As tiny animals that live widely dispersed, finding a partner of the proper sex and species to reproduce with is a problem for insects. How do they arrange a hook up?
Insects have solved that problem in a variety of ways, but sexual pheromones are one of the most common solutions. Pheromones are “chemicals emitted by living organisms to send messages to individuals of the same species.” The message transmitted doesn’t have to be about sex–there are lots of different kinds of pheromones–it’s just more appealing to human prurient interest when sex is involved
By making a species-specific blend of chemicals and releasing it into the air, insects are able to communicate over great distances. With sex pheromones, the message is usually from the female, and has the content “I’m here and ready to get it on, big boy!”
So, how do insect sex pheromones work?
Male moth antennae are exquisitely sensitive to even individual molecules of a female sex pheromone. (You can see an animation of what happens neurologically in a moth antenna when pheromone hits a receptor here, courtesy of UC Davis. Davis also has a movie of how male silk moths react to pheromone in a commonly used test apparatus.)
Reception of the pheromone triggers a series of behaviors in the receiver (male), leading to the two insects meeting and, hopefully, reproducing.
My illustration shows a theoretical situation.* The female (on the right) emits her pheromone from a gland, and the wind disperses it. Think of each pixel of color as an individual pheromone molecule. The more concentrated the pheromone, the darker the color.
Males fly upwind following a pheromone concentration gradient, and eventually find the female. (animation version here). Moving into of an area of lower pheromone concentration is a signal to turn, to try to follow the concentration as it increases.
This model makes a lot of assumptions–that the female is a stationary point, for example–but it’s a pretty good description of how the system generally works.
In fact, this simple system has been used in robotics to develop a robot that follows a smell to the source of a chemical leak!
How do synthetic pheromones muck up this process and provide insect control?
Many types of biological control in the last few decades have developed out of identifying and using an insect’s natural biochemistry and metabolism against itself. Insect sex pheromones are commonly marketed for control or monitoring of insect pests.
There are several ways that synthetic pheromones are used for insect control, but the one I’m interested in today is mating disruption. The theory is that if you saturate an area with pheromone, the males will be unable to follow a chemical trail, and will be unable to find the females. Males will also be attracted to the synthetic pheromone dispensers, and try to mate with them–with predictably unsatisfactory results for the male. Sometimes synthetic pheromone dispensers are combined with traps to kill the male insects, and really make sure no mating goes on.
No male + female = no eggs = no pest insect reproduction. It’s a type of insect birth control.
The beauty of the system is that it is specific to one species. A problem with pesticides or biological toxins are “off target” effects. You spray to kill the bollworms, and also kill all the predators that would help you out by eating the bollworms. Oh, and any butterflies and pollinators that happened to be in the area, too. Bummer.
Mating disruption involves very small amounts of a chemical, released in a small area, that results in lots of horny males and frustrated females of one species, and no fertilized eggs. Because males are so sensitive to the pheromones, micrograms are used, not pounds of active compound/acre. When it works, it’s been pretty awesome–pink bollworm and tomato pinworm are two success stories.
The problem is, mating disruption doesn’t always work 100% of the time. (What does, really, in any biological system?)
This is usually because while synthesizing a species’ pheromone is relatively easy with new molecular tools, understanding the complexity of a species’ behavior and ecological dynamics isn’t that simple. As we learn more about insects and their population dynamics, we continue to get better at figuring out why this works well on some insects, and not others. Entomologists also have seen evolution in action, as it’s turned out that insects have variation in their chemical blends, and are not as chemically monolithic as the model suggests.
All in all, it’s a fascinating system, and I’ll talk more about it tomorrow.
For additional info:
- Read a whole textbook chapter on sex pheromones!
- Visit the UC Davis chemical ecology site, and look at more cool movies and research
- A National Academy online book about pheromone communication in insects
*Disclaimer: the graphics and examples used here are for illustration only. Females don’t emit pheromone in a rectangle, and males are not a blue blob. If you can’t deal with this much abstraction in the purpose of communicating with lay readers, get a life. Or, make the graphics for me :p