What is Chemical Ecology?

One striking thing that  I realised from the discussions about Rothamsted’s wheat trial is how little was known about chemical ecology - it’s very easy to get so absorbed in your own field that you forget that what you’re working on isn’t common knowledge in the wider world.  It’s also a fascinating example of how words can have very different associations to different people – to me chemical ecology is a fascinating field of study, but I actually noticed one protestor I was talking to recoil in horror at the juxtaposition of the friendly, positive word “ecology” with the word “chemical”, with all its unnatural connotations.

Colloquially, the word “chemical” has come to mean something artificial, some unpronounceable synthetic substance with unpredictable effects cooked up in a lab somewhere, but it’s important to realise that in the technical sense the word chemical simply means a collection of atoms.  Using the technical definition every physical object we encounter is made up of chemicals – water is a chemical, so is the oxygen we breathe, the vitamins, sugars and proteins that we eat, the keratin in our hair.  (The fact that this field of study was named something that seems so off-putting to some perhaps just goes to show that science doesn’t have access to the sort of slick PR machine many assume it does!).

The science of ecology involves the study of the relationships between different organisms, and between organisms and their environment.  Chemical ecology is a particular subsection of this discipline, which studies those interactions that are mediated by chemicals; semiochemicals which convey information, for example smelly compounds which alert organisms to the presence of suitable or unsuitable food, mates, or danger, pheromones which allow individual organisms of the same species to coordinate behaviour (for example the queen mandibular pheromone of honey bees that prevents workers from laying eggs), and defensive compounds that species use to wage chemical warfare on one another – the formic acid wood ants squirt at attackers, the antibiotics secreted by some fungi to prevent the growth of competitor bacteria on their food or the signals used by the parasitic weed Striga to parasitise its host plant  for example.

Queen bee surrounded by workers, image from Wikipedia.

 

My own research on the smells that attract the fly that transmits trachoma to the tears it feeds on and the faeces it lays its eggs on, and so the aspect of chemical ecology I’m most familiar with, involves semiochemicals – the volatile molecules that diffuse through the air and convey information to the creatures that smell them.  These are often oil-soluble chemicals – if you try to think of some of the strongest smelling things you encounter aromatherapy oils are probably on the list somewhere – but they can also be things that humans can’t smell, like carbon dioxide or water.  Insects in general have very acute sense of smell – a bee, for example can smell the equivalent of a single grain of salt in an Olympic-sized swimming pool.  (This highly acute sense of smell, incidentally, is why bees are being trained to sniff out drugs and explosives.)  So if you’re hoping to learn how insects interact with their world, and maybe to control how they do so, smell is a good place to start.

 Sniffer bees in action

There are various ways to find the odours insects can detect, but one of the most direct is to eavesdrop on what’s going on in their brains using a technique called electroantennography.  A nerve impulse is fundamentally just a spike of electrical charge, so by very carefully inserting one electrode into the tip of an insect’s antenna, and another into the area of an insect’s brain responsible for smell, you can measure how the difference in charge between the two electrodes varies when the insect is exposed to different smells you think might be important to it and by doing so find out which ones trigger a nerve impulse – which ones it’s smelling.  From there you can go on to do laboratory and field tests to find out how the insect reacts to these smells; does it fly towards them, away from them, or do something in response to them, like feeding on what smells like tasty food or laying eggs on what smells like a good place for its young to develop?

Exploiting an insect’s sensory word has one great advantage over many other pest control methods – as different smells mean different things to different insects, taking an approach informed by chemical ecology allows you to target one particular pest species without affecting others, unlike blanket insecticides for example which may be just as harmful to beneficial insects or a pest’s natural predators as they are to the pest itself. Take the coddling moth for example, a pest of apple trees whose caterpillar is the traditional “worm in the apple”.

Coddling moth larva damage, from Wikipedia.

Adult females of this species produce a characteristic pheromone which the male can smell from a great distance away, and he can then follow the perfume trail to find her and mate.  Instead of spraying orchards with insecticide farmers can now use traps baited with synthetic versions of this pheromone in a technique called mating disruption – overwhelmed by the strong perfume wafting from the traps the male can no longer find the female, they both eventually die alone and frustrated and the apples are protected.

