by Pat Kelley, BCE Vice President
Insects smell odors with their antennae rather than a nose. In general, if an insect is looking for a mate and it picks up the correct pheromone, it will immediately react and begin seeking out the source of the plume. Like a lock accepting a key, the antennae of some insects are specially designed to pick up the specific pheromone of their own species. Often times, the sensory hairs on antennae are so sensitive to a potential mate’s pheromone that they can detect pictogram (one trillionth of a gram) quantities in the air.
Pheromone plumes are an intriguing and complex part of nature. Knowing a bit more about them gives us a lot of respect for how amazing insect communication can be and it also helps us become better at setting up trap monitoring programs and providing integrated pest management.
As cooler temperatures arrive, the vision of curls of smoke coming out of the chimneys of people’s homes becomes quite common. Quaint scenes of families sitting around warm wood fires, enjoying each other’s company, comes to mind.
In the insect world, the sex pheromones released by female insects are a lot like the smoke emanating from those chimneys. Pheromone scents are whisked away from the female and carried on air currents to surrounding locations. The physical shape or structure that pheromones make after they emerge from the female are called “plumes”. This terminology arose from the fact that odor plumes often resemble the shape of a feather as they start narrow at the source and spread out as they become diluted in the air.
Female Indian meal moths release sex pheromone that is carried away in the form of a “pheromone plume” that will attract any males in the area.
Plume structures can be quite complex, determined by the molecule size and density of the pheromone as well as the atmosphere and air currents. As pheromones radiate from their source, they are initially very concentrated in the air. Typically, the odors widen and expand in a type of molecular diffusion as they move away from their source. Occasionally though, finger-like strands of the scent remain in high concentrations as they float away, leaving odorless gaps in between. In either circumstance, insects are really good at picking up plume odors and determining their origin.
Once an insect locks onto the plume, it will move rapidly, making constant turns to remain within the plume. As these oriented movements bring it closer to the source, the quantity of pheromone molecules in the air increases. The increase in pheromone concentration causes the insect to slow its rate of speed and reduce its rate of turning as it nears the source. Eventually the male insect makes physical contact with the female insect or in some circumstances, they make contact with a pheromone lure in a sticky trap. Chalk it up as another successful account of the “Power of the Plume”.
Note: One reason mating disruption pheromones work well for some insects is the fact that the entire area becomes a giant pheromone plume with no actual source for the moths to find.