflies can see about 200 feet ahead: That is about when they begin to veer away from obstacles.

The team found that the loop-de-loops so characteristic of butterfly flight are especially common after taking off and when fluttering around patches of flowers. That suggests the moves are not playful or random but are part of what Cant calls a "systematic searching strategy" that helps them find -- or return to -- prime feeding spots. "It seems to help them navigate and orient," Cant said.

Elizabeth A. Capaldi, a Bucknell University animal behaviorist who once rode around on a bicycle with a transponder attached to her barrette to test a radar system she ultimately used to track honeybee flights, said radar tracking of insects can yield insights into where they go as they perform their ecologically crucial role of pollinating plants. And for those contending with insect pests, it can be helpful to know where they mate and lay their eggs.

Applications could go well beyond agriculture and ecology, added John Hildebrand, a neurobiologist at the University of Arizona. Hildebrand was part of a military-sponsored project involving night-flying moths that had been trained to "sniff out" a chemical in exploded munitions. In wind tunnel tests, the moths -- wearing electronic backpacks with prototype transmitters -- performed beautifully, he said.

When the insect smells its target, "it sticks out its proboscis, which is four inches long, and literally points," Hildebrand said, referring to the insect's curled up tongue.

He anticipates a day when tiny transducers will convert the energy from buzzing wings into electricity for bug-borne transmitters, obviating the need for tracking radar. "A motivated moth can be trained in 10 minutes" to home in a specific target, Hildebrand said.

With microphones and video cams ever shrinking, bugging devices may someday live up to their names.


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