Building a Better Moth Trap.
A little-known theory that insects use their antennae to perceive electromagnetic signals from sexual scents provides the conceptual framework for a new tool to combat the number one worldwide pest of stored food products.
"Adult fleas are tiny and flea larvae can be even tinier, so it takes a few hundred of them inside a petri dish to make a clump the size of a quarter," says Thomas Dykstra, describing the research subjects that threw him off the track of mainstream science. Dykstra's career-changing experience with fleas occurred in 1994 at the University of Florida at Gainesville.
Dykstra noticed odd behavior inside a closed petri dish containing a clump of flea larvae. Minutes after he gently shook the dish, scattering the larvae, they formed a new clump against the side of the dish closest to him. When he repeated the process and moved to a different part of the lab, the newly formed clump was oriented toward his new position.
This simple behavior, never before reported in the scientific literature and not readily explainable by mainstream science, led Dykstra to a topic for his Ph.D., which he received in 1997, and to his present employment. Flea larvae, according to the conventional view, use chemical signals to orient themselves and so should have been responding to odor molecules from Dykstra. Considering the closed petri dishes, he chose to evaluate the possibility that the larvae were responding to an electromagnetic signal transmitted through the plastic dish.
By the time he completed his research, Dykstra was convinced that flea larvae are sensitive to electromagnetic radiation, probably infrared, and he had become thoroughly schooled in unorthodox views about electromagnetic perception by insects. These views had been developed primarily by Philip S. Callahan, a local entomologist and biophysicist, who returned from retirement to chair Dykstra's Ph.D. committee and helped secure financial support for his unorthodox studies.
To some scientists, the seemingly neurotic and self-destructive attraction of moths and other insects to electric lights, candle flames, and gas flares in oil fields is a window into their ability to detect electromagnetic radiation. Insect perception of electromagnetic signals through the antennae would rank as a previously unidentified form of perception, if it is proved true.
Such perception, when directed toward emanations from sex pheromones (organic molecules of sexual attraction, produced, in this case, by female moths), would be critical to species formation and mating behavior. In this model, moths (particularly males) rely on their antennae (covered with microscopic spicules called sensillae) for detecting infrared, far-infrared, and microwave emissions from energetically excited molecules.
The mainstream view holds that moths see bright lights (ultraviolet and visible) with their eyes and smell sex pheromones and other scent molecules via chemical interactions with olfactory organs on the antennae. Therefore, their antennae must be molecular nets, pulling molecules from the air and transporting them inside, where they are identified chemically. Indeed, the sensillae covering the antennae are riddled with holes, which allow pheromone molecules inside.
Neither camp disputes the clear morphology and physiology of insect antennae. Nor do they dispute the fact that insects display a wide variety of antennae shapes. Among the moths alone, antennae can be grouped into several variations. Of particular interest are insect-scale analogues of the classic rooftop TV aerial. These "log-periodic" sensillae arrays are carried by small, night-flying moths, including crop-ravaging corn earworm and cabbage looper moths and pantry-infesting Indian meal moths.
For electrical engineers, physicists, and military communications experts grounded in antenna theory, there is nothing random about the shape of an antenna. It's all basic physics, whether it be a log- periodic array--with a single shaft bearing crossbars of progressively greater length and separation between them--or a horn configuration that receives microwaves. Based on shape and size, it is possible to predict the electromagnetic wavelengths being transmitted or received by any antenna.
Indeed, as Dykstra studied Callahan's published papers on the electromagnetic-perception hypothesis, the pieces fit together ever more tightly--the antennae gross morphologies as well as the placement and size of the microscopic sensillae. Furthermore, the properties of the cuticular (skin-like) material of which the antennae were formed made it a suitable insulating material for making dielectric antennae. The log- periodic arrays, according to Callahan, could even provide information about distance to the calling female.
The function of form
In most contexts, biologists agree with the notion that form follows function. In the world of ornithology, for example, birdwatchers know that the shape of a finch, curlew, or hummingbird beak is not random but is rather a clue to feeding habits. By such logic, one would expect the wide variety of insect antennae morphologies and their sensillae to serve some useful, nondecorative purpose. In the entomological world, however, the idea that insect antennae might be shaped for tuning into varying frequencies of the electromagnetic spectrum is generally ignored or dismissed.
