|Saw palmetto (Serenoa repens (Bartr.) Small), a prostrate
palm endemic to the southeastern coastal plain of the United States, is
common in a variety of habitats from seasonally flooded pine forests to
xeric coastal dunes and inland scrub (Tanner et al. 1996). Saw palmetto
flower and fruit production is important both ecologically and commercially
in the region. Over 300 species of insects have been observed visiting saw
palmetto flowers (M. Deyrup, Archbold Biological Station, pers. comm.); some
of these flower visitors collect nectar and/or pollen, while others find
mates or prey near or on the flowers. Where present, European honeybees (Apis
mellifera) are prominent flower visitors that produce commercial “saw
palmetto honey.” Saw palmetto fruits are eaten by many species of wildlife,
including black bear (Ursus americanus), white-tailed deer (Odocoileus
virginianus), raccoon (Procyon lotor), wild turkey (Meleagris gallopavo),
northern bob-white (Colinus virginianus), gray fox (Urocyon cinereoargenteus)
and gopher tortoise (Gopherus polyphemus; Maehr & Layne 1996). In addition,
fruit demand for medicinal use has increased because saw palmetto fruit is
used to treat benign prostatic hyperplasia (enlarged prostate; Berry et al.
1984). Since 1996, annual harvests of saw palmetto fruits in Florida have
totaled at least 7,000,000 kg (M. Huffman, Plantation Medicinals Inc., pers.
Saw palmetto flowers are borne on densely-branched, interfoliar
inflorescences, 0.5–0.75 m long (Fig. 1). Saw palmettos can produce over
five inflorescences at one time, but commonly produce one to three. Each
inflorescence contains several thousand individual flowers (Fig. 2). The
bisexual flowers are 5–6 mm long, with three white, partially connate petals
that are reflexed at anthesis (Fig. 3). Each flower has six stamens and one
pistil, with a 3-ovulate, superior ovary. Usually only one ovule matures
into a seed (Godfrey 1988).
Although seasonality of saw palmetto flowering and fruiting has been
described (Hilmon 1968), very little is known about pollination biology.
Knowledge of pollination biology should help land managers maintain
biodiversity and natural functioning of ecosystems in which saw palmetto is
prominent, while making informed decisions concerning management for wild
fruit harvesting. In this study we (1) documented timing of flower bud
opening, anther dehiscence, stigma receptivity and nectar production, (2)
observed insect visitors to flowers and (3) experimentally characterized
some aspects of the breeding system. Saw palmetto exhibits several
characteristics consistent with biotic pollination (Faegri & van der Pijl
1979), such as a conspicuous floral display, sticky pollen, floral fragrance
and nectar production. Since biotic pollination is usually associated with
outcrossing (Proctor 1978), we expected that the breeding system would
include at least some outcrossing. This study addressed the following
specific questions: 1. How long do flowers function? 2. When is pollination
likely to occur? 3. Are insect pollinators required for seed set?
Materials and Methods
Phenology. We conducted all fieldwork at the University of Florida
Southwest Florida Research and Education Center, in Collier County, Florida.
To characterize timing of bud opening, we observed a total of 16
inflorescences on February 10 and 13, 1998 and March 3 and May 18, 1999.
During the afternoon before each observation, we marked mature buds by tying
nylon sewing thread around the base of each bud. On observation dates, we
checked each marked bud every 20 min from 02:00 to 14:00. We recorded bud
opening, presence of nectar in flowers, anther dehiscence and insect
visitors to inflorescences. We attempted to collect insect visitors, and
sampled pollen loads on collected insects using fuchsin glycerine jelly
Beattie 1971). We also observed insect visitors to five additional
inflorescences during March and April 1998. For these inflorescences, we
recorded insect visitors for 10 min out of every 30 min, from 09:00 to
On February 13, 1998 and May 26–28, 1999 we collected and characterized
1- and 2-day-old flowers. We defined 1-day-old flowers as those that had
opened earlier (either pre-or post-dawn) on the collection date, and
2-day-old flowers as those that had opened the day before the collection
date. On each date we collected 10, 1-day-old and 10, 2-day-old flowers at
10:30 and 13:30. We observed each of the 160 flowers under a dissecting
microscope, and recorded anther dehiscence, presence of pollen in anthers,
presence of moisture and/or pollen on the stigma and presence of nectar in
the flower. For the 120 flowers collected in 1999 we determined stigma
receptivity by applying 3% hydrogen peroxide to the stigmas and watching for
bubbling (indicating peroxidase activity and stigma receptivity) under a
dissecting microscope Kearns & Inouye 1993). On May 26, 27 and 28, 1999, we
also collected and characterized 3-, 4- and 5-day-old flowers, respectively.
