But "everything out there is going to be received back at the same frequency you send it out at ... what you want is a unique signal," Williams said.
The tags attached to the beetles do this by reflecting back the signal at a harmonic of the frequency, or twice the frequency.
For example, if the transceiver sends out a signal at a frequency of 917 megahertz, the tags will send back a signal at 1834 megahertz.
"That harmonic tag represents a unique signal out there from everything else you're looking at," Williams said. "Attaching that tag to an insect allows you to relocate the insect as it flies around."
In 2002 Williams and his colleagues went to the Asian longhorn beetle's native habitat of China and used harmonic radar tracking to study the invasive bugs' dispersal over a two-week period.
Williams attached the tags with dental floss, because the bodies of the 1.25-inch-long (3.2-centimeter-long) shiny black beetles are too waxy for glue to stick.
The team found that the beetles moved, on average, about 10 feet (3 meters) per day.
Interestingly, most of the females were rather sedentary during the study, Williams says.
He suspects that was because the study took place late in their breeding cycle. One female, however, moved 100 feet (30 meters) in a week.
While the findings have not modified the radii of circles entomologists draw around these invasive pests, Williams said the tracking study "gave us a basic idea of how quickly these beetles can move on their own."
Doug Landis is an entomologist at Michigan State University in East Lansing who has examined the practicality of using harmonic radar to track insects.
He said that for fairly large insects like the Asian longhorned beetle, the technique allows scientists "to get a lot of information that is simply inaccessible any other way."
However, the tags that send the harmonic signal are most effective when they have about three-inch-long (eight-centimeter-long) antennas.
"So in tracking very small insects, it has its limits," Landis said.
Nevertheless, several studies, including Williams' work with the Asian longhorned beetles, have shown harmonic radar to be an effective tracking tool to learn about the basic biology of invasive species, he adds.
"That's the basis of finding what their Achilles' heel might be."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES