Learning the Language of Insects- and How to Talk Back - Entomology

odorous papers as effectively as they wereto the natural extract We found that thecombination of two particular fractions,the EAG-active one plus another one,increased the attraction compared to theone fraction alone. Analysis of the compoundsin these fractions revealed that theEAG-active one contained one compound,(E)-ethyl cinnamate The other fractioncontained two compounds that wereboth stereoisomers of methyl jasmonate' (Nishida et a1 1982)The verification that one of these compounds,(2)-methyl epijasmonate, was thesecond pheromone component and that@)-ethyl cinnamate was also a pheromonecomponent came from obtaining syntheticsamples of the compounds and presentingthem either singly or as a blend to femaleswho responded by being attracted to theblend Synthetic samples of other compoundspresent in the emission were alsoobtained and presented to the females, yeteven though some were much more abundantthan the two pheromone components,they did nothing to influence thebehavior, and were judged not to be involvedin communication. Therefore, thekey to proving that communication hasoccurred lies in the manipulation of the signalusing synthesized components, alongwith requiring that a significant behavioralresponse be observed in response to theaddition of any of the components Toprove that communication occurs in anymodality, we must prove that we can talkback to the insectsInterestingly, the @)-methyl epijasmonatewas present in such small quantitiesthat we could not get enough for GC-MS analysis However, for months we feltthat the odor from this fraction smelled sofamiliar that if we could locate the othersource of it we might get enough to be ableto identify it This component had a slightlyherbal, slightly floral, slightly fruity odor,yet we could not remember exactly wherewe had smelled it before! Then one day oneof us, Ritsuo Nishida, sniffed a whole uncutlemon before he prepared it to squeeze itinto his tea, and immediately realized thatthe oriental fruit moth hair-pencil coponentwas in there Eventually, after analysis ofthe air borne volatiles from lemons, the (Z)-methyl epijasmonate proved to be a heretoforeunidentified odorant from lemonand, nanogram-for-nanogram, was themost powerful odorant yet identified fromthis fruit. The odor is familiar to us as thenondescript "fruit-bin'' odor that we arehardly conscious of as we walk past piles oflemons, limes, and other citrus fruits in thegrocery store The fragrance industry that isinvolved with lemon-scented cleaning and1 1 1 2 1 3 1 4 1 5 1 6 1 71 8 1 9 10 11 1Fraction numberl2 ,HFig 3. Gas chromatographzc ~ecordzng (packed column) of male G molesta hazrpenctl extract,showing the relatzue abundance of unknown components and the fractzons taken by researchers fortestzng zn electroantennographzc and behauzoral bzoassays Four compounds that were zdentzfzedfrom the extract zncluded two (compounds 1 and 4 (E)-ethyl cznnamate and (1 R,2S)-(+)-(Z)-methylepzlasmonate, respectzuely), that when added together evoked szgnzficant levels of attractzon offemale G molesta females zn behauzoral assays Compounds 2 and 3 were behauzorally znactzue, eventhough 2 was over 10 tzmes as abundant as the other compounds, and even though 3 dzffers from 4only by uzytue of being a steyeozsome~ To the human nose, 3 zs uzrtually odorless, whereas 4 has apleasant, herbally fruzty aroma lzke that of an uncut lemonpolishing agents became interested in manufacturingthis epime~ to add to theirproducts for communicating to humans thesensation of "lemon-freshness."Another interesting sidelight camewhen Ritsuo Nishida read that methyl jasmonatewas known in the perfume industryas the "queen of aroma" and was one of theoldest and most widely used compounds byperfumers, extracted initially from jasmineplants Further ~esearch by Terry Acree andRitsuo Nishida revealed, however, that ofthe four possible epimers, only lR, 2s-(+)-(Z)-methyl epijasmonate, the compoundinvolved in the courtship pheromone blendand contributing to attraction, was stronglyfragrant to the human nose (Acree et a11985) The other three stereoisomers comprising97% of the "queen of aroma,"proved to be virtually odorless! The beautifulfragrance attributed for years to themethyl jasmonate actually came from animpurity present at about 3% in the methyljasmonate, and this impurity is the orientalfruit moth courtship phe~omone component!Who would have thought that mothand human courtship fragrances would beso closely intertwined, and both could involvea signal gathered initially from plantchemicals? We found subsequently thatmales lacking the hairpencil compound@)-ethyl cinnamate can imbibe it fromsugar water and incorporate it into theirhairpencils for use as a pheromone, anexample of a reflected chemical signal(Nishidaet a1 198 5,Lofstedtet a1 1989).The responses of insects receiving soundemissions must be assessed in the samefashion to understand which elements aremost critical to successful communicationFirst, a discriminating way to measure theresponse must be developed, and an exampleof this is with Teleogryllus oceanicusfemales made to walk on a styrofoampretzel and choose whether to go right orleft at choice-points during their walk. Responsesto natural emissions replayed fromaudio tape were compared in this way andshowed that the females could discriminatefrom among hybrid and even hybrid siblingmale songs, based on slight differences inthe patterns of amplitude modulation(Bentley & Hoy 1974) One useful featureof this assay is that the responder is notallowed to take itself into an area where thesound pressure level from one of the twoloudspeakers becomes greater than theother, thereby biasing the choice withoverall amplitudeThe rule in acoustic communicationthus far, whether it be for crickets, grasshoppers,drosophilid fruit flies, and eveninsects such as leafhoppers and lacewingsthat transmit their pressure disturbancesthrough a substrate such as a plant, has beenthat signals among species differ in patternsWinter 1993215