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The pollination ecology of durian ( Durio zibethinus, Bombacaceae) in
southern Thailand
Article in Journal of Tropical Ecology · January 2009
DOI: 10.1017/S0266467408005531
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Journal of Tropical Ecology (2009) 25:85–92. Copyright © 2008 Cambridge University Press
doi:10.1017/S0266467408005531 Printed in the United Kingdom
The pollination ecology of durian (Durio zibethinus, Bombacaceae)
in southern Thailand
Sara Bumrungsri∗1 , Ekapong Sripaoraya∗ , Thanongsak Chongsiri∗ , Kitichate Sridith∗
and Paul A. Racey†
∗
Department of Biology, Prince of Songkla University, Hat-Yai, Songkhla, Thailand
† School of Biological Sciences, University of Aberdeen, Aberdeen, UK
(Accepted 10 October 2008)
Abstract: The floral biology and pollination ecology of durian, Durio zibethinus, were determined in eight semi-wild trees
in mixed-fruit orchards in southern Thailand during April-May 2003 and 2005. Flowers open fully at 16h00–16h30
and most androecia drop around 01h00. Anthers dehisce at 19h30–20h00 when the stigmata are already receptive. In
a series of pollination experiments, fruit was set in all treatments within 10 d. The greatest pollination success occurred
after hand-crossed (76.6%), open (54.4%) and emasculation pollination (53.3%). Consistently, hand-crossed (12.2%),
emasculation (8.7%) and open pollination (5.1%) yielded a substantial fruit set 2 mo after the pollination experiments.
Very low pollination success in facilitated autogamy suggests that most durian trees are highly self incompatible. No
mature fruit was found after insect pollination and automatic autogamy. Fruit bats, especially Eonycteris spelaea, are
the major pollinators of this durian although the giant honey bee (Apis dorsata) was the most frequent visitor to the
flowers. Bats visited durian flowers at the rate of 26.1 (SD = 20.7) visits per inflorescence per night. Since this semi-wild
durian depends on fruit bats as its pollinator, protecting fruit bat populations and their roosts is vital for the production
of the durian fruit crop.
Key Words: Durian, Eonycteris spelaea, giant honey bee, insect pollination, mixed-fruit orchard, self-incompatibility,
Thailand
INTRODUCTION
Durian (Durio zibethinus L.) is one of the most popular and
economically important fruit crops in South-East Asia.
Thailand is the leading country for durian production,
with a yield of 686 500 Mg in 2007 (http://www.
dit.go.th/agriculture/product/agri_5/ agri_50650.htm).
Durio zibethinus was introduced to Thailand more
than 300 y ago (Brown 1997, Subhadrabandhu &
Ketsa 2001) and it is hardly surprising therefore
that at least 200 cultivars, resulting from human
selection, have been reported in Thailand (Hiranpradit
et al. 1992). Some of these (e.g. ‘Mon Thong’,
‘Chanee’, ‘Kan Yaw’, ‘Kradum Thong’) are more
popular than others and are grown commercially
in large plantations. However, unnamed seed-planted
1
Corresponding author. Email: sara_psu@hotmail.com
semi-wild durian has been grown traditionally for
household consumption in southern Thailand, Malaysia
and Indonesia. Although extensive information on the
floral biology and pollination ecology of commercial
durian is available (Honsho et al. 2004a, b, 2007a, b;
Lim & Luders 1998, Salakpetch et al. 1992), little is known
about semi-wild durian which is probably closest to the
ancestral form.
Although durian flowers conform to the syndrome of
chiropterophily, and fruit bats have been postulated to
be the major pollinators by previous authors (Soepadmo
& Eow 1976, Start & Marshall 1976), only Soepadmo &
Eow (1976) confirmed that with pollination experiments.
However, their experiment was based on only a single
tree and the pollination results were assessed 5 d later.
This is questionable since a recent study suggested
that the durian breeding system is characterized by
late-acting self-incompatibility which appears to work
within 4 wk of pollination (Honsho et al. 2004a). In
addition, some authors have suggested that insects are
SARA BUMRUNGSRI ET AL.
