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Tom Baker, Entomology, (515) 294-1610
Skip Derra, News Service, (515) 294-4917


AMES, Iowa -- The question of how animals distinguish different smells is kicking up a stink again.

Do moths respond to a "coding" process built on high-frequency brain oscillations to discern the smell of sex pheromones or do they process each randomly arriving odor strand in their brains? It appears the latter is true according to a paper in the most recent issue of Nature (March 22, 2001).

The find is important not only in understanding animals' sense of smell but also because it could help those who try to control crop damaging pests in their estimates of insect infestation and the timing of insect larval emergence.

The research was carried out in part by Tom Baker, professor of entomology at Iowa State University and a former post doctoral student, Neil Vickers, now at the University of Utah, Salt Lake City. It shows that a moth's processing of olfactory scent is more complex than what had previously been thought. The research shows that an insect's ability to detect scents is directly related to the highly irregular arrival rate of odor, which, in turn, is determined by wind and the moth's own movements.

The researchers describe experiments that show how moths (Heliothis virescens, or tobacco budworm) respond in flight to a female pheromone, a sexual attractant. The research was carried out by Vickers, Baker and Thomas Christensen and John Hildebrand of the University of Arizona, Tucson. Vickers is the lead author of the paper.

Recent laboratory research has shown that various networks of nerve cells in the brain develop high-frequency oscillations when an insect's antennae are stimulated with odor. Some researchers believe that these oscillations tell the insect which smell is which. Whether this holds true in the wild, where fine strands of scent blow around in an odor plume, was unclear until now.

In their experiments, Vickers, Baker and their colleagues blew a plume of sex pheromone over male moths and recorded the pulse output from the moths' smell-sensitive nerves in the antennae. They reported that the timing of the firing patterns of the antennae and brain are highly irregular and tightly coupled to the patterns imposed on them by the shape of the pheromone plume itself, not to brain oscillations. The firing patterns change rapidly as the insects encounter each smelly patch followed by patches of clean air.

Baker said the new work shows that as the strands of scent that make up a typical odor plume flow over the moth's antennae, the information about each strand is reported faithfully to the brain.

"This shows that where the moth is within the odor plume and how fast the wind is blowing determines the rate with which the odor quality assessments are done in the brain," Baker said. "Also, the moth appears, by it's own behavior, to give itself the kinds of stimulation that will be most optimal for upwind flight.

"The brain then is able to sample these strands. Every strand that comes down is sampled for odor quality," Baker explained. "Before it wasn't known if every strand was reported accurately."

The researchers performed three experiments in the new study, focusing on the insect's noses -- their antennae. An insect's antenna has thousands of tiny hairs containing cells that sense odors. Using antennae detached from a male moth and hooked up to electrical amplifiers, the researchers first determined the physical fine structure of an odor plume as it would be sensed by a stationary moth.

In the second experiment, an "electro-antennogram" (EAG) was attached to the back of a live moth using a tiny piece of Velcro®. The EAG monitored the plume of the scent over space and time as the moths flew upwind.

"We basically gave them hood ornaments," Baker said. "This is a very difficult thing to do, but it allowed us to measure the fluctuations in scent as they flew in the odor plume."

The experiment showed that the intensity of odor sensed by the moths depends -- to a larger degree than previously thought -- on their flight path through an odor plume.

In the third experiment one of a live moth's two antennae was wired to an electrode, while a tiny microelectrode was inserted into the insect's olfactory lobe, the part of the moth's brain at the base of that antenna that analyzes odor quality.

The moth was restrained in a tiny glass tube and placed in the larger tube that served as a wind tunnel. As a natural plume of female pheromone, placed far upwind, was allowed to flow over the moth's antenna, the scientists recorded electrical activity both from the olfactory cells in the moth's antenna and the olfactory nerve cells in the brain. The results showed that the dynamics of the odor -- namely changes in the odor's intensity over space and time -- reported by the antenna correlated closely with subsequent electrical firings of neurons or nerve cells in the moth's brain.

Baker said this research is important because it could help people in pest management design the most sensitive monitoring traps to detect the presence of pests. It also can verify that the trapping of moths is representative of the number of insects in the fields.

The tobacco budworm is a major agricultural pest that damages tobacco, tomatoes, cotton and other crops. The principles developed from the new research would also be applicable to closely related species like the corn earworm, and to the European corn borer, considered by some to be the biggest pest of corn in the Midwest.

"We want a sensitive monitoring tool for the activity of adult moths," Baker said. "If we know when the adults are reaching their peak in numbers then we know when eggs are being laid and shortly after that larvae will be out there. It helps time control tactics against the damaging larvae."


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