yucca moths and yucca plants: discovery of “the most...

32 AMERICAN ENTOMOLOGIST • Spring 2004

HERITAGE

Yucca Moths and Yucca Plants:Discovery of “the Most Wonderful Caseof Fertilisation”

Carol A. Sheppard and Richard A. Oliver

The relationship between yucca plants(Yucca and Hesperoyucca spp.:Agavaceae) and yucca moths

(Tegeticula and Parategeticula spp. [Lepi-doptera: Prodoxidae]) is often cited as a clas-sic example of insect–plant coevolution and,in particular, obligate mutualism (Powell1992, Thompson 1994, Price 1996, Proc-tor et al. 1996, Pellmyr 2003). Female yuccamoths exhibit morphological and behavioraladaptations that ensure pollination of yuccaplants, which have highly modified flowersthat reduce the possibility of self-pollinationor passive pollen transfer by other insects(Fig. 1). The ovaries of the plants serve as aprotected food source for the females’ off-spring, which feed on seeds that develop as aresult of the pollinating activity of the femalemoths.

This relationship was first explored bythe distinguished entomologist Charles V.Riley1 (Fig. 2) during the early 1870s, abouta dozen years after the 1859 publication ofthe Origin of Species. At the time, CharlesDarwin and Alfred Russel Wallace’s revolu-tionary theory of descent with modificationwas undergoing vigorous debate, in partbecause scientists questioned the significanceof the role played by natural selection. Inaddition, most older naturalists (i.e., thoseof Darwin’s generation) clung to natural the-ology, which held that organisms were placedon earth for man’s benefit and that theircountless and wondrous adaptations evi-

denced the Creator’s handiwork. In contrast,Darwinian theory, and natural selection inparticular, removed all supernatural expla-nations in accounting for the diversity of lifeon earth and thus was problematic for manynaturalists with strong religious beliefs(Mayr 1982, 1991; Ruse 1982; Bowler 1989;Moore 1993).

This transitional phase in the scientificcommunity’s conception of the natural worldwas articulated by Riley in his earliest publi-cation on the yucca moth:

Of late years, and more especially sincethe publication of Mr. Charles

Darwin’s interesting work on thefertilization of Orchids,2 we have cometo understand more and more theimportant part which insects play inthe fertilization of plants; and the oldidea, that color and perfume in flowerswere intended for man’s especialpleasure, is giving way to the morenatural and philosophic view that theyare useful to the plants by attractingthe needed insects. (Riley 1873a, p. 57)

1For biographical treatments of Riley, reputedly themost important entomologist of the 19th century(Sorensen 1995), see Smith and Smith (1996) andSheppard and Weinzierl (2002).

Fig. 1. Illustration by C. V. Riley (1892b) of Tegeticula yuccasella (Riley) ovipositing on Yuccafilamentosa. In this rendering, Riley shows a portion of the petals (top) removed to demonstratethat the anthers point away from the stigmatic tube, making self-pollination highly unlikely.

2Riley made reference here to Darwin (1862).

AMERICAN ENTOMOLOGIST • Volume 50, Number 1 33

The genus Yucca (Family: Agavaceae) comprises plantsnative to North and Central America, with approximately halfof all yucca diversity occurring in Mexico. Several yuccaspecies have been cultivated in Europe since the late 1500s.Y. filamentosa, the pollination of which first captured theattention of Engelmann and Riley, has been naturalized inthe United States for more than 150 years. Yuccas aregenerally associated with arid biomes (chaparral, grassland,shrub desert), but the two southernmost Yucca spp. occur inrainforests. According to Pellmyr (2003), Yucca taxonomy andsystematics are in a state of flux and badly in need ofrevision, a formidable task because of a historically confusednomenclature, use of horticultural material and cultivars ofunknown origin, a lack of herbarium material, and a dearthof phylogenetic analyses.To view Yucca spp. images and distribution maps for theUnited States, Puerto Rico, and the Virgin Islands, consult theUSDA, NRCS PLANTS Database (http://plants.usda.gov). To view a color image of one of the first yucca mothspecimens collected by Engelmann in 1872, see the webpage by Pellmyr (http://www.webpages.uidaho.edu/~pellmyr/Firstmoth.htm); a black and white image appearsin Pellmyr (2003).

Riley was 27 years old when he read thispaper before his fellow members of the Acad-emy of Science of St. Louis.3 A strong advo-cate of Darwinian theory, Riley rebutted thecriticism of many of his colleagues who ques-tioned the validity of the theory. He toutedthe yucca plant–yucca moth relationship asa remarkable example of Darwinian evolu-tion; and, as the decades passed, Riley’s nameand this storied case of plant–insect mutual-ism became conjoined. Yet Riley might neverhave become interested in the yucca moth,had it not been dropped in his lap by GeorgeEngelmann (Fig. 3), a senior botanist andcolleague of Riley who was far from a Dar-win enthusiast (Goldstein 1989).

In this article, we revisit the interactionsof Engelmann and Riley when both were ac-tive members of the Academy of Science ofSt. Louis. We then focus on Riley’s elucida-tion of the relationship between yucca plantsand yucca moths, an endeavor that engagedhim for over two decades and captured theinterest of foremost scientists of the 19thcentury, including Asa Gray, Charles Dar-win, and William Trelease. We close with abrief consideration of the more recent find-ings on this topic.

Engelmann: Biographical sketch andinitial yucca observations

George Engelmann (1809–1884) wasborn in Frankfort-on-the-Main, Germany(Gray 1884, Gray and Trelease 1887, Bek

1929, White 1902, Long 1995), and at-tended the University of Heidelberg and Ber-lin University, receiving his M.D. from theUniversity of Wurzberg in 1831. Engelmann’sfriends and classmates included AlexanderCarl Braun (1805–1877) and Karl FriedricSchimper (1803–1867), both of whom be-came well-known botanists, and the re-nowned naturalist Louis Agassiz (1807–1873), who achieved great stature as a popu-lar lecturer and founder of the Museum ofComparative Zoology at Harvard Univer-sity.

In 1835, Engelmann relocated to theUnited States at the request of his uncles,who wished to invest in land in the Missis-sippi Valley. Engelmann’s emigration facili-tated his desire to explore the New World,following in the tradition of famed natural-ist and explorer Alexander von Humboldt,who sought to construct a holistic view ofnature by teasing out relationships amongthe natural and physical sciences (Long1995).

Engelmann eventually settled in St. Louis,Mo., where he became a leading physician.His medical career notwithstanding, Engel-mann is a towering figure in the early days ofAmerican botany. Regarded as the indisput-able authority of the Cactaceae andConiferae, he also earned recognition as theprincipal American authority on yuccas, aga-ves, and American Vitis spp. (Rodgers 1944).He traveled to Europe and various regions

of the United States, conducting field excur-sions and visiting herbaria; and he producednumerous botanical publications, many ofthem taxonomic/natural history mono-graphs.

In an era marked by U.S. exploration andexpansion, Engelmann received botanicalspecimens for study and classification fromnumerous federally funded expeditions. Formore than 40 years, he contributed to theassociated official reports including: C. C.

Fig. 2. C. V. Riley, ca. 1873.

3Organized in 1856, the academy resided in thebustling portal city to the West and was the center ofMissouri’s scientific activity (Hendrickson 1966).

Fig. 3. G. Engelmann (from Gray and Trelease1887).


Parry’s of the Colorado region and the RockyMountains (Parry named a peak in theRocky Mountains for Engelmann), F. V.Hayden’s of the upper Missouri Valley, C.Wheeler’s of the territory west of the 100thmeridian, A. W. Whipple’s of the region west-ward along the 35th parallel, J. H. Simpson’sof the territory of Utah, J. C. Ives’s of theColorado river basin; and the reports of theU.S. and Mexican Boundary Survey and thePacific Railway Expeditions (Gray 1884;Gray and Trelease 1887; Bek 1929, part 1).In addition to his work as a physician andbotanist, and in true Humboldtian fashion,Engelmann also cultivated a serious interestin meteorology and published articles on thealtitude of Pike’s and Long’s peaks in Colo-rado (Hendrickson 1966).

Engelmann first took notice of the genusYucca in 1842, when acquaintances sent himspecimens from Texas, northern Mexico, andNew Mexico (Engelmann 1873). In the yearsthat followed, he paid particular attentionto yuccas on his U.S. and European expedi-tions. In Europe, he was “first struck withthe ‘fact’ that ‘[y]uccas do not bear fruit’,”whereas he had seen yucca fruits in someU.S. collections and observed yucca seedpods in St. Louis gardens (Engelmann 1873).He speculated that European yuccas weresterile because of incomplete development ofthe flowers or problems arising from self-fertilization. However, from the outset heconsidered the chances of self-fertilization tobe remote because the anthers of the yuccaflower point away from the stigmatic tube,making it extremely difficult for pollen tofall into this tube (Engelmann 1872, 1873).Engelmann also noted that the glutinousnature of yucca pollen rendered it highlyimprobable that the pollen could leave theanthers unassisted.