86
potential pollinators of durian (Boonkird 1992) while
nectarivorous birds were confirmed as pollinators in some
species of Durio (Yumoto 2000). Furthermore, almost
complete pollination failure occurred in open pollination
of a commercial durian cultivar planted in a horticultural
station (Honsho et al. 2004a), which may reflect the loss
of natural pollinators in the area. If this is the case, it is a
matter of serious concern for future durian yields since
decreased crop production resulting from pollination
failure has been found in other animal-dependent crops
(Kevan & Phillips 2001). Indeed, the actual pollinators of
D. zibethinus need to be clearly identified. Therefore, the
objectives of the present study are to determine the floral
biology of semi-wild durian and to test the hypothesis
that fruit bats are the principal pollinator of this crop
plant.
METHODS
Study species
Durio zibethinus is suggested to be native to Borneo,
Sumatra and Peninsular Malaysia (Morton 1987,
Subhadrabandhu & Ketsa 2001). From at least 28
recognized species of Durio, D. zibethinus is the most
common cultivated species, although no wild extant D.
zibethinus has been reported. In Thailand, a few cultivars
of D. zibethinus are planted on a commercial scale, using
trees cloned from the vegetative parts of a selected variety
to preserve the original characters. Farmers in southeast Thailand also carry out hand-crossed pollination
of commercial cultivars. On the other hand, unnamed
semi-wild durian grown from seed and exposed to little
or no artificial management, is commonly planted in
southern Thailand, Malaysia and Indonesia. Since the
seed from which it grows results from open pollination,
this semi-wild durian is assumed to be phylogenetically
close to extinct wild durian. The genetic diversity of the
semi-wild durian is reflected in a variety of taste and
aril characters. Semi-wild durian trees are robust, and
resist drought and fungal infection much better than
commercial varieties. Thus, it is normally used as grafting
stock for commercial cultivars, and also as stock for
breeding selection. Generally, trees begin to flower when
they are about 8 y old. The fruits of semi-wild durian are
found on old branches (i.e. durian is ramiflorous). They
are small and round, and there are a large number per
tree. They are typically collected from the ground after
they have ripened and fallen. Semi-wild durian trees are
usually large and some trees have a diameter at breast
height in excess of 1 m. Currently, large semi-wild durian
trees are selected for furniture wood, while young trees
are rarely planted.
Study sites
The present study was undertaken in lowland
traditional mixed-fruit orchards 0.5–3 km from Ton Nga
Chang Wildlife Sanctuary, Songkhla Province (6◦ 56 N,
100◦ 14 E, 100 m asl). Tropical lowland rain forest covers
most of this wildlife sanctuary. A cave containing a colony
of the fruit bat Eonycteris spelaea is c. 18 km from the
study orchards. In mixed-fruit orchards, a large number
of fruiting plants are planted to form several vertical
layers. Canopy-top species are planted well-spaced, while
subcanopy and understorey species are planted in the
spaces between. Durian and petai (Parkia speciosa Hassk.)
are canopy-top species while duku (Lansium domesticum
Corr.), mangosteen (Garcinia mangostana L.), banana
(Musa spp.) and ginger (Zingiber spp.) predominate in the
subcanopy and understorey. In these orchards, natural
streams, if present, are maintained as a source of water for
plants. Farmers normally do not use herbicide to control
weeds, and organic fertilizer is rarely applied. This kind of
orchard supports high biodiversity (Round et al. 2006).
The study orchards are surrounded mainly by Hevea
brasiliensis (A. Juss.) Müll.Arg. plantations, and other
mixed orchards. Durian trees in the study orchards varied
in age from 15 y to more than 80 y old, with the majority
more than 50 y old. Tree height ranges from 15–25 m. In
the study area, although it rains every month, the rainy
season is from mid-April to December and rain is heaviest
in late October to mid-December. Average annual rainfall
in Ton Nga Chang Wildlife Sanctuary is 1801 mm.
