Yucca Moths Research Articles

Host Specificity and the Genetic Structure of Two Yucca Moth Species in a Yucca Hybrid Zone

Host Specificity and the Genetic Structure of Two Yucca Moth Species in a Yucca Hybrid Zone

Inbreeding and outcrossing in Yucca whipplei: consequences for the reproductive success of plant and pollinator

Unlike most pollinators, yucca moths are active pollinators of their host plants. Females lay their eggs in the flowers they pollinate, and their larvae feed solely on the resulting seeds. Previous evidence suggests that the yucca moth Tegeticula maculata avoids self‐pollinating their host Yucca whipplei. Other yucca moths may self‐pollinate more frequently. When pollinating, yucca moths are also reported to fly large distances between plants, bypassing neighbouring plants in the process. We experimentally verify the suggestion of Pellmyr et al. that yucca is more likely to retain fruits from self‐pollination if overall fruit set is low. Thus, selection on moths to avoid self‐pollinating should be density dependent. We found no evidence that mating with close neighbours resulted in inbreeding depression, thus the moth's long‐distance flights between plants are yet to be explained.

Genetic diversity and population structure of Yucca filamentosa (Agavaceae)

Using 19 allozyme loci we studied genetic diversity in 18 populations of Yucca filamentosa (Agavaceae) from the southeastern United States. Of the 19 loci surveyed, 17 (89.5%) were polymorphic in at least one of the populations sampled. There was considerable variation among populations in the percentage of polymorphic loci (range = 31.6-84.2%, mean = 67.6%). Similar heterogeneity among populations was observed for mean number of alleles per polymorphic locus (range = 2.0-3.0; mean = 2.48) and mean expected heterozygosity (range = 0.113-0.288; mean = 0.213). On average, 83% of the total genetic diversity was found within populations. Duplications of three allozyme loci were detected in several populations. The life-history characteristics of Y. filamentosa (a long-lived, semiwoody, predominantly outcrossing monocot with a large geographical range) may contribute to the maintenance of such high levels of genetic diversity. These results contradict expectations of the genetic structure of Y. filamentosa based on observations of the dispersal and pollination behavior of its sole pollinator, Tegeticula yuccasella, the yucca moth.

Regulation of Mutualism between Yuccas and Yucca Moths: Population Level Processes

This study addresses five population level processes associated with regulation of the pollination/seed predation mutualism between Yucca kanabensis and two species of pollinating yucca moths, one each from the deep and shallow groups of species within the Tegeticula yuccasella species complex. 1) The adult yucca moth population is apparently relatively constant from year to year, as shown by the inverse relationship between the proportion of rosettes flowering and the proportion of flowers receiving oviposition/pollination visits from yucca moths. 2) The relative success of yucca moths as measured by the proportion of visited flowers retained as fruit, is an inverse function of the ratio of moths to flowers, as measured by visitation. 3) The interactions between yucca moths and yuccas is different for deeps and shallows. Not only was there differential retention of flowers visited by shallows relative to flowers visited by deeps, but the differential appears to be frequency dependent. 4) Within seasons, fruiting and therefore the success of yucca moths may be concentrated into a period of just a few days. This non-random pattern of fruiting differed from both the pattern of flowering and the pattern of visitation, and was a function of higher than average retention of visited flowers. However, no set of factors correlated consistently with these population level windows of fruit retention. 5) The interaction between yucca moths and yuccas shifts with the density of yucca moths: the number of eggs laid per flower increases with visitation, and the amount of pollen carried by yucca moths decreases with moth density. I discuss these results in the context of hypotheses for the regulation of the yucca/yucca moth mutualism, and hypotheses for how these population patterns should affect the life histories and behavior of yucca moths.

Regulation of Mutualism between Yuccas and Yucca Moths: Is Oviposition Behavior Responsive to Selective Abscission of Flowers?