After observing yucca flowers in theUnited States, Engelmann concluded that“insects (in these night-blooming flowers, ofcourse, nocturnal insects) must be the agentswhich introduced the pollen into the tube”(Engelmann 1873, p. 19) and suspected thatthe pollinator was “a white moth of the ge-nus Tortrix” (Engelmann 1872, p. 33). Inthe summer of 1872, Engelmann passed tohis young colleague, C. V. Riley, the insectshe found most frequently in the yucca flow-ers at night.

“Such Theories Would Lead Us Astray”Given Riley’s entomological expertise and

proximity to Engelmann, it is not surprisingthat the latter chose to give the yucca mothsto his associate for further investigation.History, however, marks the irony of theevent because Riley quickly recognized theunique and mutually adaptive significanceof the yucca plant–yucca moth relationship,

which he accounted for unequivocally ac-cording to Darwinian principles.

A year before Engelmann gave Riley theyucca moth specimens, the two men sparredover Darwinian evolutionary theory at the 6March 1871 meeting of the academy. Wemight imagine the scene: the august Dr. En-gelmann, then age 62, a founding memberof the academy and its first president (towhich post he was re-elected 15 times) ver-sus the young challenger, Riley, age 26, serv-ing at his third meeting as the academy’s re-cording secretary (Goldstein 1989). Accord-ing to the academy’s Journal of Proceedings,the exchange unfolded as follows. Dr. Engel-mann presented three specimens of a weedthat were extraordinarily similar despite theirhaving been collected from St. Louis County,Mo., France, and Italy. This prompted Rileyto ask Engelmann how he accounted for thefact that many European plants and weedshad become well established in the UnitedStates, yet very few American plants had be-come established in Europe. Riley added thatinsects in Europe vs. the United States exhib-ited the same pattern. Engelmann replied thatno theory was necessary to explain thesephenomena; the answer lay in the “fact thatsome plants are more vigorous than others”(Anonymous 1868–1877, p. xlii). Then, theProceedings relate that

Mr. Riley undertook to account for iton the Darwinian theory, that fromthe greater competition and strugglefor existence that had gone on inEurope under the civilized conditionsof man, their species were, many ofthem, better able to thrive under similarconditions here than our ownindigenous species. Dr. Engelmannfeared such theories would lead usastray. (Anonymous 1868–1877, pp.xlii–xliii)Also present at the meeting was Dr.

Frederick Adolphus Wislizenus, age 61, an-other founding member of the academy andEngelmann’s long-time friend, fellow medi-cal practitioner, meteorologist, and botani-cal explorer. Wislizenus attributed the spreadof European plants to the greater space af-forded them in the New World, making nomention of Darwinian theory.

This sparring match marks the first ofseveral recorded between Riley and oldermembers of the Academy of Science of St.Louis, who were grappling with Darwin’srevolutionary theory. The historianGoldstein (1989) characterized Riley as “analmost combative Darwinian” who intendedto “inject Darwinian theory into Academicdiscussions whenever possible.” Given thatRiley joined the Academy in 1868, Goldsteinspeculates that similar disputes may havetaken place but went unrecorded by the

academy’s earlier recording secretaries be-cause of implicit or explicit censorship.

In his role as president of the academyfor 1876, Riley was the first to speak on thetopic of evolution (Hendrickson 1972). Hepresaged the widespread acceptance of Dar-winian theory in his presidential address,following the custom of reviewing the previ-ous year’s scientific developments:

The visit of Prof. [T. H.] Huxley toAmerica must be looked upon as anevent in the progress of evolution inthis country—…because he broughtin succinct form, and so conspicuouslybefore the public, some of the bestarguments in favor of the doctrine. Allgreat truths that oppose longestablished popular belief must needsbelong to the few when nascent…inmy humble opinion, the idea ofevolution is founded on fact, and, likea gem freed from the deposits whichfor ages have hidden its lustre, willshine all the brighter as the obstacleswhich surround it are removed by thelight of truth. (Anonymous 1868–1877, p. ccxlvii)By this time, even Engelmann may have

moved somewhat closer to Riley’s thinking.At a March 1876 meeting of the academy,Engelmann commented on the ability of cer-tain hybrid oaks to propagate “when re-moved from the struggle for existence” pre-sented by more hardy competitors(Hendrickson 1972).

Riley Embarks on his Yucca Moth–Yucca Plant Studies

Riley reputedly pursued his research withenormous zeal and dogged determination(Smith 1992, Smith and Smith 1996), traitsevidenced by his record of investigations ofyuccas and their associated insects. He set towork immediately, examining the behaviorof Engelmann’s little white moth; and withinthree months, he reported his initial findingsin a paper read before the 21st meeting ofthe American Association for the Advance-ment of Science (AAAS) in August 1872.4

The next month he presented his paper tothe members of the academy; and a few

4This particular AAAS meeting is of historicalsignificance in that the Entomological Sub-section wasformed, at whose first meeting was discussed SamuelH. Scudder’s “Revision of American Butterflies,” putforth in advance of his Butterflies of North America.According to a report of the meeting, “There was aunanimous expression of regret anddisapprobation…at the wholesale and radical changesproposed by this distinguished author in the genericand specific names of the butterflies.…The feeling wasmanifested by all, that changes so radical and sosweeping in the received nomenclature were uncalledfor, and would prove of great detriment to the studyand popularity of this department of entomology.”(Anonymous 1872, p. 183)


months later it was published in theacademy’s Transactions (Riley 1873a), pre-ceded immediately by a report by Engelmann(1873) on the genus Yucca.

In his paper, Riley acknowledged his debtto Engelmann for having drawn his atten-tion to the fact that filamentose-leaved spe-cies of yucca “must rely on some insect orother for fertilization” (Riley 1873a, p. 59).Riley erected the genus Pronuba for the yuccamoth, described both sexes and the larvalstage of Pronuba (now Tegeticula) yuccasella(Riley), and related what he had ascertainedof the insect’s natural history. He observedthat male and female moths meet and copu-late on the yucca flower. The female thenscrapes the pollen from the anthers with apair of highly specialized, prehensile, hairytentacles (Fig. 4), modifications of the basaljoint of the female’s maxillary palps not foundin males (Fig. 5). As he explained, “With hermaxillary tentacle [palp]…she collects thepollen in large pellets, and holds it under theneck and against the front trochanters…shesometimes carries a mass thrice the size ofher head” (Riley1873a, p.60). Once the fe-male has collected a ball of pollen, she “clingsto the top of the pistil, bends her head, thrustsher tongue into the stigmatic nectary andbrings the pollen-mass right over its mouth”(Riley 1873a, p.60).

The female Tegeticula moth was the onlyspecies that Riley found to be actively en-gaged in pollinating yucca flowers,5 but dur-ing the 1872 season, he was unable to wit-ness any females in the act of oviposition. Heknew of no Lepidoptera that oviposited bypuncturing the flesh of fruit, and found noevidence that any punctures led to eggs northat eggs were deposited externally. Giventhis, Riley hypothesized that T. yuccasella eggswere inserted into the fruit through the pol-len tubes. In addition, because he had exam-ined “hundreds of [yucca seed] capsules”around St. Louis and southern Illinois, ofwhich “not more than four or five percentwere uninfested,” he conjectured that ovi-position immediately followed, not preceded,pollination (Riley 1873a, p. 62).

Reportedly Riley’s AAAS paper was “lis-tened to with marked attention, and was fol-lowed by an animated discussion” in whichAsa Gray, then out-going AAAS President,and others took part (Anonymous 1872).Gray, a renowned Harvard University taxo-nomic botanist and the most influentialAmerican advocate of Darwinian theory(Dupree 1959, Ghiselin 1969), was held inhigh esteem by Riley. A few years later, Rileysent Gray yucca moth cocoons to help test

his most important yucca-related hypoth-esis: that yucca moths serve as the sole polli-nators of yucca plants.

Tegeticula yuccasella Pupae DescribedRiley was anxious to procure T. yucasella

pupae for taxonomic purposes. Because theinsect overwinters in the larval stage, he at-tempted to accelerate metamorphosis (i.e.,break diapause) by maintaining larvae atabout 80o F throughout the winter of 1872–1873. His efforts met with modest success,but in June 1873, he wrote a publicationdescribing the chrysalids of the male and fe-male moth (Riley 1873b).

Meanwhile, the report of his yucca mothpresentation at the AAAS meeting hadpiqued people’s interest, such that in his fifthannual report as Missouri state entomolo-gist, Riley (1873c) provided excerpts fromletters sent by several readers, relating per-sonal observations of seed production, orlack thereof, by yucca plants. The letters,which had been published in various peri-odicals, represented divers geographic re-gions in the United States and Europe. ThusRiley began the long process of amassinginformation relevant to his hypothesis aboutthe unique pollinating activity of T.yuccasella.

Oviposition and Pollination: Revision ofHypotheses

Riley worked sedulously and with greatcare, as Pellmyr has commented:

[I]n contrast to the records of most ofhis contemporaries, there are very fewinaccuracies in [Riley’s] accounts,simply because of his reliance onempirical observation. In this, hearguably belonged in the exclusivegroup of exceptional naturalists with

5Riley (1873a) listed several other non-pollinatinginsects (Coleoptera and Heteroptera species) that hehad collected from yucca flowers.

Fig. 4. Female T. yuccasella gathering pollen(from Riley 1892b).