Floral biology
Flowers of eight semi-wild durian trees were accessed
using climbing gear. Floral biology was determined
including flower opening time, nectar secretion rate,
time of anthesis and time of corolla drop. Hydrogen
peroxide was applied to determine whether the stigma
was receptive when the anthers dehisce, and the presence
of a bubble of oxygen is an indicator of stigma receptivity.
Nectar was collected using 80-μl microcapillary tubes
once an hour until the corolla dropped. Nectar standing
volume was also measured before the corolla dropped.
Nectar concentration was measured with a pocket
refractometer.
Pollination experiments
Since up to 100 flowers are tightly packed in an inflorescence, flowers were cut until there were 15–30
flowers per inflorescence to minimize the effect of
number of flowers on pollination success. The pollination
experiments comprised: (1) open pollination: all potential
Pollination ecology of durian in southern Thailand
pollinators were allowed to access flowers; (2) automatic
autogamy: all pollinators were excluded by bagging
flowers from 15h00–18h00, before anthesis occurred;
(3) insect pollination: inflorescences were covered with
plastic nets (16 mm mesh size) allowing access by insects
but not bats; (4) hand-crossed pollination: anthers were
removed before anthesis, and stigmata were rubbed
directly with dehisced anthers of the ‘Mon Thong’ durian
cultivar and bagged; (5) facilitated autogamy: stigmata
were rubbed with anthers from the same tree and then
bagged; (6) emasculation pollination: the anthers were
removed before anthesis with sharp scissors and the
flowers were left uncovered allowing access by pollinators.
Flowers were subjected to hand-crossed pollination and
facilitated autogamy at 20h00 after anthesis. Large semipermeable cloth bags (diameter 20 cm, 35 cm high) with
a plastic net inside to stop the flowers touching the cloth
were used for bagging flowers. In seven sampling trees,
three to six replicates per treatment were applied, while
only one replicate per treatment was undertaken in the
eighth sampling tree. It takes about a week for all flowers
in the whole inflorescence to finish blooming. Fruit set was
determined at 10, 20, 30 and 60 d after the experiment.
Since D. zibethinus appears to have a late-acting selfincompatibility breeding system (Honsho et al. 2004a,
Lo et al. 2007), the fruit set at 10 and 20 d may include
both persistent pistils or unfertilized fruits as well as those
fertilized fruits. The study was undertaken during April
to May of 2003 and 2005. Nested ANOVA was applied
to examine the difference in pollination success between
treatments in each period. Inflorescences were nested
within tree. All statistical analyses were carried out using
SPSS 15.0.
Observation of flower visitors
Nocturnal visitors were observed with a night-shot video
camera equipped with infrared light at a distance of c.
15 m from inflorescences. Observations were undertaken
continuously between 19h00 until the corolla dropped
(c. 01h00), from 28 April to 5 May 2003 on 11
inflorescences. The number of visits was counted.
Crepuscular visitors to flowers were also noted.
Long-distance observations by video camera may be
biased toward larger objects like bats, so an alternative
technique, a scouting camera, was used to observe visitors
to durian flowers at a closer distance. Four digital scouting
cameras (Game Spy I-40, Moultrie Feeders, USA) were
set in five durian trees for 1–2 nights in each tree on
19–23 April 2008. This observation was carried out
in a mixed durian plantation that included both the
‘Mon Thong’ cultivar and semi-wild durian, with most
scouting cameras set on the former, since they were
more accessible and fewer semi-wild durian trees were
87
in flower during that period. It was assumed that visitor
community structure was similar between semi-wild
and ‘Mon Thong’ cultivar trees. Infrared cameras were
pointed at two to five inflorescences of opened flowers
at a distance of c. 2 m. Both 5-s video and still pictures
were taken when cameras were triggered. The movement
of some objects, such as bats, birds or branches, but
not insects, up to 16 m from a sensor can trigger the
camera. Thus the number of pictures of insects visiting
the flowers largely depended on other moving objects.
