Alien Beetles Tracked with "Ray Guns," Dental Floss

John Roach
for National Geographic News
October 16, 2006
Scientists are combining space-age ray guns with dental floss to get a read on how wood-boring beetles such as the Asian longhorned beetle invade new countries.

"These pests have become a problem in the last 20 years or so because of all the foreign trade," said David Williams, an entomologist with the U.S. Department of Agriculture's Animal and Plant Health Inspection Service in Cape Cod, Massachusetts.

The insects hitch rides across oceans in wooden crates and other solid wood materials used in shipping, he explains. Once the containers are unpacked, the insects fly off, mate, and spread.

The Asian longhorned beetle that invaded New York in 1996 has since killed thousands of the state's hardwood trees, including maples, elms, willows, and poplars.

Williams' primary responsibility is to study how to eradicate or at least control invasive insects such as the Asian beetle. (Related photos: "Attack of the Alien Invaders" in National Geographic magazine.)

Typically when an invader arrives, researchers identify its location and search a wider circle around it for other bugs, he says.

"In the case of wood-boring beetles, we'll cut down all the host trees that seem to be infested in an attempt to eradicate it," he said.

"What we need to know is how big the radius of the circle needs to be—how far these bugs can fly."

Harmonic Radar

Harmonic radar is one trick entomologists are using to understand the range of large insects such as the Asian longhorn.

The technique involves a transceiver, which Williams says looks like a ray gun the fictional space hero Buck Rogers might use, along with tiny wire tags attached to the beetles. (Related:
"Dragonfly Migration Tracked With Tiny Radio Tags"
[October 12, 2005].)

The transceiver sends out a radar beam into the environment that reflects off whatever it hits. Sensors then measure those reflections.

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."

Basic Biology

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."

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