Of the Silk Moth

FLASHBACK_Attractants

On the Trail of Love of the Silk Moth

The females of many moths and butterflies attract the males with a beguiling scent. In 1959, Adolf Butenandt unraveled the chemical nature of the attractant he had recently investigated at the Max Planck Institute of Biochemistry.

TEXT BIRGIT FENZEL

This is also a long-distance relationship of It is no coincidence that many different this exotic laboratory species (it had not sorts: over distances spanning several theories regarding these enigmatic, silent been used for such experiments before) kilometers, the males of many butterfly siren calls of the female butterflies were cir- would have certain advantages. The species species find females that are willing to culating at the time. Some assumed that is easy to breed, and the creatures are mate. How they accomplish this was long “vibrations that can travel over incalculable undemanding in terms of food, climate a well-kept secret of the butterflies, and one distances in connection with actual diffu- and light conditions. In addition, mature that spurred on insect lovers and entomol- sion of matter” were behind this phenome- specimens cannot fly – a fact that greatly ogists in equal measure. It was almost non – and indeed Butenandt mentions this facilitated the experiments in open dishes. exactly 50 years ago that a Nobel laureate theory in an article dating from 1959 in The other behavior of the fat moths also in chemistry found the right approach to which he presented his knowledge about made them a suitable test species in solving this mystery. A year before Adolf the love-life of butterflies, but probably the search for the sexual attractant of Butenandt took office as the third President simply for the sake of completeness. lepidoptera. First of all, the females emit- of the Max Planck Society, he published the As a renowned expert in hormonal ting the scent carry their secretion store chemical structure of the scent with which questions, he was more interested in an- around with them in two yellow sacs at the female butterflies attract their suitors. other approach that other researchers were end of their abdomen, just like perfume The question of how male butterflies also following. In a series of experiments, bottles in a handbag. Second, because of find their way to the females had already they had already found out that the females their extremely phlegmatic lifestyle, the long fascinated insect lovers and entomol- of certain butterfly species produce sexual males make excellent indicators for the ogists alike. And evidently not just them – and species-specific substances in special beguiling substances. In fact, they only the words of Adolf Butendandt in his glands that are used to attract males. Ex- move if they detect the presence of a female research report reveal an early enthusiasm periments were conducted in a number of or her scent. Then they are transformed into for this subject: “Even today we enjoy read- places and with various butterfly species in living spinning tops with shimmering ing the fascinating accounts by I.H. Fabre, order to isolate the scents for more detailed wings and circular searching motions written more than 50 years ago, which tell chemical analysis. The knowledge gained gravitating in the direction of the higher of a single female Emperor moth kept in a from 30 years of butterfly pheromone concentration of the glandular scent. wire cloche at an open window from 8 pm research was, however, “discouragingly To begin with, the researchers plunder- to 10 pm attracting a total of 150 males over modest.” ed the attractant scent sac of around 7,000 a period of 8 days, some from ‘kilometers With this as their starting point, Adolf female moths. At 100 milligrams, the away’.” Butenandt and his colleagues set off on the yield of a waxy substance was not exactly Chinese researchers provided more pre- track of the scent at the Kaiser Wilhelm opulent. Further examination showed that cise details about the range of this myster- Institute in Berlin-Dahlem in 1939. The this must be a “lipid-soluble, neutral and ious attractant of the female butterflies by researchers wanted to identify the bio- unsaponifiable alcohol” that is “resistant releasing males from a train traveling away chemical properties of the gland secretions to diluted acid and alkali, but sensitive to from a gauze cage containing attracting and determine the threshold at which the oxidants.” They named the new substance females. A good quarter of the male butter- enticing substances began to take effect. Bombykol. flies found the females over a distance of They focused on the silk moth, Bombyx mori In order to determine the quantity of eleven kilometers – an achievement border- – a furry white type of moth with a squat substance required to trigger arousal in the ing on the miraculous as the Münchner body shape and a pronounced propensity males, Adolf Butenandt and his colleagues Merkur newspaper commented at the time. for lethargy. Butenandt anticipated that placed the fat Bombyx males in flat dishes.