Visitors to sampled flowers at 18h00–06h00, the period
during which pollination is most likely to occur in this
durian, were observed and identified, and the frequency
percentage of each was calculated. This may be underestimated for bats as the trigger time of the cameras was
3 s which may not be rapid enough to capture flower visit
by bats that usually last for 1–2 s. Bats were identified
to species, when possible, by size and hair colour. Three
species of bat that are mainly nectarivorous occur in
this area: Eonycteris spelaea, Macroglossus sobrinus and M.
minimus (nomenclature according to Corbet & Hill 1992).
Eonycteris spelaea is the largest with fore-arm (FA) length
of 61–78 mm and body mass (BM) of 45–60 g with dark
fur while M. sobrinus and M. minimus are much smaller
(FA = 44–50 mm, BM = 13–23 g and FA = 36–44 mm,
BM = 11–16 g respectively, Corbet & Hill 1992, Payne
et al. 1985) with brighter red fur. Insects were identified
to groups such as bees, moths, and to species in the case
of the giant honey bee (Apis dorsata). The behaviour of
visitors at flowers was also noted.
Bat sampling at flowering trees
Mist-nets (2.6 m × 9 m) were set 6 m high in durian
plantations where scouting cameras were set during the
flowering period to confirm the species identity of bats.
Nets were checked every 15 min for netted bats. Captured
bats were identified to species following Corbet & Hill
(1992).
RESULTS
Floral biology
Semi-wild durian had a short flowering period which
occurred at the same time in three different years. Its
main flowering period was mid-March to late April. In
each individual tree, flowering lasted for c. 10 d. Petal
lobes began to separate and the style obviously exserted
at 16h00: the petal lobes gradually recurved outward,
the anthers gradually exserted and recurved outward
followed by the petal lobes which fully separated from the
stigma by 20h00. When fully open, most of the study trees
88
SARA BUMRUNGSRI ET AL.
Figure 1. Per cent fruit set at 10, 20, 30 and 60 d, respectively after anthesis in six pollination treatments carried out in eight semi-wild durian trees
during April–May 2003, and 2005. The different shading represents different times after anthesis. The box represents lower quartile, median and
upper quartile. The whiskers and star represent minimum, maximum and extreme outlier values.
had a flower with a style exserted beyond the anthers (i.e.
herkogamy), although one tree had a style and anthers
nearly at the same height. Anthers dehisced at 19h30–
20h00. The stigma was moist when the anthers released
pollen, and was already receptive. Nectar secretion
began in the late afternoon (c. 16h30) after flowers
opened, and the c. 0.37 ml of nectar had accumulated by
19h00. The secretion rate was about 0.05 ml h−1 from
that time until 01h00 when the corolla dropped. The
average (± SD) total nectar volume was 0.65 ± 0.14 ml
(range = 0.48–1.00 ml, n = 18). Sucrose concentration
was highest in the early evening, 21.9%, and gradually
decreased until flowers dropped (13.3%) with an overall
average concentration of 17.2%. Nectar standing volume
at 01h00 was 0.97 ± 0.26 ml (range = 0.65–1.4 ml,
n = 9) with average concentration of 15.4% ± 1.14%
(range = 12.8–16.4%). The androecium of most flowers
(>50%) dropped at 01h00, and by 06h00, all androecia
had dropped.
Pollination experiments
Pollination experiments were carried out on 3089 flowers of 127 inflorescences from eight trees. The average
number of flowers in each sampled inflorescence was
23.7 ± 9.5 (mean ± SD, range = 9–71) and the number
of inflorescences in each treatment was 16–30
(mean ± SD = 21 ± 4.9). Ten days after the pollination
experiments, most fruit had set as a result of handcrossed pollination (mean = 76.6 %, median = 80.6 %,
n = 8 trees), followed by open pollination (mean =
54.4%, median = 64%), emasculation pollination
(mean = 53.3%, median = 64.4%), facilitated autogamy
(mean = 26.3%, median = 31.9%), insect pollination
(mean = 22.6%, median = 24.2%) and automatic autogamy respectively (mean = 12.8%, median = 8.9%,
Figure 1). There was a statistically significant difference
between treatments (F = 5.11, df = 36, P < 0.001),
with the exception of hand-crossed pollination and
open pollination (F = 4.2, df = 1, P = 0.062), and the
latter and emasculation pollination (F = 0.01, df = 1,
P = 0.923). Fruit abortion occurred in all treatments.