We collected 10 flowers at 10:30 and 10 flowers at 13:30 on each of the
three days, and recorded the same information that we recorded for 1- and
2-day-old flowers in 1999.
Breeding System. Saw palmetto is a clonal species with branching,
prostrate stems and a spreading growth habit similar to tillering in
grasses. At the terminal end of each stem branch is a meristem with a
rosette of leaves, hereafter called a ramet. On February 22, 1999 we located
25 saw palmetto ramets of similar height and width that had initiated
inflorescences. We verified that each ramet originated from a separate stem,
and thus were reasonably confident that the ramets were from different
genetic individuals. To characterize the breeding system, we used an
experimental design consisting of five treatments: (1) emasculated and
open-pollinated (allowed xenogamy, geitonogamy), (2) caged and hand
(self)-pollinated (tested for geitonogamy), (3) emasculated and caged
(allowed agamospermy, possible geitonogamy), (4) caged (tested for
self-pollination without flower visitors), and (5) non-manipulated (hereafter
these treatments will be referred to as (1) – (5)). We did not include a
caged, cross-pollinated treatment, because we assumed that the emasculated,
open-pollinated and non-manipulated treatments included xenogamy. We marked 5
ramets for each of the 5 treatments, and marked 20 buds on one inflorescence
of each ramet by gently tying nylon sewing thread below the base of each
bud, for a total of 100 buds per treatment.
For the caged treatments, we placed cylinders (16 cm diameter)
constructed from chicken wire and wrapped twice with white bridal veil (mesh
<3 mm diameter) on inflorescences before anthesis began. To help exclude
pollinators, we used metal wire to fasten excess bridal veil material on the
bottom of each cage around the inflorescence rachis, and applied Tanglefoot
(The Tanglefoot Co., Grand Rapids, MI), a sticky material that excludes or
traps crawling insects, around the base of the rachis. Each cage covered
approximately half of an inflorescence, consisting of 800–1000 buds.
For the hand-pollinated treatments, we monitored buds daily until they
opened. Between 08:00 and 10:00 on the day of bud opening, we carefully
removed cages and obtained stamens from other flowers on the same ramet with
dehisced anthers containing pollen. We then rubbed these anthers over the
stigmatic surface of each treated flower until we observed pollen on the
stigma using a hand lens. We completed this treatment during the same time
period that the emasculation treatment was completed.
We monitored marked flowers until they either fell off or produced fruit.
We removed cages and counted all fruits on May 13, 1999, after all fruits
began to develop. We recorded fruit set when a flower’s style and stigma had
withered but the flower remained attached to the rachilla, and the ovary
wall had turned green. We used a Kruskal-Wallis test to test for differences
in numbers of fruit set among treatments and performed two planned
comparisons. First, to determine if cross-pollination increased fruit set, we
compared treatments (4) and (5). Second, to determine if geitonogamy
increased fruit set, we compared treatments (2) and (3).
Phenology: Flowers opened asynchronously within an inflorescence
over a period of approximately 1 month, with anthesis progressing from the
base to the top of the inflorescence. During the 1998 and 1999 12-hr
observation periods, a total of 37 marked buds opened. Over half of the buds
opened from 02:00 to 04:00. Very few buds opened between 04:00 and 07:00.
Buds opened at a faster rate after 07:00, with approximately 40% of buds
opening between 07:00 and 14:00 (Fig. 4). Nectar was visible at the base of
the gynoecium as soon as buds opened. Anther dehiscence began at 8:00, with
the number of flowers with dehiscing anthers increasing rapidly until 11:00
(Fig. 5). Anthers of shaded flowers and later-opening flowers dehisced
somewhat later than flowers in the sun or flowers that opened earlier.
Median time between dehiscence of the first anther in a flower and
dehiscence of all anthers was 2 hrs (range = 40 min–5 hr). Of 26 flowers for
which we quantified anther dehiscence, 11 had all anthers dehisce by 14:00.
Flowers collected 1, 2 and 3 days after opening of buds showed similar
timing for anther dehiscence. Virtually all anthers of collected flowers had
dehisced during the first day of anthesis. Maximum amounts of pollen were
available on anthers immediately after dehiscence, also during the first day
of anthesis. Stigma receptivity, however, occurred somewhat later than
anther dehiscence, indicating that saw palmetto flowers are weakly
protandrous. During the first day of anthesis, stigmas were receptive in
only 14% of flowers collected. By the morning of the second day of anthesis,
however, over 80% of flowers had receptive stigmas. Proportions of flowers
with receptive stigmas continued to be high (70-100%) through the morning of
the fourth day of anthesis Fig. 6). The three-lobed stigma appeared open and
moist in receptive stigmas. The lobed stigma in non-receptive flowers was
closed, with one lobe appearing as a hood over the stigmatic surface. Nectar
was consistently present in flowers through the second day of anthesis and
sporadically present through the fourth day of anthesis. After the fourth
day of anthesis, styles, stigmas and petals browned and withered.
