Life for the leafminer moth is short and sweet. They emerge in early spring, and flutter their life away in a few weeks. Which means that the pressure is on to find a mate! Female moths don't leave that `all-important encounter' to chance; they release a potent mixture of pheromones to attract suitable suitors. But how do the males home in on their own females, rather than getting distracted by other species that smell similar? Mattias Larsson and his colleagues in Sweden set about getting to the root of the male moth's delicate sense of smell. After close inspection, they discovered that male Eriocrania semipurpurella leafminer moths may have as many as five phermone receptors, but only two are needed to direct them to their mates(p. 989). So what do they use the extra receptors for? Warning them off when they're on the wrong track.
Leafminer moths emerge around the same time that the birch tree buds burst in early spring. This was the signal for Larsson to head out into the field,prepared to trap enough male leafminer moths to test their sensitivity to a variety of pheromones. Back in the lab he looked for pheromone receptors on the moth's antennae and tested their responses to pheromone compounds from leafminers and other closely related species.
In the case of most female moths, their pheromone scent is a blend of volatile organic chemicals. The leafminer moth's organic perfume is a mixture of two enantiomers of a single compound: (R,Z) and (S,Z)-6-nonen-2-ol. Larsson found that E. semipurpurella males have one receptor per pheromone enantiomer, which accurately discriminate between the almost identical compounds, and three other receptors that recognise a variety of other scent molecules.
But why does this moth recognise other smelly molecules? Because other moths in the neighbourhood might be using the same basic pheromone mix, which they spice with traces of other compounds. The male leafminer moth doesn't want to waste time being lead astray by ladies of another species, even if they do smell tantalisingly like one of his own. He can tell when he's on the wrong track, because the other receptors act as antagonists! As soon as the male picks up a hint of another pheromone compound, he loses all interest in that female, and begins a new search for the elusive scent that will lead him to his own lady leafminer.
Other insects, like the leafminer's close relative the caddisfly, also rely on pheromones for communication. But for some caddisflies, pheromones send messages of danger rather than romance. The family resemblance between both insects' antennae and pheromone blends makes it likely that the insects inherited the same sense of smell from a common ancestor, but today's leafminers have modified their defence to friendlier uses. For them pheromones register allure rather then alarm.