Fig. 5. Illustration by Riley (1873a) from his first yucca moth publication, showing key featuresof adult T. yuccasella: (a) denuded head of female with (1) pollen mass, (b) female maxillary palpwith (c) tubercle, (j) ovipositor.


whom he regularly corresponded, suchas Charles Darwin, Alfred RusselWallace, Henry Walter Bates, ThomasBelt, Fritz and Hermann Müller, andAsa Gray. (Pellmyr 2003, p. 37)By the close of the 1873 summer season,

Riley reported his further observations ofthe oviposition behavior of T. yuccasella. Asnoted earlier, he had initially hypothesizedthat oviposition occurred after pollination,and that the ovipositor was not used to punc-ture the fruit of the plant (Riley 1873a). Thesehypotheses proved incorrect.

In collaboration with Engelmann, Riley(1873d, 1874) discovered that the female’sovipositor does, in fact, puncture the ovary.Over time, the aperture of the puncture formsa depression and, as the fruit enlarges, theoviposition passage becomes obliterated,which explains why Riley was misled by hisinitial observations. Indeed, only by dippingthe pistil in ink a day or two after oviposi-tion and examining the stained tissue undera magnifying lens was he able to trace thepassage created by oviposition that led tothe egg locus. Riley explained that he couldobserve the oviposition of hundreds of eggsbecause once the female selects a suitable siteand her ovipositor penetrates the pistil, “thewhole perigon6 may be detached, some ofthe encumbering petals and stamens re-moved, the insect brought within focus of agood lens and all her movements observedto the greatest advantage, without disturb-ing her” (Riley 1873d, p. 620).

Also contrary to his original hypothesis,Riley determined in the second season thatthe female first oviposits, and then, “nosooner is the ovipositor withdrawn into theabdomen than the moth runs up to the topof the pistil, uncoils her pollen-bedecked ten-tacles, thrusts them into the stigmatic open-ing, and works her head vigorously…up anddown” (Riley 1873d, pp. 620–621). Headded that the female may deposit two, three,or more eggs before pollination occurs, andhe suspected that “the converse of this isequally true” (Riley 1873d, p. 621).7

At the conclusion of his paper, he stronglyreiterated his view:

[My observations this summer] haveconvinced me more than ever thatPronuba [Tegeticula] is the only insect

by the aid of which our yuccas can befully fertilized; for I have studied thisfertilization diligently night after night,without seeing any other species gonear the stigma. (Riley 1873d, p. 622)

Engelmann’s Further Studies of T.yuccasella and Yucca spp.

During the summer of 1873, while Rileypursued his yucca moth oviposition studies,Engelmann made in situ observations of T.yuccasella in various yucca species. His origi-nal drawings depict egg morphology andembryonic development of T. yuccasella con-comitant with early post-fertilization changesin the yucca ovule (Fig. 6). He observed thatthe developing embryo, with segmentation,is evident at 60–66 hours after oviposition(Fig 7). Engelmann also found that very soonafter oviposition, the young seeds “begin toswell up to three or four times the thick-ness” of the normal seeds and “are thus pre-paring the sustenance of the young larva,which feeds on one or usually both of themuntil able to attack the meanwhile more orless developed young seeds” (Fig. 7; Engel-mann 1875, p. 211; Riley 1873d).

Engelmann (1875) reported that the stig-matic opening closes immediately after thefirst night of flowering (Fig. 6). Riley (1873d)thus noted that it is during this one nightthat female T. yuccasella must pollinate theyucca and lay her eggs; subsequently, Riley(1892a) stated that, on occasion, thistimeframe could be 1–2 nights. These find-ings are substantiated in a recent review(Pellmyr 2003).

Influence of Darwin and Gray on Riley’sScientific Conceptualization

The work of Darwin strongly influencedRiley’s scientific conceptual framework. Heunequivocally supported the Darwiniantheory of species origin versus the older viewof special creation and immutable species.Moreover, Riley immediately recognized thatthe mutualism between yucca moths andyucca plants represented “one of the firstand strongest examples of evolution bymeans of natural selection” (Pellmyr 2003,p. 36). Riley openly expressed this opinionat scientific meetings and in publications, andoften interpreted his entomological findingsin light of evolutionary theory. For example,

6"Perigon” (Latin form of “perigone”) refers to aflower in which the calyx and corolla (collectivelytermed the perianth) are not clearly differentiated.(Swartz 1971)

7The latter scenario, which has never been documented(Olle Pellmyr, personal communication), wouldaccount for the pollination of capsules that contain nolarval Tegeticula nor evidence of ever havingcontained any.

Fig. 6. Engelmann’s unpublished sketches, dated July1873 (Missouri Botanical Garden, WilliamTrelease Professional Papers, Agavaceae, Box 8). Far left, T. yuccasella egg, 60–66 hours post-oviposition in Y. filamentosa. The elongate egg (1.5 mm long, 0.08 mm diam) holds the developing“curled up larva” at one end. Sketches at right compare relative size and form of an ovule 60–66hours post-ferilization (top) vs. an unfertilized ovule from a bud (bottom). In the former, thestigmatic tube is closed, whereas in the latter it is still open.


in his first yucca moth publication, he wrote:We have in this little moth a remarkableadaptation of means to an end. Thereis between it and its food-plant amutual interdependence which at onceexcites our wonder, and is fraught withinteresting suggestions to those whoare in the habit of reasoning fromeffect to cause. Whether we believe, asI certainly do, that this perfectadaptation and adjustment have beenbrought about by slow degreesthrough the long course of ages, orwhether we believe that they alwayswere so from the beginning, they areequally suggestive of that same law andharmony so manifest throughout therealm of Nature. (Riley 1873a, p. 63)In this same paper, Riley elaborated on

the adaptive advantages to the yucca plantand the yucca moth: The plant was guaran-teed pollination with a high degree of prob-ability, and it lost only a few seeds in return;the insect received a safe larval feeding site.As a good Darwinist (and with clear refer-ence to plant–insect coevolution), Riley ac-counted for this arrangement on the basis ofgradualism, postulating that this mechanismcould have resulted in the development ofthe “peculiarities” present in the plant andthe insect:

[I]t is quite easy to conceive, onDarwinian grounds, how both thesecharacters may gradually have beenproduced in the course of time fromarchetypal forms which possessed

neither. These peculiarities are,moreover, mutually and reciprocallybeneficial, so that the plant and theanimal are each influenced andmodified by the other, and the samelaws which produced the beneficialspecialization of parts would maintainthem by the elimination of all formstending to depart from them. (Riley1873a, p. 63)Riley recognized the significance of cross-

pollination largely because of Darwin’s workin this area.8 In a paper summarizing his manyyears of research on the relationship betweenyucca moths and yuccas, Riley made refer-ence to several of Darwin’s botanical publi-cations from the 1860s:

The importance of insects as agents incross-fertilization was scarcelyappreciated, however, until the lateCharles Darwin published the resultsof his researches on Primula, Linum,Lythrum, etc., and his elaborate workon the fertilization of orchids. Thepublication of these works gave toflowers a new significance and to theirstudy almost as great an impulse asdid his immortal “Origin of Species”to the general study of biology.9 (Riley1892a, p. 101)Riley’s early thinking about pollination

also drew upon the work of Asa Gray. Riley

cited Gray’s 1872 work, How Plants Behave,in his first yucca moth publication to sup-port the fact that, typically, the flowers ofangiosperms are “curiously and elaboratelyconstructed so as just not to do of them-selves what must necessarily be done forthem in order to prevent degeneracy or ex-tinction of the species,” thereby facilitatingcross-pollination (Riley 1873a, p. 57). Hepointed out that many plants depend onother organisms to transfer their pollen fromplant to plant and stated that the yuccas hehad been studying “seem to depend for as-sistance, so far as we now know, on the singlelittle Tineid [Prodoxid] which I have de-scribed” (Riley 1873a, p. 58).

Evidence for the ExceptionalPollinating Activity of T. yuccasella

Whether Riley anticipated that his yuccamoth research would span two decades andraise the hackles of many a colleague is moot;undoubtedly, he was cognizant from theoutset of the enormity of his claim, which hehad couched in cautious terms in his firstyucca moth publication (Riley 1873a). Headmitted that he knew of no other case inwhich a single insect species was solely re-sponsible for pollinating a given plant, andhe had witnessed the discussion that his pre-sentation provoked among his respectedcolleagues at the 1872 meeting of the AAAS.

Riley based his hypothesis on several linesof evidence: His observation that female T.yuccasella was the only insect found activelypollinating the flower; the appearance of thefemale’s highly modified maxillary palps andher behavior regarding pollen collection anddeposition; the fact that exclusion of the fe-male moth (by netting) did not result in yuccaself-pollination; and the glutinous nature ofyucca pollen. He also had additional infor-mation from Engelmann, Gray, and others,who reported that yucca plants introducedto northern United States and European gar-dens where the yucca moth is absent fail toyield fruit and seed, but that pods of wildyucca (Yucca angustifolia, [now Y. glauca]from the Black Hills of Colorado, Y. rupicolafrom Texas, and Y. whipplei from Califor-nia) show “unmistakable holes of egress ofthe larvae” (Riley 1873a, p. 64).

