Love at first sniff: Male moths go by first impressions

A European corn borer male. (Credit: Cardé Lab, UC Riverside.)

An international team of researchers, including an entomologist at the University of California, Riverside, has an explanation for why we see so many hybrid moths in nature. The team closely examined the behavior and the olfactory circuitry of male moths and found an answer in female-produced pheromones — chemicals generally consisting of a blend of two to several derivatives of fatty acids.

Male moths use pheromones to find females. To avoid mating with the wrong moth species, the pheromone blends are specific for each moth species, with only males of the same species “understanding” these volatile messages — a result shown many times during the last 40 years of research on moth pheromones. The researchers focused on the European corn borer, a moth species in which males often mate with females from a different strain.

Strains are variants, forms of the same species. While two different species cannot mate with each other, strains, being from the same species, can.

To understand the mating behavior of the European corn borer, first, the researchers followed the flights of males to female pheromones in a wind tunnel. Each strain of the European corn borer uses a blend of pheromone components in a very specific ratio. But to their surprise, the researchers found that as the male moth flies upwind along the pheromone plume, its olfactory circuitry loses the ability to measure this ratio.

“This happens because receptors in the moth brain for each pheromone component have differential rates of sensory adaptation and each type of receptor begins to fire at a different rate, causing the input into the moth’s brain to change as the moth flies along the plume,” explained Teun Dekker, a former UCR graduate student and now an associate professor at the Swedish University of Agricultural Sciences, and a coauthor on the study. “To overcome this mismatch, moths rely on the ratio they detected in their first encounter with the plume.”

According to the researchers, this “mental short cut” is needed for male moths to continue their orientation along the plumes which, from a sensory input viewpoint, seem to be changing in pheromone component ratio.

“Once male moths lock onto a pheromone plume, they are much less attuned to blend quality,” Dekker said. “In other words, males fly even to blends that were initially unattractive, and so can mate with females of different strains that they would not have approached otherwise, explaining why we find hybrid moths in nature.”

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