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What might the silk moth attractant look like? This question interested Adolf Butenandt, seen here in the 1950s at his desk at the Max Planck Institute of Biochemistry.

They then held glass rods moistened with larger quantities of butterflies could still be extracted. “It is a double-unsaturated alco-

a petrol ether solution containing various obtained via Japan and Spain, the resulting hol of the formula C16H30O and contains the concentrations of the tantalizing phero- yield of the substance was far too low attractant in 10-10 γ!” wrote Butenandt. mone in front of the olfactory organs of the to solve the mystery. Nevertheless, the re- At that time, the unit γ was used for a test moths. searchers were able to accumulate valuable microgram. In the preparation, the attrac- In these experiments, the scented knowledge about the “behavior of the tant was thus 100,000 times more concen- rod proved to be a genuine magic wand. attractant in the application of chemical trated than in the product of the previous Beginning at a certain threshold, it brought and physical separation methods.” analysis: just one ten-billionth of a micro- the male moths – that had been decidedly The 1950s heralded the return of fruitful gram bewitched half the males used for the inert until then – to life and even made them times for butterfly research at the institute. scent test. dance at higher doses. After several years In 1953, Butenandt and his colleagues had After 20 years, Butenandt had finally and many experiments, the researchers had 313,000 female moths available that had found the substance that constituted developed a solid qualitative test for hatched from 1,000,000 cocoons. The re- the love navigation system of moths and the comparative evaluation of attractant searchers enriched the glandular secretion butterflies and produced it in pure form – a preparations of varying degrees of purity. to such an extent that they obtained three pioneering achievement in the field of pheromone research that would influence gen- Münchner Merkur from July 9/10, 1960 erations of researchers to come. Although Professor Adolf Butenandt had approximately 500,000 hormone glands processed in designed as basic research, Butenandt had » order to obtain a full 12 milligrams of this hormone and to explain its chemical structure. already been thinking of the practical bene- (...) He set out the following perspective: With the aid of the attractant, all male insects fits, such as combating pests with specific within a certain area can be enticed into a lethal trap so that the females remain sexual attractants. “But that is still all in the unfertilized. future,” he wrote at the time. Now, 50 years later, scent traps for moths are found in the According to Butenandt, the researchers milligrams of a preparation of which kitchens and wardrobes of many house- defined the unit of attractant as “the quan- a hundred-thousandth of a microgram – holds. At least in this respect, practical tity by weight of substance (...) contained a millionth of a gram – was sufficient to applications have become a reality. in 1 ccm of a petrol ether solution, with send half of the tested males into raptures In other respects, scents have been used which (...) a clear positive reaction can be of love. According to Butenandt, this quan- only in literature. Luckily, one might say – triggered in at least 30 out of 60 animals.” tity was not sufficient for a full analysis, as everyone who has read Patrick Süsskind’s Experiments showed that the original “but it was concluded from their behavior novel Perfume will know. That might have version of the pheromone obtained from that the attractant must contain two also comforted the American researcher’s the female butterflies “had to be enriched conjugated double bonds and more than wife who met Butenandt at a Nobel laure- by several powers of ten to produce the ten carbon atoms.” ate congress in Lindau in 1951. In his article, pure attractant.” Six years later, the researchers finally he recalls her question about when the “sex- Years would pass before Butenandt and isolated enough substance for a more pre- attractive substances” could be used. The his colleagues succeeded in identifying the cise examination. From around 500,000 woman’s response to his comment that the precise chemical structure of the phero- glands of female silk moths, they obtained substances affected only butterflies was a mone. The war years had their impact on 12 milligrams of a crystallized colored ester disappointed: “Oh, Doctor B., why did you

Photos: Archive of the Max Planck Society, Berlin-Dahlem the work of the researchers, and although from which the colorless attractant itself is waste your time with butterflies?”

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