The majority of fruit abortion occurred during the
first 20 d after pollination experiments, and after that
period, fruit abortion gradually decreased (Figure 1).
At 60 d when fruit was almost mature, the pattern
of pollination success was generally similar to that
recorded at 10 d. It was greatest after hand-crossed
pollination (mean = 12.2%, median = 13.7%) followed
by emasculation (mean = 8.7%, median = 5.6%), and
open pollination (mean = 5.1%, median = 4.3%).
Facilitated autogamy also yielded a very small percentage
of fruit (mean = 1.2%, median = 0.7%). No fruit was
found from automatic autogamy and insect pollination
(Figure 1). Pollination success was significantly different
between treatments (F = 5.29, df = 5, P = 0.001).
Although pollination success from hand-crossed
pollination was higher, it was not significantly different
from open pollination (F = 3.35, df = 1, P = 0.087) and
emasculation pollination (F = 0.68, df = 1, P = 0.418).
Also, pollination success between open pollination and
Pollination ecology of durian in southern Thailand
89
20
18
Number of visits
16
14
12
10
8
6
4
2
0
19.00-20.00
20.00-21.00
21.00-22.00
22.00-23.00
23.00-24.00
24.00-01.00
Time
Figure 2. The frequency of bat visits at inflorescences (n = 11) of durian for the period 28 April–5 May 2003. Bats were observed with infrared
night-shot video at a distance of c. 15 m. Error bar represents 1 SD.
emasculation pollination was not significantly different
(F = 0.59, df = 1, P = 0.45).
Observation of flower visitors
From video observations, the commonest nocturnal
visitors were bats, probably E. speleae from its size and
typical ‘tseet’ call. Bats visit flowering trees either as a
flock, more than 30 bats when trees were in full bloom, or
only a few individuals after the main flowering period of
the tree had just passed. Visits by bats were sporadic and
erratic. Some inflorescences were rarely visited by bats,
others were repeatedly visited. From 11 inflorescences
observed over four nights, bats made 26.1 visits per
inflorescence per night (SD = 20.7, range = 6–72 visits).
The peak of visits occurred at 20h00–21h00 (Figure 2).
The number of visits was then lower and relatively stable
until 01h00 when visits by bats ceased. Bats mostly spent
c. 1–2 s, and occasionally, up to 1 min in each visit. Beetles
and moths were also observed. Some nectarivorous birds
(flowerpeckers, Nectariidae) were rarely found feeding on
flowers during the late afternoon.
From 64 h of observation with infrared scouting
cameras, 1964 clips of 5-s video and 2078 still pictures
were taken. No visitors were seen in most of these clips
and pictures (75% of video clips and 77.3% of pictures).
From 490 video clips and 471 pictures with observed
visitors, bees predominated both from video (80%) and
still pictures (92.5%). These bees were large, presumably
giant honey bees (Apis dorsata) which visit during both
day and night. Bats were observed in only 11.4% of
video clips and 5.7% of still pictures. Moths and other
visitors were least observed, in 8.5% of video clips and
1.8% of still pictures. With identifiable pictures from
scouting cameras, bats visiting flowers were all identified
as Eonycteris spelaea (n = 25). Bats land on inflorescences,
head up or nearly horizontal in the face-up position,
thumb claws holding opened flowers, and they insert their
muzzle into the corolla tubes of flowers. For visits longer
than 1 s, the bat fed on several flowers consecutively.
However, bats appear to collect pollen or check for the
presence of nectar rather than feeding on nectar during
those shorter visits (≤ 1s). At every visit, the momentum
of the flying bat shook the inflorescence and consequently
the bat’s wings, body and face rubbed against anthers and
stigmas. No bats were observed at durian flowers after the
corolla dropped.
Bat sampling at flowering trees
Five bats were captured during 10 net hours. All were
identified as Eonycteris spelaea. Three were juvenile and
two were mature males. Pollen was found on the body of
all captured bats.