In his first yucca publication, Riley ex-pressed hope that “the next blooming-sea-son of our Yuccas will find other eyes thanmy own watching [Tegeticula’s] ways andmethods,” because much remained to be dis-covered about the insect (Riley 1873a, p. 63).Such professional generosity may soundsomewhat specious, given the potential im-

Fig. 7. Engelmann’s unpublished sketches, dated July 1873, of T. yuccasella development inYucca flaccida (Missouri Botanical Garden, William Trelease Professional Papers, Agavaceae,Box 8). Top left, larva (1 mm in length) at 60–66 hours after oviposition, showing segmentation;top and lower right, seeds with spherical holes from feeding larvae, which eclose from the egg4–5 days after oviposition. Lower left, seed pod.

8Darwin’s influence in this regard has been noted bymany; see Ghiselin, 1969, Faegri and van der Pijl1971, Bowler 1990, Proctor et al. 1996.

9Riley (1882) articulated the same thoughts in anearlier address to the Biological Society of Washing-ton.


pact of such an extraordinary discovery,but perhaps he believed that the opportu-nity to obtain corroborating evidence out-weighed the possibility of another scientiststealing his thunder. In any event, the mostmeaningful publications on yucca mothsand mutualism would come from Riley, andT. yuccasella and other insects closely asso-ciated with yuccas would continue to be apart of Riley’s scientific investigations for20 years.

Riley Sends T. yuccasella Cocoons toColleagues

Riley hypothesized that if T. yuccasellawere the sole pollinator of yuccas, then yuc-cas that never produce fruit should be foundin areas where yuccas had been transportedbut the moth was absent. He further rea-soned that if his hypothesis were correct, in-troducing yucca moths to areas devoid ofmoths would yield fruit-producing yuccas.Riley proposed to test the validity of his hy-pothesis:

It is my intention to obtain a largenumber of cocoons this yearand…distribute them among thosewho grow the yucca in those parts ofthis country or in Europe where seedis not produced. (Riley 1873d, pp.622–623)During 1873 and 1874, Riley sent larvae

(the numbers are shown in parentheses) intheir cocoons to Asa Gray, Cambridge, Mass.(150); botanist Jules-Emile Planchon,10

Montpellier, France (100); botanistHermann Müller, Lippstadt, Germany (250);Charles Darwin, Down, England (100); lepi-dopterist Henry Tibbats Stainton,Mountsfield, England (50); and MeadeWoodson (whom Riley described as a “greatadmirer” of Yucca), Kansas City, Mo. (100)(Riley 1881). Riley requested that the co-coons be buried in the ground near yuccaplants. Darwin, having no yucca plants athis home, sent his insects to Joseph DaltonHooker, noted botanist and director of theRoyal Botanic Gardens, Kew, England.11

Müller’s moths hatched, but no ornamentalyuccas were in flower at the time (Pellmyr2003) and the results of the remaining ex-periments are unknown.

We do know that Riley was mailing yuccacocoons to naturalists until just before hisaccidental death in 1895. Extant letters12 that

Riley wrote from 1893 to 1894 indicate thatthe following correspondents were sent co-coons: Professors M. Valery Mayet and M.Viala (Ecole Nationale Agronomique,Montpellier, France), D. Morris (AssistantDirector, Royal Botanic Gardens, Kew, En-gland), T. Hanbury, FRS (Ventimiglia, Italy),and Professor G. L. Goodale (Cambridge,Mass.). Riley asked that the cocoons beplaced around yuccas in the filamentosegroup, or those which typically bloomed inMay and June.

Criticism and Confusion: True YuccaMoths, Bogus Yucca Moths, and More

In the years following his first publica-tion on the yucca moth, T. yuccasella, Rileyencountered much criticism, from botanistsand entomologists, who questioned his sci-entific description of the moth, his observa-tions of its behavior, and his conclusionsabout its unique relationship with yucca. Aswe will discuss, many of the disagreementsarose because naturalists unwittingly con-fused T. yuccasella with other insect species.Regardless of the reasons underlying a givencontentious issue, Riley’s record of scientificpublications and presentations attest to theconsiderable time and energy he expendedcountering those who impugned the accu-racy of his findings.

Riley vs. ZellerThe first dispute occurred when Professor

P. C. Zeller, an entomologist from Prussia,questioned Riley’s assertions that larval yuccamoths lack anal prolegs and that the adultfemale moth collects pollen with her maxil-lary tentacles. (Pellmyr [2003] has pointedout that Zeller’s experience with yucca mothswas “limited to three pinned specimens givento him.”) Zeller proposed that the female’s“tongue” (proboscis) could, by itself, accom-plish the task of gathering pollen.

In his rebuttal, a clearly vexed Riley (1876)reiterated his published statement, in whichhe had indicated that all of his conclusionshad been based on careful, repeated observa-tions conducted over the course of three sum-mers. He also took umbrage at Zeller’s lastsentence13 : “In my opinion, Riley, while mak-ing his most interesting discovery, hasn’t seeneverything yet; and other observers will benecessary to explain satisfactorily the curiousgoings-on during reproduction of the moth.”

Riley wrote:…as for the reflection in the lastsentence which I have quoted, I amvain enough to believe that there is no

other provocation for it than a certainill-will on my critic’s part. It comes, too,with all the less grace from one whoconfessedly made hasty and incorrectobservations on the position of thespecies. From one less honored andesteemed, such captious remarkswould have received no notice at myhand. (Riley 1876, p. 326, fn.)In response to his publication, Riley received

a personal letter from Zeller14 that began,You do me a great injustice by thinkingthat I wrote the notes on Pronuba[Tegeticula] with any ill feeling towardsyou. I am quite convinced that ifyou had written them in the samemanner in opposition to me I wouldnot have felt injured at all, but wouldonly have felt impelled to repeat myobservations, for, least of all, do I holdmyself to be infallible, but am of theopinion that every observation mustgo through the crucial test of repetitionby several observers before it canbe regarded as correct andincontrovertible.In his letter, Zeller stated that he had asked

Stainton for his opinion of the polemic.Stainton replied that Zeller’s statements re-garding Riley’s observations were “trulyZellerian, i.e., a little too dogmatic. You saythat it is quite impossible that Yucasella cando what R. says it does.” Zeller explained toRiley that because “even great investigators,as for example Réaumur, have made mis-takes,” the remarkable features that Rileyhad described caused him to question Riley’sobservations. In closing his letter, Zellerwrote: “As you have up to this time onlyshown a kind spirit toward me, I cannotimagine at all how I could return it with ill-will.” Whether Riley responded to the letteris unknown.

Riley vs. BollJacob Boll of Dallas, Tex., attempted to

refute Riley’s oviposition and pollinationfindings with assertions based on cursoryobservations and a poorly designed experi-ment. Moreover, Boll had never bothered toread Riley’s yucca moth publications andeventually admitted to Riley that he “knewabsolutely nothing of [Riley’s] writings onthe subject except what he learned throughProf. Zeller’s notice” (Riley 1877, p. 572).Unlike Riley, Boll (1876) made no effort toobserve the behavior of the moth under fieldconditions, but instead he had caged femaleyucca moths with cut yucca flowers that hadapparently already been fructified. Boll alsoput forth the fallacious notion that the fe-

10Planchon collaborated with Riley on grapephylloxera research (Smith 1992).

11Darwin to Hooker, 7 April 1874 (CambridgeUniversity Library, classmark DAR 95:321).

13"Meines Erachtens hat Riley bei seiner höchstinteressanten Entdeckung noch nicht Alles gesehen,und noch andere Beobachter werden erforderlich sein,um die sonderbaren Vorgänge bei der Fortpflanzungder Motte ganz genügend zu erklären.”12Missouri Botanic Gardens: William Trelease papers.

14Missouri Botanical Garden, William Treleaseprofessional papers, Box 8, folder 1; letter dated 3April 1876.


male yucca moth used pollen or stamen hairsto seal her oviposition puncture. He furtheropined that the exclusive fertilization of yuc-cas by yucca moths had not been provenand that “internal fertilization, if one shouldtake this for such, would belong to the realmof fable” (cited in Riley 1877, p. 573). Rileycalled Boll’s publication “palpably superfi-cial and erroneous” (Riley 1877, p. 570) andconcluded with a biting rejoinder:

Mr. Boll should increase his knowledgeby perusing what has been written onthe fertilization of flowers by insects.He should also learn something morethan he has done in this instance of asubject he intends to treat, andespecially of observations which he hasundertaken to criticize. Investigations,however instigated, should be carriedon, not under the warping influenceof individual motive, but solely for thelove of truth and knowledge. (Riley1877, p. 573)Riley vs. ChambersAnother clash occurred in 1877, when V.

T. Chambers published his account of theTineina of Colorado in Hayden’s Geologicaland Geographical Survey of the Territories.Chambers (1877) claimed that he had dis-covered a “spotted version” of adultPronuba [Tegeticula], thereby bringing intoquestion Riley’s taxonomic description of themoth’s front wings as “uniformly silvery-white.” Chambers sent all of his specimensto Hagen at Harvard, a few of which wereforwarded to Riley, whose incredulity uponexamining the specimens is evident in thispassage:

…what is more remarkable, they areone of Mr. C.s’ own describedspecies—[H]yponomeuta 5-punctella.Of the six specimens submitted to me,there was but one Pronuba[Tegeticula], and that was immaculate,as the species always is. …I have rearedupward of 500 specimens of Pronuba[Tegeticula], and have it from SouthCarolina, Texas, California, Coloradoand Missouri, and there is never thefaintest tendency to maculation. (Riley1877, p. 569)To make matters worse, Riley was asked

by Hagen not to denude the specimens and,as a result, was incorrect in his identificationof the “spotted” Tegeticula as H. 5-punctellaChambers. Riley would eventually be in aposition to correct his error and, in so do-ing, erect a new genus.