The coddling moth isn’t the only insect species whose chemical communication can be its downfall.  Contrary to popular belief bedbugs don’t actually live in bedlinen, but spend the day hiding in “refuges”, cracks in walls or furniture.  Dozens huddle together in these refuges, waiting out the day, and find each other using their characteristic smell, sometimes described as reminiscent of cilantro or coriander (although I won’t be sprinkling bedbugs on my Thai curry any time soon).  They’re not the only ones who can use this smell though – bedbug infestations mean big business losses for hotels so they need to be tipped off at the first sign of infection.  The most sophisticated bedbug detectors out there use the smell that bedbugs produce to find them, and then signal that they’ve done so...by wagging their tails.

Bed bug detection dog

That’s right, the best bedbug detectors out there are dogs.  Well, the sniffer dogs need something to do if the bees are displacing them at airports.

In Kenya a novel farming system exploiting chemicalecology is being pioneered to control stem borer caterpillars.  These are the larvae of a number of different moth species (Chillo partellus, Eldana saccharina, Busseola fusca, Sesmia calamistis) that basically do exactly what they say on the tin; chomp their way through the stems of maize plants, boring out the centres, which obviously doesn’t do a lot of good to either the maize or the farmers who want to eat it.  The moths find the maize plant to lay their eggs on by smell, and that’s where chemical ecology comes in, using a push-pull strategy of interplanting a plant that smells repellent to the moths with the maize, to mask its naturally attractive smell, and surrounding the maize crop with a plant that smells attractive to the moths to lure them away.  Cunningly the repellent-smelling intercrop is a plant called Desmodium, a member of the bean family that enriches the soil with nitrogen and as a bonus kills the parasitic weed Striga which also reduces maize yields, and the attractive plant is a grass which can be fed to cattle and which traps the stemborers with sticky sap.

An understanding of chemical ecology isn’t just helpful for plant growing either.  Kenyan cattle herders dread nagana, a disease spread by tsetse flies which causes weightloss and death in their herds.  Attractant traps for tsetse flies already exist – blue sheets (a colour that the flies find attractive) baited with carbon dioxide mimicking the exhaled breathe of the animals tsetses feed on. (Incidentally this is why tsetse flies chase cars: a tsetse’s prey is a large moving object breathing out carbon dioxide, and a car is a very large, fast-moving object pumping out large amounts of carbon dioxide).

Tsetse trap, from Wikipedia

But these traps alone aren’t sufficient to protect the Kenyan cattle herds.  The solution came in the form of repellent collars for the cattle, which mimic the odour of animal species that tsetse don’t find attractive.  These are being rolled out at the moment and, in conjunction with traps, serve as a push-pull system for the tsetse.

How about insects pests that transmit human diseases, could chemical ecology be used to control them?  It’s a possibility.  Like tsetse flies, mosquitoes are attracted to the carbon dioxide in exhaled breath, and also to various odorous chemicals evaporating from human skin.  We may already have something that could serve as the pull component of a push pull system – a trap baited with carbon dioxide and a synthetic blend of these chemicals that could be more attractiveto mosquitoes than humans are, and we’re on our way to develop a push.

At present the only effective wearable mosquito repellent is DEET, developed by the US military to protect its soldiers.  Although highly effective it has its drawbacks, sometimes causing irritation or damaging clothes.  Repellents made from lemon eucalyptus look promising but evaporate too quickly from the skin to be very useful at the moment.  The solution may lie in chemicals that we ourselves produce naturally – it turns out that as well as the attractive chemicals we all produce some of us also produce natural mosquito repellents. 

This is not in fact an oven ready scientist but me in a survival bag.  These are used to capture the odours that human beings produce, to analyse for chemicals that are attractive or repellent to mosquitoes, as they’re airtight and have very few odours of their own.  Air that has had all its own odours purified out with a charcoal filter is blown into the bag, and the, umm, miasma sucked out and analysed.  Maybe someday we’ll be able to use these odours to make everyone smell utterly repellent, at least to mosquitoes.

Using chemical ecology to study the stimuli useful to insects gives us a greater understanding of the world from their perspective, and the more we understand about what chemical information they use to find resources important to them the more we can manipulate those resources that are also useful to us, like crop plants or even our own bodies, to reduce the conflict between us.  In an increasingly resource-constrained world, gaining a better understanding of natural systems in order to make fewer, more sophisticated changes to better meet our needs is surely the way to go.