Ask an Entomologist: Sex Pheromones and mating disruption

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.)

sex pheromones #1

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. pheromone trail 2

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. frustrated malesThe 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:

*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

8 thoughts on “Ask an Entomologist: Sex Pheromones and mating disruption

  1. Note from Bug Girl: I removed 4 paragraphs of what is clearly a pasted, spam message. If you had actually -read- my post, I might have been more inclined to let some of it remain. I also removed your address and phone number, because I think it’s a bad idea to let that loose on the internet.

    In order to try to help “stem the tide”, I have just finished re-writing my IPM encyclopedia entitled: THE BEST CONTROL II, that contains over 2,800 safe and far more effective alternatives to pesticide POISONS. This latest copyrighted work is about 1,800 pages in length and is now being updated at my new website…….All of these copyrighted items are free for you to read and/or download. There is simply no need to POISON yourself or your family or to have any pest problems.

  2. Hi,
    If you are advocating the use of pheremones, then you should know that when these are used in an aerial spray on us people, they do effect our hormone levels causing irregular menstruation. I know from first hand experience, living in Monterey where recently was sprayed Checkmate ORLF after which I experienced irregular signs of oncoming menstruation in the middle of my cycle. Many other women experienced similiar disruptions including a good friend of mine who menstruated for more than one month. Such changes in hormone levels are what cause breast cancer and other physical ailments, and will surely effect the sexual development of our children, possibly causing them to bear children with birth defects.
    Sincerely, Heather Chang

  3. And, I should add, pheromones are marketed for use in the home to monitor all sorts of household pests–mostly clothing moth and Indian meal moth.

    Pheromone molecules are ALREADY in the air every day as we walk from place to place, or in our homes. You probably have cockroach pheromone on you somewhere, if your office is like mine. :)

    It’s not a scary thing, it’s just a new (to the public, anyway) thing.

  4. This is to inquire where can I purchase a synthetic male butterfly hormone? What are the procedures in applying the hormone as pest control and does possible target insects includes beetles, spider mites, etc..

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