Almost every entomologist can recite the classic textbook explanation that sex pheromone perception is a matter of pheromone molecules chemically binding with the insect's antennae receptors to trigger a flow of neural information toward the brain. The idea that moth antennae perceive far-infrared and microwave emissions from energetically stimulated scent molecules is viewed as "really off the wall" stuff from the 1960s and '70s. Most believe it was "debunked" long before the 1990s retirement of its chief proponent, Callahan, who held joint positions at the University of Florida and the USDA.
Regarding his decision to work with Callahan, Dykstra recalls, "I knew I would in effect be shooting myself in the foot." Nonetheless, he decided that he was willing to give up his dreams of being a professor. Instead, he would plunge deeper into Callahan's theories, explaining sex pheromone and scent perception in terms of masers, lasers, electromagnetic wavelengths, and antenna theory.
Dykstra was awed by the stunning variety of insect antennae and the more than 400 distinct and widely repeated types of insect sensillae. If the function of the antennae and their sensillae was primarily to snare hydrocarbon scent molecules, he reasoned, they would be of a few different designs best adapted for that purpose.
By the time Dykstra finished writing and defending his thesis in 1997, it was clear that he would have no chance of securing an academic position unless he turned away from the study of electromagnetic perception by insects. Faced with the need to earn a living, Dykstra decided to set up shop outside academia. With Callahan's assistance, he found a venture capital angel and started a company to do experiments with laser technology and pheromones. Instead of fighting a losing battle against fixed academic views, Dykstra took up the challenge of developing commercial insect traps based on Callahan's pheromone- perception theories. With this approach, the proof of the theory, he hopes, will be established first through the vote of the marketplace.
The standard model
For entomologists studying pheromone perception, a highly simplified explanation starts with female moths beating their wings as they emit a genetically determined blend of pheromone molecules from an abdominal gland. A gentle breeze carries the perfume-like pheromone molecules downwind from the calling female. The antennae of a male moth that is physiologically ready to mate detects the scent of the female pheromone, triggering a cascade of internal neural events. They progress from the male antenna to the brain and "initiate" an upwind zigzag flight pattern in search of the female of the species.
In this stylized mating scheme, where the female is the caller and the male is the pursuer, the male moth antenna is the detection device recognizing the correct blend of female sex pheromones. (Species are differentiated by distinct blends of similar pheromones.)
The importance of male antennae in sex pheromone perception can be demonstrated in various ways. If male moth antennae (which include specialized structures leading up to the brain) are surgically implanted into females, they respond like males and fly upwind in search of other females. Also, by wiring male antennae to the appropriate equipment, researchers have confirmed that they respond to the appropriate sex pheromone blends for the species. Researchers have even confirmed that single olfactory cells respond to a single specific pheromone molecule. But there is one nagging loose end.
In the chemists' lock and key model of olfaction, the receptor (a molecule or molecular complex) is the lock, and a matching pheromone molecule is the key. When the key nestles into the lock, the receptor initiates a signal to the brain. The problem is that despite decades of research, no one has been able to identify an actual pheromone receptor in male moth antennae, or anywhere else for that matter.
Thus, pheromone receptors are theoretical constructs. Nonetheless, argue proponents of this mainstream view, an excellent case can be made for their existence. Knowing what we know about molecules and receptors from other systems, such as neurotransmission, it is hard to believe otherwise. Indeed, receptors for hormones and other types of molecules are commonplace in nature, and researchers and pharmaceutical companies routinely design molecules to bind with target receptor sites. Hence, the scientific consensus is that a pheromone receptor site will eventually be identified in a male moth antenna.
It could be a very long time before a male moth pheromone receptor is identified, because pheromone perception is unlike anything else in our world. "Pheromone perception is much more complex than any neurotransmitter and receptor system or other olfaction system that we know," says Cornell University's Wendell Roelofs, one of the world's leading pheromone researchers.