As fate would have it, in July 1879, Rileyhad occasion to pass through Dallas, where-upon he visited Boll, who had bred what hethought to be T. yuccasella from the flowerstem of Y. rupicola (Riley 1880a). Upon ex-amining the insects, Riley determined that

Boll’s moths were not, in fact, T. yuccasella,but a closely related species. One month later,Riley attended the annual meeting of theAAAS held in Saratoga, N.Y. There he an-nounced to the Entomology Club that hehad likely discovered the cause of the dis-crepancy between his findings and those ofBoll and Chambers—namely, that the mothsthe latter two had been observing wereProdoxus decipiens Riley15 (Lepidoptera:Prodoxidae) (Riley 1880b) (Fig. 8). Al-though very similar in appearance to theyucca moth, P. decipiens oviposits on theflower stem and is not a yucca pollinator(Fig. 9).

Ever the careful observer, Riley had re-ported the presence of the then-unnamed P.decipiens in his first yucca moth publication.He wrote of finding “a smaller white,apodous larva” in the pulp of Y. aloifolia,“sometimes in considerable numbers,”which “occasionally gnaws into the seed fromthe outside,” adding that this larva has hy-menopterous affinities and that its leglesscharacter “will at once distinguish it” fromlarval Tegeticula (Riley 1873a, p. 59, fn.).

In the publication that derived from hispresentation at the 1879 AAAS meeting,Riley (1880a) pointed out the various ways

in which Prodoxus differed from Tegeticula:The adults show an inclination to macula-tion, and the female lacks the characteristicmaxillary tentacles; the larvae are legless; andthere are differences in the chrysalis, malegenitalia, and habits of the two species.

In addition, having been unable to exam-ine Chambers’s “supposed spottedPronubas [Tegeticula], the types of which arein the Cambridge [Harvard University] mu-seum,” Riley conjectured that the specimenswould turn out to be Prodoxus, based onChambers’s description of its habits and hisdepiction of the pattern of maculation. Heopenly expressed his displeasure with thecarelessness of Chambers’s scientific work:

We assumed that Mr. Chambers hadmade a proper reference in describinghis Hyponomeuta 5-punctella, but weare now perfectly satisfied that he hadnot, since many specimens ofProdoxus agree exactly in maculationwith his description and figure. Morecareful study plainly shows thatProdoxus does not even belong to thesame family, but must be placed withPronuba [Tegeticula] in the Tineidae.…Mr. Chambers…has published some14 pp. of matter containing many

Fig. 8. Female Prodoxus decipiens Rileyovipositing in a yucca flower stalk (from Riley1892a).

Fig. 9. Riley’s rendering of a yucca flowerstem with (a) oviposition scars and (b) pupalexuvia of P. decipiens (from Riley 1892a).

15The genus name derives from the Greek, “judging ofa thing prior to experience” (Riley 1880b, p. 155,footnote).


interesting but irrelevant facts, andmore that is funny than convincingwhen it comes to the point at issue. Hebrings forward no fresh evidenceexcept such as he calls circumstantial,and admits that he does not ‘pretendthat they [the arguments] areconclusive of the question, especiallywhen opposed to the positivestatements of so competent anobserver’. (Riley 1880a, p. 143)Recent research has revealed another

twist in the P. decipiens story. For many years,this species was considered synonymous withP. quinquepunctellus (Chambers) (Chambers1875). However, Althoff et al. (2001) haveshown P. decipiens to be a distinct species,ranging from the eastern United States west-ward to central Texas; Riley’s P. decipiensseries derived from South Carolina.P. quinquepunctellus occurs in the southwest-ern United States into the northernGreat Plains states; Chambers’s P.quinquepunctellus specimens were fromColorado. These two sister taxa are sympa-tric in central Texas (Althoff et al. 2001). It isalso worth noting that although Riley (1880a)stated that a few P. decipiens larvae are foundin the fruit, this observation has not beencorroborated and whereas certain prodoxidsare gall-makers in fruit, no species is knownto do both (Olle Pellmyr, personal commu-nication).

Riley vs. MeehanMr. Thomas Meehan, botanist and edi-

tor for Gardeners’ Monthly, also attendedthe 1879 AAAS Saratoga meeting. He gave apaper in which he reported that although hehad observed T. yuccasella to be abundantat the time of flowering of Y. glauca, the plantnever produced fruit unless he artificiallypollinated it. Based on this, he concluded thatT. yuccasella must not pollinate Y.filamentosa as Riley had claimed.

Riley (1877) had known since 1876 thatT. yuccasella adults typically appear 2–3weeks after Y. glauca blooms and that thisyucca species rarely produces seed. He in-formed Meehan at the AAAS meeting thatthe moths in question most likely were P.decipiens. Riley added that the one time hedid observe a few seeds in Y. glauca in St.Louis was when it had bloomed a bit laterthan usual; upon inspection, he found thatthe capsules contained T. yuccasella larvae.As if to further frustrate Riley, Meehan, likeChambers, had sent his specimens (living lar-vae) to Hagen,16 who initially thought themto be Coleoptera; once they had pupated,Hagen realized his error. At the time thatRiley’s paper (1880a) went to press, Hagen

had been unwilling to send Riley any ofMeehan’s specimens for identification be-cause, Hagen explained, he had been study-ing them and was “about to publish it”(Hagen 1880). As we shall see, Riley wouldonce again find himself defending his find-ings (this time to Darwin) because of Meehan’sflawed scientific work and unfounded asser-tions.

Riley Erects Prodoxidae; HagenMisidentifies P. decipiens

In 1878, Riley resigned his post as thefirst state entomologist of Missouri to be-come the head federal entomologist in Wash-ington, D.C., the position he held (except fora 2-year hiatus) until 1894, the year beforehis death. During that time, Riley briefly re-vived the American Entomologist as a newseries and, with L. O. Howard, co-edited anew journal, Insect Life, published by theBureau of Entomology, USDA. He also wasfounder and first president of the Entomo-logical Society of Washington [D.C.] and afounding member of the Biological Societyof Washington [D.C.], both of which pub-lished their own Proceedings. Thus, much ofthe ensuing yucca-related controversy wasrecounted by Riley in the pages of these peri-odicals and at his presentations at AAASmeetings.

In 1881, a seminal paper by Riley ap-peared in the Proceedings of the AAAS,17

which he sent to Darwin (discussed later). Inthis publication, Riley summarized his yucca-associated findings to date; quoted exten-sively from the criticisms by Meehan and Bollof his work and his rebuttals thereof; de-scribed several new yucca-associated insectspecies (Pronuba [Tegeticula] maculata,Prodoxus intermedius [now Tegeticulaintermedia (Riley); see Pellmyr 1999],Prodoxus marginatus, Prodoxus cinereus,and Prodoxus aenescens); noted that G. H.Horn had informed him that the namePronuba was preoccupied in Coleoptera andif so, then Zeller’s Tegeticula would take pre-cedence; and erected a new family,Prodoxidae, which stands today.

Riley (1881) also set the record straighton two points of taxonomic interest. First,he corrected his earlier misidentification ofthe specimens that Chambers had sent toHagen—having finally been given permissionto denude some of Chambers’s specimens,Riley identified them as a 5-spotted form ofP. decipiens, thereby supporting the validityof his earlier conjecture (Riley 1880c). Sec-ond, Riley reported Hagen’s admission tohaving mistaken P. decipiens for T. yuccasella;thus, Hagen’s then-recent publication (1880),

in which he claimed to have bred T. yuccasellafrom yucca flower stems, was in error.

Riley’s abstract for the 1882 AAAS meet-ing resounded with confidence that his latestfindings would finally silence his critics:

This paper records some recentexperiments and observations whichestablish fully and conclusively the factthat Pronuba [Tegeticula] is necessaryto the fertilization of the capsularYuccas. It describes for the first timehow the pollen is gathered andcollected by the female Pronuba[Tegeticula]. The act is as deliberate andwonderful as that of pollination.Going to the top of a stamen shestretches her tentacles to the utmoston the opposite side of the anther,presses the head down upon the pollenand scrapes it together by a horizontalmotion of her maxillae. The head isthen raised and the front legs are usedto shape the grains into a pellet, thetentacles coiling and uncoilingmeanwhile. She thus goes from one toanother until she has a sufficiency.(Riley 1883, p. 467)Riley (1883) reported several other im-

portant findings: He confirmed Engelmann’sconclusion that the stigmatic apices of cer-tain Yucca species are impotent and that theflower morphology of Y. aloifolia, the onlyspecies known to self-fertilize, is such thatthe flowers can achieve this task; he demon-strated that the irregularities in yucca pods,then considered a character state by bota-nists, are attributable to Tegeticula oviposi-tion punctures (Fig. 10); and he determinedexactly where and how the egg of Tegeticulais oviposited by examining the internalanatomy of the yucca ovule and the oviposi-tor of T. yuccasella. His new findings not-withstanding, if Riley believed that he had atlast quelled the yucca moth controversy, hewas indeed mistaken.