In the obligate pollination/seed predation mutualism between yuccas and yucca moths, yuccas may exert considerable control over the interaction by abscising a high proportion of visited flowers. Because egg mortality in abscised flowers is 100%, yucca moths should avoid ovipositing in, or decrease the number of eggs laid in flowers more likely to abscise. The focus of out study was to demonstrate experimentally (1) the extent to which abscission is non-random with respect to the number and kinds yucca moth ovipositions, and (2) the extent to which yucca moths modify their oviposition behavior in response to the probable fate of a flower as determined by the number and kinds of previous ovipositions in a flower. To address these problems we studied the interaction between Yucca kanabensis and its two species of pollinating yucca moths, one each from the deep and shallow groups in the Tegeticula yuccasella species complex. We demonstrate that the presence of deep ovipositions increased abscission rates of flowers with shallow ovipositions, the presence of shallow ovipositions had no effect on abscission of flowers with deep ovipositions, and the probability of abscission increased with the number of deep ovipositions but was independent of the number of shallow ovipositions. These results indicate that regulation of mutualism through selective abscission of fruit works for deeps but not for shallows. We predicted that yucca moths should avoid flowers with deep ovipositions and seek out flowers with shallow ovipositions. We found, however, that shallows showed only a weak avoidance of flowers with deep ovipositions, and that deeps did not avoid ovipositing in flowers that already contained deep eggs. We consider three hypotheses that could explain the weak fit between out predictions and observations.

POLLINATING SEED EATERS: WHY IS ACTIVE POLLINATION SO RARE?

Insects whose larvae depend on developing seed are very common, but active pollination, meaning that the insect possesses specific structures and behaviors for the purpose of assuring pollination, is only known to have evolved twice in such insects, namely in yucca moths and in fig wasps. This rarity could be due to high cost of pollination, phylogenetic constraints, alternative life history shifts to reduce or avoid risk of seed nondevelopment, or ecological factors such as co-pollinators that can satiate pollen requirements and mask variation in pollinator effectiveness among ovipositing seed eaters. Ecological costs of being a pollinator were measured for a yucca moth species and were found to be low: active time allocated to pollen pickup and deposit was on average 4.1%, an average of 0.42% of female body mass was allocated to specific structures for pollen manipulation, and the average pollen load weighed <4% of moth body mass. These estimates suggest that ecological costs need not be a major obstacle to evolution of active pollination. In contrast, recent combined ecological–phylogenetic analyses for the yucca moth family suggest that the evolution of active pollination and transition to mutualism depended largely on preadaptations, and that few traits were truly novel. If general, active pollination would be predicted to be likely to evolve only in lineages with life histories that facilitate mutualism. Alternative outcomes to evolution of active pollination include delayed oviposition, detection of floral pollination status, egg placement that allows the larva to select a fruit, and modified egg dispersion strategies to balance the cost of pollination. The historical significance of these factors can be assessed only when mechanisms are documented in many lineages and analyzed in a phylogenetic framework. Active fungal inoculation among arthropods is ecologically analogous to active pollination, and offers a complement for comparative analyses. Specific structures for spore transport have evolved many times in at least three orders of insects, and several times in mites. The large number of independent lineages of active pollen and fungus dispersers jointly provide a platform for testing hypotheses about, e.g., the role of preadaptations in evolution of mutualism, reversals of mutualism, and the role of mutualism in diversification.

Non-Random Fruit Retention in Yucca filamentosa: Consequences for an Obligate Mutualism

Post-pollination selective flower abortion can provide plants with a mechanism to exact some control over which fruits are matured. In Yucca filamentosa, seed quality and quantity are limited by the oviposition and pollination behaviors of its obligate mutualist, the yucca moth Tegeticula yuccasella. The plant produces many more flowers than can ever become mature fruit and selectively retains those flowers receiving few eggs and large pollen loads. This study addresses additional factors that affect patterns of floral abortion. We document intrinsic differences in floral abortion probability in Y. filamentosa based on floral position, and abortion based on herbivory by a beetle that coexists with the yucca moths. We then ask how they affect the plant-moth interaction. Pollination experiments showed that the probability of floral retention was positively correlated with late flowering within an inflorescence, more distal position within side branches, and number of flowers on a side branch. Pollination of all flowers, early flowers only, or late flowers only did not result in different overall fruit set, suggesting that resources for fruit maturation can be allocated in response to fluctuations in pollinator availability. Competition among flowers for limited resources plays an important role in determining which flowers become fruits. Flowers attacked by larvae or adults of the nitidulid beetle Carpophilus melanopterus invariably abscised, regardless of pollination status. Beetles were estimated to be responsible for 37% of all floral abscission. Beetle damage precedes T. yuccasella visitation and any moth eggs within aborted flowers do not survive. For this reason, moths should avoid oviposition on beetle-infested flowers. However, there was no evidence that T. yuccasella avoided oviposition within beetle-infested flowers. Intrinsic differences in floral abortion probability and abortion triggered by the beetles add to the previously documented egg- and pollination-driven factors of the yucca moth. Together, these factors create a complex landscape of floral retention probabilities that may influence the oviposition behaviors of the yucca moths.