Most olfaction systems are more generalized in their recognition of the "smells" of scent or odor molecules. In contrast, insect pheromone perception is so fine-tuned that finding a mate of the right species can depend upon distinguishing between two molecules that are the mirror image of each other. Such molecules are called enantiomers. Indeed, the pheromone- perception system is so sensitive that even a subtle shift in the ratio of enantiomers in a pheromone blend can turn off the upwind sexual search of one set of males and cause males of another species to fly upwind in search of mates.
The European corn borer moth, a billion-dollar crop pest attacking over 200 plant species, illustrates the system's sophistication. Buried in the male moth's head, at the base of its antennae, is an information processing center that receives the neural signals of the antennae into seven distinct compartments, one for each component of the sex pheromone blend.
Recently, Roelofs' laboratory provided evidence that a genetic change causing a shift in the position of just one double bond in a pheromone molecule was sufficient to create a new species of corn borer moth. "An ultrasensitive pheromone-perception system is as it should be," says Roelofs, "as we are dealing with mating and the vitally important function of maintaining the integrity of the species."
Walter Leal of the University of California, Davis, echoes this view, suspecting that the sensitivity of the system has something to do with the difficulty of finding the pheromone receptor, which researchers assume is inside the sensillae. Both Roelofs and Leal argue that pheromone perception is so complex that it will not be fully understood until after a full moth genome has been sequenced.
In the meantime, Leal and his group, along with research groups in Switzerland and Japan, are studying pheromone-binding protein (PBP) molecules, which are highly concentrated in the lymph fluids inside the sensillae on insect antennae. In addition, the lymph fluids inside the sensillae also contain a large dose of pheromone-degrading enzymes. The working hypothesis is that PBPs capture pheromone molecules and protectively ferry them through the lymph fluids to pheromone receptors that then trigger "neuronal activities, the language of the brain."
"Besides being abundant inside the antenna, pheromone-degrading enzymes literally cover the insect body, including the outer cuticle and forelegs," says USDA researcher Steve Ferkovich. Almost three decades earlier, he published a paper in the British journal Nature with the then-surprising news of pheromone-degrading enzymes inside the sensillae of the pesky cabbage looper moth. Ferkovich cracked open the sensillae, collected the liquid that oozed out, and into it dipped male moth antennae coated with pheromones. He expected to find some interesting molecules in the ooze binding to the pheromones. Instead, Ferkovich found the pheromone-degrading enzymes, which immediately destroyed the pheromones.
Show us the receptor
Regardless of research in the mainstream camp, Dykstra holds to his view that pheromone perception in moths involves the male moth antennae sensing electromagnetic spectra emitted by pheromone molecules. The failure of scientists to identify a chemical pheromone receptor strengthens Dykstra's conviction that Callahan is right. Dykstra offers an explanation based on antenna theory when asked about the tiny pores of the sensillae (visible only under an electron microscope), which allow pheromone molecules inside to bind with PBPs. In antenna theory terms, the pores function as holes in the antenna's dielectric shell, making the sensillae more efficient in their role as antennae.
As to those abundant PBPs binding with pheromone molecules for a trip to the pheromone receptor, Dykstra offers an alternate explanation. "The PBPs," he says, "are garbage-collection molecules mopping up excess pheromones so that the system is not inundated and shut down." In any case, says Dykstra, the male moth antennae would have "read" the electromagnetic spectrum of the pheromone molecule long before it bonded with the PBP inside the antenna. Plus the PBPs eventually dump the pheromone molecules, which are almost immediately destroyed by the pheromone-degrading enzymes.
Lessons from the doomed moths
Callahan's ideas had a long and rigorous development. In addition to publishing dozens of papers in the field, Callahan published the 1975 book Tuning In to Nature: Solar Energy, Infrared Radiation, and the Insect Communication System.
"Callahan is a genius," says Ferkovich, who worked alongside him at the USDA laboratories in Gainesville for several years. "Probably down the road they are going to find that he is right in a lot of different areas. But demonstrating the events [pheromone perception] happening is not that easy."