Insect Visitors: We observed 34 insect species on saw palmetto
inflorescences, representing 7 orders: Orthoptera, Thysanoptera, Hemiptera,
Homoptera, Coleoptera, Diptera and Hymenoptera (Table 1). Approximately 80%
of the species were in the orders Diptera and Hymenoptera.
Diptera were common flower visitors, but usually carried no pollen (Table
1). Syrphid flies (Syrphidae) were represented by three species, Ornidia
obesa and two unidentified species. These flies tended to visit single
flowers and obtain nectar quickly while hovering. One of the unidentified
species was a common visitor that we observed at four of the five
inflorescences. It visited an inflorescence only once or twice during the
day, usually during the morning. Muscid flies (Muscidae) were represented by
two species. These flies obtained nectar while crawling from flower to
flower and usually stayed on inflorescences for several minutes per visit.
Two Diptera species carried pollen on their bodies (Table 1). The first,
Plecia nearctica (Bibionidae; lovebug) was a very common visitor. Single or
coupled individuals actively foraged for nectar, crawled over numerous
flowers, or simply lay on inflorescences, sometimes remaining for hours.
Lovebugs usually carried pollen (typically <100 grains) on various parts of
their bodies. Physoconops sp. (Conopidae) also carried pollen. Two
individuals were observed with pollen on the ventral surface of their
Other Diptera observed and collected were one species each in the
families Stratiomyidae, Bombyliidae and Dolichopodidae. Species in
Stratiomyidae and Bombyliidae were observed on only one occasion. The
Dolichopodidae species was a very common visitor to inflorescences but
presumably visited flowers as an insect predator. Hymenoptera contained the
most species of insect visitors, the most numerous visitors, and virtually
all of the presumed pollinator species (Fig. 7). The most common insect
visitors were ants (Formicidae), represented by Camponotus sp. and eight
unidentified species. Three out of four species that we observed nectaring
carried pollen (typically < 100 grains) on their bodies (Table 1).
was represented by three species: Polistes metricus, Polistes exclamans
Meschocyttarus cubicola floridana. These common visitors obtained nectar and
crawled over numerous flowers, but carried no pollen on their bodies (Table
1). However, four bee species – Colletes sp. (Colletidae), Augochloropsis
metallica and an unidentified species in Halictidae, and Apis mellifera
– were potential pollinators. All of these species carried large loads (>
100 grains) of Serenoa pollen (Table 1). Apis mellifera visited most
frequently, approximately every 30 min to 1 hr at all five inflorescences.
Colletes sp. and the halictid species were observed regularly at two
Other species that we observed visiting flowers were a cockroach (Orthoptera:
Blattidae), a true bug (Hemiptera: Largidae: Largus succinctus) and a beetle
(Coleoptera: Coccinellidae); none of these species carried pollen (Table 1).
We observed the cockroach at night on one occasion. A thrips species (Thysanoptera:
Heterothripidae) and two planthopper species (Homoptera: Flatidae) were
common at inflorescences but were not observed visiting flowers. Notolomus
basalis (Coleoptera: Curculionidae) was common on inflorescence branches.
Although we did not observe it visiting flowers, a collected individual had
fewer than 100 grains of Serenoa pollen on its body (Table 1).
Breeding System. Median fruit set ranged from 0 in treatment (4) to 20%
in treatment (5) (Table 2). A Kruskal-Wallis test showed differences in
number of fruits produced among treatments (χ24=18.58, p < 0.001). Treatment
(5), with cross pollination, had higher fruit set than treatment (1),
without cross pollination (0.01 < p < 0.025). Although no difference in
fruit set was detected between treatments (2), with 100% hand pollination (geitonogamy)
and (3), without hand pollination (p > 0.1), failure to detect a difference
may have been due to small sample size.
Phenology of anthesis, behavior of insect visitors and results of
experimental manipulation of flowers indicated that Serenoa has a
facultatively xenogamous breeding system. Because Serenoa is a clonal
species, however, apparent xenogamy may in fact be pollination from
genetically identical ramets. Whereas addition of a handcross-pollination
treatment may have helped distinguish between xenogamy and geitonogamy, much
more work would be required to characterize genotypes of ramets.
Anthesis lasted approximately 4 d, with weak protandry promoting xenogamy
or geitonogamy. Although pollination is possible throughout anthesis,
probability is highest on the second day. At this time stigmas are most
likely to be receptive, and nectar is most likely to be present as an
attractant for potential pollinators.