Erratum: "Evolution of Pollination and Mutualism in the Yucca Moth Lineage"

Previous articleNext article No AccessErratum: "Evolution of Pollination and Mutualism in the Yucca Moth Lineage"Original articleEvolution of Pollination and Mutualism in the Yucca Moth LineagePDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 149, Number 4Apr., 1997 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/286024 Views: 6Total views on this site Copyright 1997 The University of ChicagoPDF download Crossref reports no articles citing this article.Related articlesEvolution of Pollination and Mutualism in the Yucca Moth Lineage15 Oct 2015The American Naturalist

Genetic consequences of specialization: yucca moth behavior and self-pollination in yuccas.

Reciprocal specialization in interspecific interactions, such as plant-pollinator mutualisms, increases the probability that either party can have detrimental effects on the other without the interaction being dissolved. This should be particularly apparent in obligate mutualisms, such as those that exist between yucca and yucca moths. Female moths collect pollen from yucca flowers, oviposit into floral ovaries, and then pollinate those flowers. Yucca moths, which are the sole pollinators of yuccas, impose a cost in the form of seed consumption by the moth larvae. Here we ask whether there also is a genetic cost through selfish moth behavior that may lead to high levels of self fertilization in the yuccas. Historically, it has been assumed that females leave a plant immediately after collecting pollen, but few data are available. Observations of a member of the Tegeticula yuccasella complex on Yucca filamentosa revealed that females remained on the plant and oviposited in 66% of all instances after observed pollen collections, and 51% of all moths were observed to pollinate the same plant as well. Manual cross and self pollinations showed equal development and retention of fruits. Subsequent trials to assess inbreeding depression by measuring seed weight, germination date, growth rate, and plant mass at 5 months revealed significant negative effects on seed weight and germination frequency in selfed progeny arrays. Cumulative inbreeding depression was 0.475, i.e., fitness of selfed seeds was expected to be less than half that of outcrossed seeds. Single and multilocus estimates of outcrossing rates based on allozyme analyses of open-pollinated progeny arrays did not differ from 1.0. The discrepancy between high levels of behavioral self-pollination by the moths and nearly complete outcrossing in mature seeds can be explained through selective foreign pollen use by the females, or, more likely, pollen competition or selective abortion of self-pollinated flowers during early stages of fruit development. Thus, whenever the proportion of pollinated flowers exceeds the proportion that can be matured to ripe fruit based on resource availability, the potential detrimental genetic effects imposed through geitonogamous pollinations can be avoided in the plants. Because self-pollinated flowers have a lower probability of retention, selection should act on female moths to move among plants whenever moth density is high enough to trigger abortion.

Evolution of Pollination and Mutualism in the Yucca Moth Lineage

The obligate pollination mutualisms between yucca moths and yuccas are some of the most obvious cases of coevolution, but the phylogenetic origins and extent of coevolution in these interactions are little understood. Ecological and phylogenetic information from the yucca moth family, Prodoxidae, shows that pollination has evolved at least three times from separate moth behaviors. Passive pollination occurs in Greya during nectarine by one species and during oviposition by two other species. Active pollination among prodoxids has evolved only once, in the yucca moths. Several life-history traits necessary for the evolution of obligate mutualism are common traits within the Prodoxidae, and only active pollination and modified mouthparts appear to have been novel traits in the yucca moths. We use moth and host biology in a phylogenetic framework to establish hypotheses for the evolution of active pollination and reciprocal specialization in the form of functional nectarlessness in yuccas.

Non-mutualistic yucca moths and their evolutionary consequences.

Interspecific mutualisms are regarded as having evolved from antagonistic or commensalistic interactions, with most mutualisms remaining facultative but some having coevolved into obligate reciprocal dependency. Underlying mutualism is an intrinsic conflict between the parties, in that each is under selection for increased exploitation of the other. Theoretical models suggest that this conflict is a source of evolutionary instability, and that evolution of 'cheating' by one party may lead to reciprocal extinction. Here we present phylogenic evidence for reversal of an obligate mutualism: within the yucca moth complex, distinct cheater species derived from obligate pollinators inflict a heavy cost on their yucca hosts by laying their eggs but not pollinating the yucca. Phylogenetic data show the cheaters to have existed for a long time. Coexisting pollinators and cheaters are not sister taxa, supporting predictions that evolution of cheating within a single pollinator is evolutionarily unstable. Several lines of evidence support a hypothesis that host shifts preceded the reversal of obligate mutualism. Host or partner shifts is a mechanism that can provide a route of evolutionary escape among obligate mutualists in general.

