Pheromone Traps - Using Sex as Bait

Meera - This week, I've come down to the fruit gardens here at the Royal Horticultural Society's site in Wisley, Surrey to find out how the chemical signals that insects use to communicate with one another can also be used to stop them from damaging plants and crops. Apple growers, for instance, often find their profits quite literally being eaten into by the larvae of coddling moths. It's possible to solve the problem by spraying regularly but an alternative, more environmentally friendly approach is to use a device known as a pheromone trap. RHS entomologist, Dr Andrew Salisbury, is with me now to explain how this works. So Andrew, what is a pheromone?

Andrew - The technical definition is a chemical that mediates a response between individuals of the same species. There are several different types. The main one people have heard about is the sex pheromones where a male or a female releases a chemical and attracts the opposite sex. There are also aggregation pheromones, which attract both sexes, and even alarm pheromones which cause insects to move away from each other.

Meera - The pheromone traps actually use sex pheromones to trap the insects chemically work in order to attract a male mate?

Andrew - Basically, the female releases the pheromone and the molecules are picked up by specific receptors on the male's antennae. The actual number of molecules they can pick up can be very small. This causes a response in the male to follow the pheromone stream upwind until they find their potential mate.

Meera - Knowing this, how do pheromone traps exploit it?

Andrew - Pheromone traps basically exploit it by using a synthetic version of the same pheromone to which the males are attracted. Males fly to the trap, hopefully in preference to their female mates.

Meera - How are the pheromones actually found and known about in order for them to be used in these traps?

Andrew - Basically, entomologists and other scientists olfactometers. These are pieces of equipment that are choice chambers. In one side of the equipment you have a stimulus which, if you are looking for a pheromone, may be a female moth. A male moth is released into the equipment and he can't see the female. If he moves towards the female and significant numbers move towards the female then you can say there probably is a sex pheromone.

Meera - Ok then. How do you find the actual chemical?

Andrew - First of all, you have to collect the odour of females and this is basically done in glass vessels and air is drawn over a polymer that picks up volatile organic chemicals. You then take the volatile organic chemicals. Basically, you use things like gas chromatographs, mass spectrometers to come up with identities of chemicals in the mix that you've collected. Of course, you don't really know which of the chemicals you've collected affects male behaviour. The next stage is to actually cut off male antennae, stick them in an electroantennogram and pass various chemicals over the antennae. If you get a peak you know the antennae response to it. You've made progress in identifying the pheromone. Then you take it out to the field and do field trials. If all goes well eventually you'll have a product.

Meera - We've got a pheromone trap here in front of us. It's hanging on an apple tree and it's kind of like a tent with a base attached to it. Why do they have this particular structure?

Andrew - The open edge of the tent structure is partly to keep the weather out and larger animals such as birds actually getting into the trap. The base actually contains a sticky substance, a non-drying glue to which the insects you're trying to attract get trapped on.

Meera - How does this help pest control?

Andrew - There is obviously going to be some mating disruption here. Those males which get trapped don't mate with females. The main way that these sticky traps work is that it can tell you when the peak emergent or peak flying period time of this particular moth is which can tell you when or even if it's worth using a chemical. Coddling moths can be around for two months and laying eggs for two months and spraying several times during that period is uneconomic and not very environmentally friendly.

Meera - So you say this sticky trap is more associated with amateur gardeners. We've got another one hanging on a tree over on this side of the garden. This uses another method in order to affect mating, does it?

Andrew - Yes, this has no sticky base on this one. It's just a tray with little dimples full of powder. The powder in this case contains pheromone and the idea with this one is males fly in, get covered in the powder, which contains the female sex pheromone, fly off and they now smell like a female. This causes massive mating disruption and hopefully eliminates the need for chemicals.

Meera - I love that. So there's going to be a male with loads of other males following him around?

Andrew - That's right. That's the sort of thing we hope will happen with this sort of control.

Meera - What other sorts of traps are available out there? What other species can be targeted?

Andrew - One quite exciting development which is going on at Rothampstead research is that they have found the aphid alarm pheromone (greenfly alarm) which makes them get up and walk away. It's a very similar chemical. It's found in catmint. They've been growing catmint, extracting the chemical from the plant and this process has the added advantage that cat toys can be made out of the product as well.

Meera - So a dual purpose?

Andrew - Yes, indeed. Much more is in development. Many species of insects we've investigated produce a pheromone of some type. Watch this space for further developments.