Riley vs. HulstIn 1886, the Reverend George D. Hulst

instigated what would become a contentiousdebate with Riley. Hulst, editor of the jour-nal Entomologica Americana, argued in itspages that honey bees must be extensivefertilizers of yuccas because he had observedhoney bees “plentiful about the flowers,”the majority of yucca seed capsules he ex-amined gave no indication of the presenceof Tegeticula larvae, and yuccas are some-times fertile in foreign countries (Hulst1886). In response, Riley began by under-scoring the depth and breadth of his yuccamoth studies:

For over 16 years now I have verycarefully studied the habits of Pronuba[Tegeticula] in connection with Yucca,not in one locality alone but in nearly

16According to Hagen (1880), Meehan sent thespecimens at Engelmann’s request.

17This paper was read by Riley at the 1880 AAASmeeting (Anonymous, 1880).


every State east of the RockyMountains where that genus occurseither indigenously or by introduction.I have also had occasion to study it inmany places in Europe in which it iscultivated, and the results have beenpublished in several papers. (Riley1887, p. 233)Riley continued, stating that he had seen

neither honey bees nor other nectar-feedingHymenoptera frequenting yuccas and thatthis was not surprising, given certain char-acteristics of the flowers—they are half-closed in the daytime, opening when beesare done foraging, and their glutinous pol-len would not stick to hirsute Hymenoptera.He reiterated that years earlier, he had listedthe insect species found in yucca flowersand that his associates and others had cor-roborated his findings. In response toHultz’s criticism that T. yuccasella larvae arenot always found in the fruit, Riley empha-sized that pollination can (albeit rarely) oc-cur without oviposition, and that sometimesthe eggs may fail to hatch successfully orthe larvae may perish (in which cases thefruit bear evidence of oviposition activity).Riley cited his earlier publications, whichprovided substantiating evidence in re-sponse to others who also had questionedhis findings, to rebut Hulst’s other criti-cisms.

Unconvinced by Riley’s reply, Hulst(1887) wrote a rejoinder that was printedimmediately beneath Riley’s response. Hulstpersisted in his belief that honey bees couldact as yucca pollinators because the flowers

remain open until 10 a.m., and “businesshours for honey bees begin long before thattime” (Hulst 1887, p. 237). He added thatMeehan’s work with artificial fertilizationsuggested that self-fertilization in yuccas waspossible.

The beleaguered and clearly annoyedRiley penned his response in August 1887,while en route to Europe, and mailed it toHulst from England (Riley 1888, p. 150, fn.).But according to Riley, Hulst “exercised hiseditorial prerogative in declining to publish[Riley’s] communication…” (Riley 1888, p.150, fn.), which prompted Riley to send hisresponse to the Proceedings of the Entomo-logical Society of Washington. Riley ex-plained that he had deferred in his responseto Hulst mostly because he had hoped thatHulst “would himself gracefully amend hisopinions to accord with the facts” becausethe yucca fruiting season had ended, and hewould have been “able to make more carefulobservations” (Riley 1888, p. 150). Rileythen countered each of Hulst’s arguments infull, stating that he had “followed up on[Meehan’s] experiments, and made manyothers during the past seven years, on [Yucca]filamentosa and aloifolia.” At a time whenrelatively few entomologists were perform-ing experiments of any sort, Riley’s brief de-scription of the pollination experiments heconducted speak to his scientific acumen andthe thoroughness of his approach:

My experiments have been made inthe afternoon, evening, and morning,with flowers one day, two days, andthree days after opening; with pollen

from the same flower or from otherflowers either on the same or otherracemes, by touching the mere apiceswith anther or brush, and by forcingthe pollen by either conveyance intothe stigmatic tube. In theseexperiments…I have endeavored toguard against all influences, such asthe condition of the plant and theweather, which might affect or vitiatethe results. (Riley 1888, pp. 151)Riley also contrasted Hulst and “his good

friend” Meehan, stating that the latter hadwritten much on the fertilization ofYucca—much, too, that has notshown the keenest penetration nor thestrictest accuracy. But, in candidlyadmitting his errors when shown tobe wrong (as he has done to the writer,and, I have reason to believe, to Mr.Hulst, who sought his support in thebelief here combated), he has provedhimself to be the true naturalist. (Riley1888, p. 150–151)Riley pointed out that Meehan had been

working with Y. glauca, whose flower mor-phology and abundant stigmatic liquor likelymade it more susceptible to artificial pollina-tion; a quote from a letter of Engelmann toRiley supported this assertion. Riley also citedthe research of his trusted colleague, botanistWilliam Trelease, who had shown that femaleTegeticula, in the act of pollinating, repeatsthe process several times, “first from one ofthe angles between the apices, then from an-other, and…the tongue18 is used, in additionto the tentacles, to push the pollen down tothe bottom of the tube” (Riley 1888, p. 152).

In addition, Riley (1888) provided morerecent evidence to discount Hulst’s belief thathoney bees could serve as pollinators of yuc-cas. He had those in his employ (L. O.Howard, T. Pergande, O. Lugger) observemore than 200 yucca stalks at the USDA inWashington between 9 and 10 a.m., and nohoney bees were seen around the flowers.Riley noted that he himself had seen a singlebee on two occasions, but each time it merelyprobed the flower on its outer base and flewoff without entering it. Furthermore, Rileynoted, these findings had been corroboratedby Professors Cook and Beal at MichiganState Agricultural College (now MichiganState University).

Lastly, Riley related the findings of Trelease(1886), who reported that the stigmatic cav-ity of Y. filamentosa is not nectariferous andthat the small amount of nectar it secretes is

Fig. 10. Illustration by Riley (1892a) of mature pods of Y. angustifolia (now Y. glauca): (a)artificially pollinated (thus lacking punctures and constrictions due to oviposition), (b) pollinatedby Tegeticula (showing oviposition punctures, constrictions, and larval exit holes), (c) larvawithin the pod.

18Trelease (and Riley 1873a) was incorrect, as the“tongue” is not used to pollinate, but is held straightout above the tentacles. However, he subsequentlydescribed its proper position during pollination for T.synthetica (Trelease 1893).


located at the base of the pistil or the petals,not near the stigma. Riley stated that he hadcorroborated Trelease’s findings “by dissec-tion [of the pistil] and study of the insectsseeking this scant nectar” (Riley 1888, p.154).

Without doubt (and understandably so),what most infuriated Riley was Hulst’s alle-gation that “[to] Dr Engelmann…we are in-debted for the discovery of the fact thatPronuba [Tegeticula] is an agent in the fer-tilization of Yucca (though he did not workout the history which Prof. Riley has doneso well)…” (Hulst 1887, p. 236). Regardingthis, Riley remarked: “Whatever may haveled Mr. Hulst to make this assertion, it issimply untrue….” Riley acknowledged thatit was quite likely he might never have stud-ied the yucca moth had Engelmann not calledit to his attention, but he correctly assertedthat Engelmann had “made no observationwhatever upon insect pollination.” Riley con-tinued:

The discovery that Pronuba[Tegeticula] was the agent was my own,as were all the subsequent discoveriesin reference to the insect made thatyear; but they were alwayscommunicated to him, and oftenshared with and witnessed byhim.…Dr. Engelmann was, during myresidence in St. Louis, at once myfriend, companion, and master innatural-history matters, and I have toomuch reverence for his memory toallow to pass unchallenged what hehimself would repudiate were he stillamong us. (Riley 1888, pp. 153)With the remark that he had “already

devoted more time to Mr. Hulst’s opinionsthan they justify,” Riley recapitulated hisdenouncement of Hulst’s assertions. He con-cluded,

I await with interest and curiosity anynew discoveries in this connection,but, so far as present knowledgejustifies anticipation, I should expect,where neither Pronuba [Tegeticula]nor Pronuba [Tegeticula]-like insectexists, to find the plant modified tomore readily permit self-fertilizationsooner than to find Apis mellifica thepollinating agent, the opinion…ofMr. Hulst, to the contrarynotwithstanding. (Riley 1888, p. 154)

Darwin: “It is the most wonderful caseof fertilisation”

On 7 April 1874, Darwin wrote to J. D.Hooker, the celebrated botanist and one ofhis closest friends, about the yucca moth–yucca plant relationship as elucidated byRiley, stating: “It is the most wonderful caseof fertilisation ever published.”19 By this

point in time, Darwin was well acquaintedwith Riley’s work because the latter had senthim copies of his Annual Report of the Mis-souri State Entomologist. Indeed, accordingto Darwin’s son, Francis, and A. C. Seward,“These reports were greatly admired by Mr.Darwin, and his copies of them, especially ofNos. 3 and 4, show signs of careful read-ing.” (Darwin and Seward 1903, p. 385, fn.2).