Windows of Fruit Production in Yuccas: A Response to Pellmyr

James et al. (1994) studied patterns of flower and fruit production in 38 Yucca elata (Agavaceae) inflorescences, finding a clumped pattern of fruit production within the flowering period of each inflorescence. James et al. (1994) found no relationship between the timing of fruit production and the phonological sequence or vegetative characteristics of the plants. The timing of fruit production within the flowering period of an inflorescence was variable, and fruit production was not concurrent among inflorescences flowering at the same time. Also, neither environmental conditions, nor the relative abundance of the sole pollinator (the moth Tegeticula yuccasella, Prodoxidae) on the night that fruits were initiated explained variation in the timing of production. In short, James et al. (1994) were unable to identify any obvious mechanism for the window of fruit production. James et al. (1994) acknowledged that an alternative mechanism accounting for clumped fruit production was that moths only pollinated flowers on an inflorescence during a specific period, but this hypothesis could not be tested because of a lack of data. Pellmyr (1995) has identified the following areas of inadequacy in James et al's (1994) study: (1) failure to identify the causes of flower abortion and specifically not to test for a correlation between pollinator activity and fruit production; (2) failure to positively identify the species and sex of moths in flowers during censuses, making estimates of pollinator density invalid; (3) failure to account for species of invertebrate other than yucca moths or their close relatives, that damage flowers and cause fruit to abort; and (4) failure to ensure that the insertion of an ovipositor by a female yucca moth resulted in oviposition. In this paper we respond to these four areas of criticism, devoting most of the discussion to the first criticism, which was the primary theme of the paper by James et al. (1994). In summary, our responses are these: (1) Pellmyr incorrectly interprets the description of patterns of fruit production by James et al. (1994) as an attempt to attribute mysterious cause, failing to acknowledge that variation in pollinator activity was in fact considered as a possible mechanism. We assert that Pellmyr over-emphasises the effect of variation in pollinator activity as a cause of clumped and asynchronous fruit production by individual inflorescences and we provide two analyses not included in James et al. (1994) to lend weight to this assertion. (2) Pellmyr appears to have incorrectly interpreted the moth census as an attempt to est mate density, when it was intended to assess relative abundance (an index of abundance). However, Pellmyr correctly indicates that temporal change in the sex ratio of the moth population, if it occurred, may have been a weakness in the data. (3) The role of invertebrates other than T. yuccasella is irrelevant to the results because inflorescences were bagged during the study. And, (4) whether the insertion of the ovipositor does or does not result in oviposition is minor and does not bear greatly on the results the study.

Multiple origins of the yucca-yucca moth association.

The association of species of yucca and their pollinating moths is considered one of the two classic cases of obligate mutualism between floral hosts and their pollinators. The system involves the active collection of pollen by females of two prodoxid moth genera and the subsequent purposeful placement of the pollen on conspecific stigmas of species of Yucca. Yuccas essentially depend on the moths for pollination and the moths require Yucca ovaries for oviposition. Because of the specificity involved, it has been assumed that the association arose once, although it has been suggested that within the prodoxid moths as a whole, pollinators have arisen from seed predators more than once. We show, by using phylogenies generated from three molecular data sets, that the supposed restriction of the yucca moths and their allies to the Agavaceae is an artifact caused by an incorrect circumscription of this family. In addition we provide evidence that Yucca is not monophyletic, leading to the conclusion that the modern Yucca-yucca moth relationship developed independently more than once by colonization of a new host.

Cheating in an Obligate Mutualism: How Often Do Yucca Moths Benefit Yuccas?