More than 50 years ago, Callahan began the close field observations of nature, particularly night-flying moths, which are at the heart of his work. On breaks from maintaining homing-beam transmitters guiding U-boat bombing planes to safe landings in Lough Erne, Ireland, during World War II, Callahan went moth watching. Ireland is famous for its ghost moth males, which somehow know to hover over patches of grasslands where females are hidden. Callahan wondered whether this might be explained by a natural equivalent of the homing-beam transmitters that sent out radio waves to guide Allied pilots.
While wandering the world after World War II, Callahan was briefly jailed in Basra by the Iraqis as an American spy. When he was released, he set out walking by night across the desert to Syria. In The Soul of the Ghost Moth (1981), Callahan recounts seeing piles of dead insects and witnessing "the dance of the doomed moths at Basra Petroleum Field No. 1":
"The street lights ... were a mere flashlight compared to the powerful torches that lit the Iraqi sky from hundreds of burn-off pipes. Hydrocarbons combusted and burst into the cool desert air ... Many of the deranged six-legged dancers did not fly to the flame but swirled about like mad whirling dervishes in the night air at the edge of the light. The huge moths with cross-armed antennas seemed to vibrate them in unison to the flickering light. Why do not insects fly to the sun and moon if visible light is the reason for such madness?"
Thus, by the late 1940s Callahan's observations were already telling him that something in the electromagnetic spectrum other than visible light was motivating these strange, suicidal insect behaviors around bright lights. Years later pheromones were discovered, and many turned out to be hydrocarbon molecules similar to those combusting over the Iraqi desert.
Callahan spent almost two decades painstakingly mapping every sensilla on cabbage looper and corn earworm moth antennae, publishing the results in entomological journals into the 1970s. "He [Callahan] was an excellent morphologist," says Ferkovich. "He did a lot of work on describing shapes of insect sensillae." But Callahan's stock in the entomological world plummeted when he openly theorized that sensillae shapes and spatial arrangements were indicative of their functions.
Callahan had undergone 1,000 hours of intensive radio technical training in the U.S. Army Air Force during World War II. For him, short electromagnetic wavelengths, "where the physics and mathematics of optics overlap with the physics and mathematics of microwave [radar] engineering," were part of the comfort zone. Not so for the rest of the entomological community.
So Callahan began publishing titles such as "Solid state organic (pheromone-beeswax) far infrared maser" and "Moth and candle: the candle flame as a sexual mimic of the coded infrared wavelengths from a moth sex scent" in journals like Applied Optics. Although respectable physics journals published his articles, Callahan failed to find an audience for his ideas among entomologists.
On to the marketplace
Seeing Callahan's science relegated to the backwaters of academic discourse, Dykstra instead aims to make his mark by building a better moth trap. His target is the Indian meal moth, the number one worldwide pest of such stored products as nuts, grains, chocolate, and birdseed. Dykstra is designing reusable pheromone traps that he expects to last over two years, versus a rated lifetime of about three months for conventional pheromone traps.
"My traps amplify infrared emissions from scent molecules," says Dykstra. "All molecules give off infrared emissions, with no exceptions that I know of, as long as they are above zero degrees Kelvin, where all motion stops." By running the emissions through a spectrometer, scientists produce for each molecule a distinctive spectrum of frequencies. Dykstra likens the peaks of the spectrum to the bar codes on the food that we buy.
"We believe," he says, "that insects selectively identify molecules based on their infrared spectral peaks. By amplifying and projecting the key pheromone frequencies, our better moth trap attracts more moths than would be attracted by the unassisted molecules available in today's commercial pheromone-lure traps."
Dykstra claims that his prototype traps are already hitting a moth- trapping efficiency of 60--90 percent, and their efficiency will be even higher once he adds the sticky paper used in standard pheromone-lure traps. "Compared with the industry standard of 50--70 percent capture efficiency for a pheromone lure with sticky paper," he says, "our better moth trap is looking very good.
"Our patents are nearing approval; we are negotiating with several potential manufacturers; and signs point toward commercial success. Beyond that, there are many possible new developments based on nonvisual electromagnetic perception. Eventually," he sighs, "we hope that marketplace success will help to gain academic credibility for Callahan's ideas."
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|Publication:||World and I|
|Date:||Apr 1, 2004|
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