Darwin (1876) cited Riley’s yucca workin his book, “The Effects of Cross and SelfFertilisation in the Vegetable Kingdom;” andsoon after its publication, Meehan sent Dar-win a letter calling into question the signifi-cance of Riley’s findings. Meehan wrote thatGray had admitted in a letter to him that“there is far more self fertilization amongflowers than he [Gray] had supposed.”Meehan added that although only T.yuccasella can fertilize the yucca, “as you areno doubt aware, the Pronuba [Tegeticula]fertilizes the flower with its own pollen,—and it seems to me a great mystery why theplant should not be arranged to use any it-self, and not be necessitated to have insectaid in the use of its own pollen.”20

Subsequently, when Riley sent Darwin acopy of his 1881 AAAS publication, he re-ceived the following response from Darwin:21

I must write half a dozen lines to sayhow much interested I have been byyour “Further Notes” on Pronuba[Tegeticula], which you were so kindas to send me.—I had read the variouscriticisms, & though I did not knowwhat answer could be made yet I feltfull confidence in your result, & now Isee I was right.—What an inaccurateman Mr. Meehan is! His Epitaphought to be “He retarded naturalscience in the U. States as much as anyone man advanced it.” …If you makeany further observation on Pronuba[Tegeticula]—it would I think be wellworth while for you to observewhether the moth can or doesoccasionally bring pollen from oneplant to the stigma of a distinct onefor I have shown that the Cross-fertilisation of the flowers on the same

plant does very little good; & if I amnot mistaken you believe that Pronuba[Tegeticula] gathers pollen from thesame flower which she fertilizes—.In his reply,22 Riley clarified his findings

regarding Tegeticula and cross fertilization23:My language is perhaps ambiguouson this point, but in fact she not onlypollinizes with the same load of pollendifferent flowers on the same panicle,but often flies from plant to plant. Ihave never seen her gather the pollen,but have watched her thus go fromflower to flower and from plant toplant during a single evening and withone and the same load.24

In the same letter, Riley voiced a harshassessment of Meehan’s scientific prowess:

His trouble is that he has not been sotrained that he can appreciate a truthwhether it conflict with his ownopinions or not. There is a bias in mosteverything he writes or says, and he isespecially fond of controvertinggenerally-accepted doctrine, uponhis own observations—oft-timesinsufficient and even inaccurate. He is,withal, unfair, not to say, untruthful.In exasperated tones, Riley recounted at

length what had transpired between him andMeehan. Riley had visited Meehan at his in-vitation during the summer of 1881, whenY. glauca was finishing flowering and Y.filamentosa was about to commence doingso. Riley observed numerous P. decipiensovipositing into the stems of Y. filamentosa.Riley also had collected several hundredmoths, every one of which he determined tobe P. decipiens, as he subsequently informedMeehan. To Riley’s surprise, that August,Meehan read a paper in Cincinnati in whichhe reported that he had collected from asingle Y. glauca flower 17 moths, 4 of whichwere Prodoxus, the remainder beingTegeticula. Because Y. glauca were not fertil-ized, Meehan inferred that Tegeticula was in-capable of pollination. When Riley askedwho had identified the moths, he was in-formed that Meehan had sent them to Hagenthe day after Riley’s visit. A patently annoyedRiley continued,

[Meehan] made no mention whateverof my visit or my determinations

19Darwin to Hooker, 7 April 1874 (CambridgeUniversity Library, classmark DAR 95: 321). In aletter to C. V. Riley, Hermann Müller echoed Darwin,calling it “the most wonderful instance of mutualadaptation” yet detected. (Private letter to C. V. Riley,cited in [Riley 1881, p. 623, fn. 4])

20Meehan to Darwin, 1 July 1877. (CambridgeUniversity Library, classmark DAR 171: 112)

21Darwin to Riley, 28 Sept. 1881. (CambridgeUniversity Library, classmark DAR 147: 303)

22Riley to Darwin, 18 Dec. 1881. (CambridgeUniversity Library, classmark DAR 176: 158)

24Trelease corroborated this finding (letter fromTrelease to Riley, quoted in Riley 1892a, p. 125).

23Pellmyr et al. (1997) state that Riley erred in hisassertion that yucca moths do not performgeitonogamous pollinations and provide evidence tothe contrary.


[emphasis Riley’s]…[Hagen] knowslittle outside of Neuroptera and onlythe year before had made a mostunpardonable blunder in pronouncingcertain moths which he had bred fromYucca stems to be Pronuba [Tegeticula]yuccasella where afterward heconfessed they were Prodoxus, and onewould suppose that a fair man seekingonly truth would have at least put myown determinations, in relation togenera of my own creation, along sideof Hagen’s.Riley noted that Hagen had determined

the moths to be males, adding that “malesare not so easily distinguished as the femalesand Hagen may again have blundered; buteven if he was correct the explanation issimple.” Riley continued, stating thatTegeticula males often emerge before the fe-males and that, as Y. filamentosa had not yetbloomed, the males likely took refuge in thefew remaining Y. glauca flowers. He added,

But nothing is more certain, from aseries of experiments and observationswhich I made this past summer, thanthat Prodoxus female oviposits in thestem and that her race is run essentiallyby the time the plant begins to bloom,for she can only successfully ovipositin the tender stem and perishes in theact when attempted after the flowersbegin to open and the stem gets hard;further that she is absolutely incapableof pollinizing and never attempts it.Approximately six months after Riley

penned this letter, Darwin died on 19 April1882, and one month later, the BiologicalSociety of Washington, D.C., held a memo-rial meeting in his honor that was attendedby an estimated 700 people. Riley and sev-eral other prominent members of the Soci-ety, including its president, Major JohnWesley Powell, delivered addresses.

In his presentation, Riley (1882) pro-claimed Darwin’s greatness, enumeratinghis many original contributions to and in-sights into the entomological world. Rileyclosed with a heartfelt remembrance of thetwo occasions (in 1871 and 1875) onwhich he had visited Darwin at his resi-dence in Down:

Going to him as a young entomologistwith no claim on his favor, he seemedto take delight in manifestingappreciation…an interest in naturalscience was an open sesame to hisgenerous soul. His consideration,without aggression, was the secretof the gratitude and respect whichall felt who had the honor to knowhim, either personally or throughcorrespondence. (Riley 1882, pp. 79–80)

Final Years: Riley’s Work with Treleaseand Others

Riley maintained a steadfast interest in thecomplex of moths associated with the genusYucca through the last years of his life. Extantletters25 indicate that in February 1893, Rileywrote to J. T. Mason (Houston, Texas, Land& Trust Co.) expressing his keen interest inobtaining a Tegeticula species that he felt cer-tain must be associated with Y. filifera, a largetree yucca of northeastern Mexico; Riley hadevidence of the moth’s existence from seedpod samples. A few months later, Riley wroteMason to inform him that the plant specimenhe had sent Riley was most likely Y.guatemalensis [now Y. elephantipes] and thatthe associated moth was a new species, whichRiley named Prodoxus intricatus (Riley1893). Riley added,

I am quite confident that furtherintelligent research will give us aninteresting form of Pronuba[Tegeticula] in this Yucca guatemalensis[elephantipes] and if you have a chanceof studying Y. filifera you willundoubtedly secure a number ofinteresting species of the family, of bothgenera, Pronuba [Tegeticula] andProdoxus. The Pronuba [Tegeticula]associated with filifera must be a largeand interesting species and I am quitesure will prove different from any ofthe three so far characterised [sic].Recently, Parategeticula elephantipella

Pellmyr & Balcázar-Lara was described andappears to be the exclusive pollinator of Y.elephantipes (Pellmyr and Balcázar-Lara2001); information on Y. filifera pollinationremains incomplete (Olle. Pellmyr, personalcommunication).

Riley collaborated with other profes-sional scientists in his investigations. He gaveJ. B. Smith (1893) Tegeticula specimens toinvestigate morphological homologies be-tween their maxillae and those of other in-sects. As chief entomologist, USDA Divi-sion of Entomology, Riley directed one ofhis field agents, D. W. Coquillett, to observethe behavior of female T. maculata on Y.whipplei in southern California (Coquillett1893). The flower stalk of this yucca spe-cies can reach spectacular heights (Fig. 11).Riley had a particular interest in Y. whippleibecause he thought that certain of its traitsmight facilitate ordinary pollination, yet heknew that T. maculata serves as a pollinator(Riley 1893). Riley also described severalProdoxus spp. from Y. whipplei and, in re-cent times, Powell (1992) cited Y. whipplei andits associated prodoxids as one of the twomost complex yucca/yucca moth systems.

In his final years, Riley worked closelywith botanist William Trelease, first appoin-tee to the George Engelmann Professorshipof the Henry Shaw School of Botany, Wash-ington University, St. Louis, and first direc-tor of the Missouri Botanical Garden,founded by Shaw (White 1902, Rodgers1944). Trelease (1886) corroborated Riley’searlier findings on yucca moth ovipositionand pollination, calling it “a case withoutparallel”, and at his urging, Riley (1892a)compiled a comprehensive review of his 20years of yucca moth studies.