We observed bouts of oviposition/pollination/pollen collection behaviour by the females of two species of yucca moths associated with Yucca kanabensis. Some aspects of the bouts were highly stereotyped: bouts always began with oviposition and never with pollination; two pollination events never followed each other without an intervening oviposition; and, pollen collection was almost always followed by flight to another inflorescence. However, other aspects of the bouts were highly variable: the number of ovipositions per bout varied from 1 to more than 20, and the number of pollination attempts varied from 0 to 12. Although yucca moths are essential for the sexual reproduction of yuccas, yucca moths did not transfer pollen in 50-80% of their bouts, either because moths did not carry pollen or because they failed to attempt to transfer pollen. Bouts without attempted pollination were characterized by fewer ovipositions than bouts with attempted pollination, but the absence of pollination attempts was not an artifact of short bouts. Whether a moth carried pollen did not affect the likelihood of her failing to attempt to pollinate. Moths responded to the presence of previous ovipositions in flowers by ovipositing less frequently, and by pollinating less frequently or even failing to pollinate at all. However, moths also failed to attempt to pollinate in fresh, previously unvisited flowers. Moths that visited more than one flower usually attempted pollination in some flowers but not others. Moths not carrying pollen often failed to collect pollen even when it was available. Bouts without pollen transfer must decrease the potential net seed production for flowers, because there is an increase in the number of seeds likely to be eaten by yucca moth larvae without an increase in seed set. We interpret these results in light of several hypotheses for why moths should fail to attempt to transfer pollen or fail to carry pollen

Evolutionary stability of mutualism between yuccas and yucca moths

INTERSPECIFIC mutualisms inherently possess a conflict of interests between the interacting species in that fitness increases of one species occur at the expense of the other. This holds for mutualisms as diverse as plant associations with mycorrhizal fungi or nitrogen-fixing bacteria, animals and endosymbionts, and obligate plant-pollinator associations1–6. Prevailing models of interspecific cooperation predict that mutualistic interactions are evolutionary stable only when both interacting species possess mechanisms to prevent excessive exploitation3–6. In light of this, it is paradoxical that some of the classical examples of coevolved obligate mutualism seemingly do not meet this criterion. In mutualisms involving seed parasites that actively pollinate their hosts, such as yucca/ yucca moth and fig/fig wasp interactions, there is no apparent means of retaliation on behalf of the plant. Predictions from theory suggest that a cryptic mechanism, such as selective abortion of flowers with heavy egg loads, could stabilize these interactions4,6–9. Here we present the first empirical evidence that such a mechanism in fact exists in the yucca/yucca moth interaction. A strong nega-tive effect exists between moth egg number and probability of flower retention. Furthermore, we show a strong positive effect between the number of pollinations received and the probability of flower retention. Selective maturation of fruit with low egg loads and high pollen loads provides a mechanism to increase the quantity and possibly quality of seeds produced, and simultaneously select against moths that lay many eggs per flower or provide low-quality pollinations4,6,8,10. Not only can these results explain the stability of this type of interaction, but selection for high-quality pollination also provides a mechanism to help explain the evolution of active pollination among yucca moths.

Mitochondrial Dna Phylogeny of the Prodoxidae (Lepidoptera: Incurvarioidea) Indicates Rapid Ecological Diversification of Yucca Moths

Yucca moths of the family Prodoxidae demonstrate a remarkable coevolved mutualism with their host plants, species of the genus Yucca. Phylogenetic studies of the evolution of these interactions have been hampered by a lack of a robust phylogeny for this group. We inferred a phylogeny of the Prodoxidae from 780 bp of DNA sequence from the mitochondrial cytochrome oxidase I and II genes and the intervening leucine transfer RNA. Generic relationships derived from mitochondrial DNA variation across 38 prodoxid and two outgroup haplotypes were substantially congruent with earlier morphology-based hypotheses, but molecular data provided enough characters to resolve relationships among many taxa unresolved by morphological characteristics. Similarity of sequence divergence estimates among the yucca-feeding lineages suggests that diversification of exploitative strategies in yucca moths occurred during a short period. Our results also highlight the importance of including characters that accumulate changes in both clocklike and episodic fashion in phylogenetic reconstruction.

Reproductive consequences of interactions between Yucca glauca (Agavaceae) and Tegeticula yuccasella (Lepidoptera) in Colorado

For over 100 years the association between Yucca (Agavaceae) and Tegeticula (Lepidoptera: Incurvariidae) has been accepted as a quintessential example of an obligate mutualism. The yucca moth is purported to be the sole pollinator of Yucca, while Yucca flowers provide courtship and mating arenas, and Yucca seeds provide food for developing Tegeticula larvae. We studied Yucca glauca in northern Colorado, comparing the reproductive ecology and breeding systems of Yucca in plains populations, the “preferred” habitat of Yucca, with “marginal” sites at the edge of the local elevational distribution. Tegeticula are abundant at plains sites, and fruit set is significantly higher than in the foothills, where fruit set is limited by the paucity of moths. The low frequency of moths at high elevation, coupled with behaviors of adult female Tegeticula which lead to self‐pollination, failure to pollinate, and periodic overloading of fruits with eggs, may help explain why Yucca glauca appears to maintain alternative pollinators. Some fruits lack evidence of Tegeticula infestation, suggesting that larvae die before completing development, or that fertilization of Yucca sometimes occurs without the intervention of Tegeticula. Biochemical analyses of nectar and observations of floral visitors revealed that it is highly likely that the fly Pseudocalliope sp. nov. (Lauxaniidae), which congregates and mates on Yucca glauca blossoms, acts as a secondary pollinator. Autogamy appears to occur infrequently in natural populations. We therefore propose that the yucca‐yucca moth symbiosis be viewed as a facultative mutualism.

