Plumes: How Pheromones Travel
By Pat Kelley, BCE, President of Insects Limited, Inc.
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, come to mind.
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.
What is a Plume?
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.
Plume structures can be quite complex, determined by the molecule size and density of the pheromone as well as the air currents and atmospheric conditions including what the barometer is telling us.
How do Plumes Work?
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. They can end up very weak the further they travel from the source.
Occasionally, finger-like strands of the scent remain in high concentrations as they float away from the source, leaving odorless gaps in between.
In either of these scenarios, if the plume is a sex pheromone coming from a female insect or from a pheromone lure, male insects are good at picking up plume odors and determining their origin back to that female (or a perceived female in the case of a lure).
Insects smell odors using sensory hairs in 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 will 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. Oftentimes, the sensory hairs on antennae are so sensitive to a potential mate’s pheromone that they can detect picogram quantities in the air. One picogram is equal to one trillionth of a gram. A typical paperclip weighs about 1 gram. Imagine breaking a paperclip into a trillion pieces and just how small a quantity that the male moth can detect using his antennae.
Watch this video titled “Radar Love” to learn more about how insect antennae pick up pheromone scents: Click Here to Watch
Once an insect locks onto the plume, it will move rapidly, constantly readjusting 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 (if we are monitoring with pheromone traps), they make contact with a sticky trap that contains a pheromone lure and are captured. Chalk it up as another successful account of the “Power of the Plume”.
Location of pheromone-emitting female by male moth tacking upwind. The pheromone trail forms a somewhat discontinuous plume because of turbulence, intermittent release, and other factors.
Image from Insectomania/”Reception of communication molecules” 2022 https://www.insectomania.org/natural-enemies/box-42-reception-of-communication-molecules.html (After Haynes & Birch, 1985)
Note: One reason why 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.
Insects Limited, an Insect Pheromone Company
Insects Limited, Inc. researches, tests, develops, manufactures and distributes pheromones and trapping systems for insects in a global marketplace. The highly qualified staff also can assist with consultation, areas of expert witness, training presentations and grant writing.
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