To begin the new decade in style, the first molecule of the month for 2010 is the sex pheromone of the female silk moth. It might sound a weird one, but bombykol is about as interesting as they come.
Moths have been rather a pest over the years. But when I first started to think about this, I couldn’t quite work out what it is that moths eat (except my clothes once… but that’s another story). Then I realised, it is of course their caterpillars which do the damage, not the moths themselves. They can be an absolute menace to farmers, destroying everything from cabbages to courgettes. Ever since my dad started his allotment and began bringing garden fare home for the table, I seem come in to contact with these critters a little more often than I would choose.
Bombykol – it’s a simple, long hydrocarbon, with two double bonds and an alcohol group at one end.
The common silk moth (bombyx mori)
Interestingly moths can smell incredibly well. Male silk moths can smell bombykol at concentrations of 3,000 molecules per cm2 of air , making them only slightly worse smellers than sniffing champions, dogs. They have around 10,000 times fewer nerve cells connected to their smell receptors than dogs though, which makes their amazing sense of smell quite remarkable.
Bombykol was discovered by a German named Adolf Butenandt, who painstakingly isolated the moth’s internal glands, dissolved away the tissue and other detritus and concentrated the remnants to leave a sample of the hormone. Butenandt’s hard work was to earn him the 1939 nobel prize for chemistry (although due to Nazi oppression he did not collect the award until 1949). Admittedly, this award did skim over Bombykol, focusing largely on his more relevant discoveries of the human female primary sex hormones, estrone and the progestagens . Bombykol came first though – it was the first ever pheromone to be isolated.
Adolf F. J. Butenandt
The discovery of bombykol also heralded in a new era where humans would begin to understand a small amount about the way the natural world works on a molecular level – and then exploit it. After the 2nd world war, Britain still had a major food deficit, and soon people were scratching their heads over how we might turn the moths super sense of smell against them, and stop them eating our sprouts.
What happened was this. Big chemical companies quickly began producing bombykol on an industrial scale and shipping it out to farmers across the UK. Farmers sprayed it liberally on their fields and soon the entire country was doused in the stuff. The result of this sex pheromone full-immersion was that the male silk moths of the 1950s went absolutely insane. The poor blokes must have briefly thought that they had just become the luckiest Lepidoptera  ever. It appeared there were women all around them, literally oozing with sexual desire. In fact, the crazed males, blinded with a huge dose of fake libido, had no idea where the females were. They simply couldn’t find anyone to mate with, and breeding levels hit an all time low. The vegetables were safe.
It might seem extremely cruel to the moths at first glance, but pheromones have become an increasingly popular choice for keeping garden nuisances under control. Compared to traditional pesticides, pheromones are rather mild. The first bonus is that they don’t kill the insects (these days we can sexually confuse more than just moths) they just drastically lower breeding levels. With many insects becoming resistant to pesticides too, pheromones are a safer bet for the future of crop protection.
Scientists don’t know for certain exactly how the pheromones work. Aside from the males being confused, some theorise that they become acclimatised to the sexual excitement, and the lower levels of hormones given off by females are no longer enough to put them ‘in the mood’ . From time to time, the males and females do meet, but scientists have found that often the eggs they produce are infertile. This seems strange, since we wouldn’t really expect higher levels of pheromones to impact on the health of the newly laid eggs. We probably have much more to learn about insect pheromones before we really know what’s going on with these subtle yet powerful chemicals.
So this seemingly obscure chemical, isolated in the 1930s was to go on to have a strange impact on the history of Britain and even the way in which the whole world protects it’s food sources. I feel like we should all join hands and shout ‘thanks bombykol!’
. K. E. Kaissling, The Sensitivity of the insect nose: the example of bomyx mori, Biologically inspired signalling for chemical sensing, 2009, 188, 45 – 52.
. He also discovered the cortisone (an important anti-inflamatory agent) and several other important hormones.
. Lepidotra is a classification name for certain insects, including butterflies and moths.
. See an article here.