Reproductive Consequences of Interactions between Yucca glauca (Agavaceae) and Tegeticula yuccasella (Lepidoptera) in Colorado

For over 100 years the association between Yucca (Agavaceae) and Tegeticula (Lepidoptera: Incurvariidae) has been accepted as a quintessential example of an obligate mutualism. The yucca moth is purported to be the sole pollinator of Yucca, while Yucca flowers provide courtship and mating arenas, and Yucca seeds provide food for developing Tegeticula larvae. We studied Yucca glauca in northern Colorado, comparing the reproductive ecology and breeding systems of Yucca in plains populations, the “preferred” habitat of Yucca, with “marginal” sites at the edge of the local elevational distribution. Tegeticula are abundant at plains sites, and fruit set is significantly higher than in the foothills, where fruit set is limited by the paucity of moths. The low frequency of moths at high elevation, coupled with behaviors of adult female Tegeticula which lead to self-pollination, failure to pollinate, and periodic overloading of fruits with eggs, may help explain why Yucca glauca appears to maintain alternative pollinators. Some fruits lack evidence of Tegeticula infestation, suggesting that larvae die before completing development, or that fertilization of Yucca sometimes occurs without the intervention of Tegeticula. Biochemical analyses of nectar and observations of floral visitors revealed that it is highly likely that the fly Pseudocalliope sp. nov. (Lauxaniidae), which congregates and mates on Yucca glauca blossoms, acts as a secondary pollinator. Autogamy appears to occur infrequently in natural populations. We therefore propose that the yucca-yucca moth symbiosis be viewed as a facultative mutualism.

Fruit Abortion in Yucca elata and Its Implications for the Mutualistic Association with Yucca Moths

The yucca moth Tegelicula yuccasella is the sole pollinator of Yucca elata, on yucca fruits to complete its life cycle. A high percentage of pollinated flowers aborts, killing the T. yuccasella eggs and larvae in them. We examined patterns of fruit production and abortion in Y. elata, and related these patterns to vegetative characteristics, moth abundance, and environmental conditions. We studied 38 inflorescences throughout their flowering period, during one season in southern New Mexico, USA. Each night we recorded the number of flowers opening, the number of fruit formed, the relative abundance of yucca moths, and climatic conditions. We monitored 11786 flowers, resulting in 699 mature fruit. Large inflorescences produced more, but proportionately fewer fruit than small inflorescences

Pollination ecology of Yucca elata : An experimental study of a mutualistic association.

The pollination biology of a population of 250 Yucca elata (Liliaceae) plants was studied in southern New Mexico. Yucca elata and the prodoxid yucca moth Tegeticula yuccasella have a mutualistic association that is essential for the successful sexual reproduction of both species. However, a wide range of other invertebrate species visit flowers during the day and at night. Our aim was to quantify the role of yucca moths and other invertebrate visitors in pollination and fruit set, using manipulative field experiments. Inflorescences were bagged during the day or night (N=12 inflorescences) to restrict flower visitors to either nocturnal or diurnal groups. Yucca moths were active exclusively nocturnally during the flowering period and thus did not visit inflorescences that were unbagged during the day. None of the 4022 flowers exposed only to diurnal visitors set fruit, whereas 4.6% of the 4974 flowers exposed only to nocturnal visitors (including yucca moths) produced mature fruit. The proportion of flowers producing fruit in the latter treatment was not significantly different from unbagged control inflorescences. In a series of experimental manipulations we also determined that: (1) flowers opened at dusk and were open for two days on average, but were only receptive to pollen on the first night of opening; (2) pollen must be pushed down the stigmatic tube to affect pollination; and (3) most plants require out-cross pollination to produce fruit. The combination of these results strongly suggests that yucca moths are the only species affecting pollination in Y. elata, and that if another species was to affect pollination, it would be a rare event.