Trelease (1892) also performed morpho-logical studies of several yucca species, anddetailed his pollination and oviposition stud-ies of T. maculata on Y. whipplei andTegeticula synthetica (Riley) on Yuccabrevifolia (commonly called the Joshua tree)(Trelease 1893). Riley (1892a, 1893)quoted extensively from these studies, in-cluding Trelease’s unpublished findings. Inhis research, Trelease was assisted by hiswife, Julia Johnson Trelease, who made be-havioral observations of the moths and ren-dered illustrations of pollination and ovi-position.26 Trelease also collected severalProdoxus species (P. coloradensis Riley, P.reticulatus [now Greya reticulata (Riley); seeDavis et al. 1992], P. cinereus, P. aenescens,T. intermedia) from various yuccas and sentthem to Riley, who detailed their morphol-ogy (Riley 1893).

Fig. 11. Yucca whipplei in bloom. (from InsectLife 1893).

25Missouri Botanical Garden, William Treleaseprofessional papers, Box 8, Folders 1 & 2.

26William Trelease Papers, #2560, Division of Rareand Manuscript Collections, Cornell UniversityLibrary.


Fig. 12. Masterly renderings from Riley (1892a) of T. yuccasella ovipositor: (A, c) file-likesurface of (b), the ovipositor base; (h) extended oviduct; (B) partial enlargement of (A); (C)ventral tip of abdomen with (k) muscular attachments. Also shown are eggs from the yucca pistil(m) and the mature ovary of the moth (o), the developing embryo (n), and the male genitalclaspers (r, s).

In February 1894, Riley wrote Treleaseof a possible joint trip to Mexico; threemonths later, he wrote that he was clearingout his USDA office in preparation for hisreplacement, L. O. Howard.27 Soon thereaf-ter, Riley was named Honorary Curator ofInsects, U.S. National Museum, and on 14September 1895, he died suddenly follow-ing a bicycling accident.

Interested readers seeking the single bestreview by Riley on prodoxids and yuccascan do no better than to peruse his paperwritten for the Third Annual Report of theMissouri Botanical Garden (Riley 1892a).28

This comprehensive work provides exten-sive excerpts from his earlier publications,taxonomic descriptions for Pronuba[Tegeticula] and Prodoxus spp., and someof Riley’s unpublished observations. It alsocontains 10 plates, each featuring several il-lustrations, including renderings of the ovi-positor of T. yuccasella (Fig. 12), and micro-scopic sections of Y. filamentosa pistils withTegeticula eggs and larvae in situ (Fig. 13).

Current Research and ClosingRemarks

After Riley had worked for several yearson various yuccas and their associated moths,he stated that there was “little doubt” thatother species of Pronuba [Tegeticula] andProdoxus would be discovered (Riley 1880a,p. 145). A dozen years later, he recognizedthat there was still “much yet to learn of thepollination of other species of Yucca”, andhe predicted that “all the [yucca] specieswhich are sufficiently distinctive in charac-ters and in range, may be expected to havespecial Pronubas [Tegeticula] associated withthem” (Riley 1892b, pp. 95–96).

Since then, knowledge of the complex ofprodoxids associated with yuccas has in-creased significantly and Riley’s predictionshave been validated. In his revision, Davis(1967) described two new Prodoxus speciesand Parategeticula pollenifera, a yucca polli-nator. Currently there are 78 described spe-cies of Prodoxidae; of these, 11 are Prodoxusand 4 are Parategeticula and, until recently,3 were Tegeticula species: T. synthetica (Riley),T. maculata (Riley), and T. yuccasella (Riley)(Pellmyr 2003). However, T. synthetica isbelieved to contain two species and T.maculata may comprise several (Pellmyr2003).

In his recent systematic revision of theyuccasella complex north of Mexico, Pellmyr

(1999) reported that T. yuccasella comprisesat least 13 species, of which 2, T. intermedia(Riley) and T. corruptrix Pellmyr, are“cheater yucca moths,” that is, moths thatcoexist with yucca pollinators and ovipositinto the fruit, but do not engage in yuccapollination. Females of these species lack the

maxillary tentacles needed for pollination(Pellmyr 1999, Pellmyr and Krenn 2002).(Because he viewed the female’s tentacles asan absolute requirement for placement inTegeticula, Riley misidentified T. intermediaas a bogus yucca moth, naming it Prodoxusintermedius.)

27William Trelease Papers, #2560, Division of Rareand Manuscript Collections, Cornell UniversityLibrary.

28For a condensed version of this publication, seeRiley (1892b).

Fig. 13. Microscopic views of Y. filamentosa and T. yuccasella (from Riley 1892a): (a) longitudinalsection through pistil, (b) oviposition punctures, (c) elongate eggs in situ, (d, e) larvae in situ.Also shown is a seed (f) at 13 days after oviposition, with a young larva visible at the funicularbase.


Riley demonstrated remarkable pre-science as he considered the phylogeny andevolutionary biology of yucca-associatedmoths:

Who, studying these two species [T.yuccasella and P. decipiens] in all theircharacters and bearing, can fail toconclude that, notwithstanding theessential differences that distinguishthem not only specifically butgenerically, they are derivations fromone and the same ancestralform?…Which of the two insects isoldest in time, or whether thedivergence from some archetypal formhas been simultaneous, are matters ofopinion which those interested inevolution will decide for themselvesone way or the other, or according asknowledge increases. (Riley 1880a, pp.144–145)According to Pellmyr’s recent review

(2003), prodoxids colonized yuccas about41.7 million years ago. Soon thereafter, thethree genera (Tegeticula, Parategeticula, andProdoxus) underwent rapid diversification.Together, these genera exhibit a species di-versity greater than 20 times that of theirsister group, Mesepiola, which arenonpollinating seed parasites (Pellmyr andKrenn 2002).

In his review of yuccas and yucca moths,Powell (1992) argued that “insect biologistsand ecologists failed to recognize the highlycomplex nature of this coevolutionary rela-tionship until recently,” stating that little re-search was done in the 70 years followingRiley’s work. He listed several areas ripe forfurther investigation, and the recent treat-ment of this topic by Pellmyr (2003) pro-vides an informed measure of the advancesthat have been made in the decade sincePowell’s review, including ideas relevant tothe evolution of obligate mutualism.

Speculation about the possibility of non-yucca moth pollinators of yuccas continuedto be raised a century after Riley’s death, yet,according to Pellmyr (2003), the only yuccaspecies for which there is even “moderatesupport” for such came from Trelease(1893), who performed pollinator exclusionexperiments on Y. aloifolia. Pellmyr (2003)argues that carefully executed experimentswould settle the recurring question of yuccacopollinators.

Our review of the first two decades ofyucca moth research provides a window onmid- to late 19th-century entomology, whenthe discipline broadened from a purely de-scriptive enterprise (taxonomic and life his-tory studies) to include research aimed at thepractical application of scientific knowledge.At the same time, Darwin’s theory of evolu-tion emerged, providing a conceptual frame-

work within which to pursue research of bothsorts. Great naturalists like Riley recognizedthis and strove to use valid scientific ap-proaches to test their hypotheses, but at timesthey labored in the midst of colleagues lessgifted and far less rigorous in their empiri-cism. Riley’s powers of observation, clarityof reasoning, and abundant self-confidenceserved him well in rebutting his relentless crit-ics. With the writings of Darwin and Graylaying the theoretical groundwork, and thecollaborative efforts of Engelmann, Trelease,and others providing the needed botanicalexpertise, Riley synthesized a corpus ofknowledge on a fascinating case of coevolu-tion. The majority of his findings have with-stood the test of time for more than a cen-tury, and yuccas and yucca moths continueto be a vibrant area of research, perhaps thegreatest testament to Riley’s labors.

AcknowledgementsWe are indebted to Olle Pellmyr, Depart-

ment of Biological Sciences, University ofIdaho, for expertise provided on yucca mothtaxonomy, systematics, and evolutionarybiology. We thank Shelley Innes, DarwinCorrespondence Project, Cambridge Univer-sity Library, for frequent and generous as-sistance; and Rick Weinzierl, University ofIllinois, for procuring copies of some rarejournal articles. We also thank MarinaMeixner, Institut für Bienenkunde,Oberursel, Germany, for German translation,and Edward H. Smith, Professor Emeritus,Cornell University, for information pertain-ing to the Riley–Trelease correspondence andcontinuing collegiality. We gratefully acknowl-edge the Syndics of Cambridge UniversityLibrary, for quotation of letters in the Dar-win correspondence; the Missouri BotanicalGarden, for citation of Riley letters and re-production of unpublished sketches by En-gelmann; and Cornell University Library, Di-vision of Rare and Manuscript Collections,for reference to letters in the William TreleaseCollection. Finally, we thank anonymousreviewers for helpful comments on our manu-script.

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Carol A. Sheppard received her M.S. andPh.D. degrees in Entomology from the Uni-versity of Illinois at Urbana–Champaign,where a course on the history of biologysparked her long-standing interest in histori-cal entomology. She is a faculty member atWashington State University (WSU), Depart-ment of Entomology, FSHN Rm. 160, P.O.Box 646382, Pullman, WA, 99164-6382(e-mail: [email protected]) and a mem-ber of the WSU Honors College Faculty.Richard A. Oliver received his B.S. degree inZoology from WSU, where he also was anHonors College student. He conducted someof this research on yuccas and yucca mothsas part of a special topics course in entomol-ogy at WSU. Currently, he is enjoying work-ing with animals at the Hogle Zoo in SaltLake City.

yucca moths and yucca plants: discovery of “the most...

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