Although Serenoa flowers were visited by a wide variety of dipterans and
hymenopterans, the primary pollinators appeared to be bees. Bees carrying
large loads (> 100 grains) of Serenoa pollen regularly visited flowers of
every inflorescence observed. In addition to carrying large pollen loads,
bees promoted xenogamy by visiting inflorescences of many different Serenoa
ramets, and by crawling over numerous flowers of each inflorescence. Through
this activity, bees not only may pollinate two-day-old flowers while
obtaining nectar, but also may pollinate older flowers with receptive
stigmas, but without nectar. This behavior is equally likely to result in
geitonogamy and self-pollination.
The most prominent insect in this study was the European honeybee (Apis
mellifera), a likely function of nearby (< 1 km) apiaries. Where honeybees
are sparse or absent, native bees are likely the primary pollinators (M.
Deyrup, Archbold Biological Station, personal communication). Although we
observed little or no pollen on bodies of most flies and wasps, they may
crosspollinate Serenoa flowers. Behavior of other insects that remain
primarily on one inflorescence (e.g., lovebugs, ants) occasionally may
result in geitonogamy or self-pollination.
We conclude from the results of experimental manipulation of flowers that
while both geitonogamy and xenogamy are possible, insects are required for
effective pollination of Serenoa flowers. Treatments (2), (3) and (4) showed
that geitonogamy is possible but results in low or only occasional fruit
set. The comparison between treatments (1) and (5) demonstrated that
xenogamy increased fruit set to normal levels. Three possible explanations
for fruit set in treatment (4) are apomixis, pollination of flowers by
thrips, and geitonogamy via gravity or wind. A caged treatment using
insecticide to exclude thrips would help to clarify the mechanism (Baker &
Cruden 1991, Kearns & Inouye 1993).
Subtle differences in breeding systems exist between Serenoa and related,
co-occurring palm species. Sabal etonia Swingle ex Nash has weakly
protandrous, primarily bee-pollinated flowers similar to those of Serenoa.
Timing of flower opening and anther dehiscence also were similar, but unlike
Serenoa, anthesis in Sabal etonia lasted only 1 day (Zona 1987).
palmetto (Walter) Lodd. ex Schult. also has primarily bee-pollinated
flowers, but the flowers are protogynous and function only for 1 day (Brown
1976). Sabal minor (Jacq.) Pers. has weakly protogynous, primarily
wasp-pollinated flowers that function for 1 d (Ramp 1989). Rhapidophyllum
hystrix H. Wendl. & Drude is usually dioecious, has self-compatible flowers,
and is reportedly pollinated by a species of Notolomus (Shuey & Wunderlin
Percentage fruit set for Serenoa in this study is low when compared to
other palm species (Brown 1973, Ramp 1989), but is comparable to natural
Serenoa fruit set from other sites. Fruit set from six ramets monitored in
two other southwestern Florida sites during a concurrent study ranged from
2-39%, and averaged 18% (M. Carrington, University of Florida, unpublished
data). Serenoa’s low fruit set may be the result of a preponderance of
pollination by geitonogamy among different genetically identical ramets.
Because Serenoa shares pollinating species (notably Apis mellifera) with
at least the two other bee-pollinated palms, competition for pollinators
could occur. However, Serenoa has a longer flowering season than either
Sabal etonia or Sabal palmetto (personal observation), its inflorescences
are longer-lived (Zona 1987, personal observation), and its flowers are
longer-lived (Brown 1976, Zona 1987). All of these characteristics should
increase the likelihood that Serenoa flowers will receive intraspecific
pollen. In addition, we identified only Serenoa pollen on insects visiting
Serenoa flowers, suggesting that constancy of flower visitors was high.
As a result of demand for saw palmetto fruits for medicinal use, interest
in fruit harvesting and in establishing commercial plantations has
increased. Results from this study indicate that insect pollination of
flowers is an essential component of managing saw palmetto for fruit
production. To encourage insect visitation to flowers, land managers should
not use insecticides in managed areas during flowering and should reduce or
suspend insecticide use in areas adjacent to saw palmettos. Although placing
apiaries in or near saw palmetto areas during flowering may increase fruit
set, introduced honeybees may out-compete native bee species, thus reducing
rates of pollination for other native plant species (Corbet 1991).
Establishment of plantations has been virtually non-existent in the
United States, and probably is not needed in the Southeast where extensive
areas of wild saw palmettos occur. Where saw palmetto is cultivated in
greenhouses, nurseries or plantations, however, this study has shown that
opportunities may exist for self- or crosspollination of flowers via
hand-pollination. Annual saw palmetto flowering and fruiting are significant
ecological events that attract hundreds of insect species, and provide food
for bird and mammal species, most notably the rare Florida black bear. Land
managers will face increasing challenges to provide human benefits (i.e.,
wild fruit picking, enlarged prostate treatment) while conserving
biodiversity and ecological phenomena. This study, through reporting on the
natural history of flowering and pollination, is a contribution toward this
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