DISCUSSION
Floral biology
Although the floral biology of semi-wild durian is broadly
similar to that reported in commercial cultivars, some
aspects, which may have a significant effect on its
pollination ecology, differ slightly from that reported for
cultivars. Previous studies indicated that anthesis and
anther dehiscence of the ‘Chanee’ cultivar in southeast Thailand occur simultaneously at 19h00 (Boonkird
1992, Honsho et al. 2007a) or even earlier (17h45) in
90
some cultivars (Salakpetch et al. 1992, Subhadrabandhu
& Ketsa 2001). All floral organs except gynoecia of
durian are shed the morning after flowering (c. 09h00)
in the ‘Mon Thong’ and ‘Chanee’ cultivars (Honsho et al.
2004b, 2007b, W. Thawiphon unpubl. data) or within
16–48 h after anthesis in commercial durian cultivars
planted in Australia (Lim & Luders 1998). The timing
of anther dehiscence in the present study is comparable
with that reported by Soepadmo & Eow (1976) at around
19h30–20h00, whereas that of commercial cultivars
is earlier. Androecium abscission of semi-wild durian
occurs much earlier than in commercial activars in our
study and that of Soepadmo & Eow (1976) (01h00)
or even earlier (23h00) according to Valmayor et al.
(1965). The time of anther dehiscence of the ‘Mon Thong’
cultivar planted sympatrically in the study orchards
is similar to that reported previously in south-east
Thailand, but androecium abscission is earlier, as in semiwild durian (S. Bumrungsri, pers. observ.). The results
from hand-crossed pollination suggest that the effective
pollination period (i.e. the interval of time during which
pollination can occur) of some commercial cultivars (e.g.
‘Chanee’) is between 6 h before anthesis and 12 h after
(Honsho et al. 2007b), so it is synchronized with flower
longevity although stigma receptivity was reported to be
much longer (Salakpetch et al. 1992). Compared with
commercial cultivars, the effective pollination period in
open pollination of semi-wild durian is much shorter than
that in the natural pollination of commercial cultivars,
which is only 5.5 h, from 19h30 to 01h00. Thus, all
pollination success in open pollination is the result of
nocturnal pollinators. This challenges the suggestion of
Boonkird (1992) that stingless bees (Trigona spp.), the
major diurnal flower visitors, are potential pollinator of
D. zibethinus.
The concentration and volume of nectar in semi-wild
durian are comparable to those reported in commercial
cultivars (Boonkird 1992, W. Thawiphon unpubl. data).
The optimal sucrose concentration for pollen germination
was 10–15% in the ‘Mon Thong’ and ‘Chanee’ cultivars
(Honsho et al. 2007a, W. Thawiphon unpubl. data).
Pollination experiments
Although a relatively high percentage of fruit set was
found in all treatments 10 d after pollination experiments,
considerable fruiting (>5%) at 60 d was found only
in three treatments, namely open, hand-crossed and
emasculation pollination, while a significantly smaller
fruiting percentage was observed in facilitated autogamy.
The latter yields a lower fruit set or no fruit set at all
compared with cross-pollination in commercial cultivars
(Honsho et al. 2004a, Lim & Luders 1998, Lo et al.
2007). Lim & Luders (1998) also indicated the poor
SARA BUMRUNGSRI ET AL.
quality of fruits from facilitated autogamy (e.g. misshaped, distorted, up to 50% lighter flesh weight, and
fewer arils). The result from the present study supports the
previous suggestion that most durian trees are highly self
incompatible (Honsho et al. 2004a, Lim & Luders 1998).
A different degree of self incompatibility was recognized
in different D. zibethinus cultivars from partially selfincompatible to completely self-incompatible (Brown
1997, Honsho et al. 2004a, Lim & Luders 1998, Lo et al.
2007, Valmayor et al. 1965, S. Somsri unpubl. data).
Late-acting self-incompatibility which was suggested to
characterize D. zibethinus, was considered to take place
within 4 wk of pollination (Honsho et al. 2004a). Most
self-pollinated fruits of the ‘Chanee’ cultivar dropped
within 10 d and all such fruits drop within 35 d after
pollination (Lo et al. 2007). Cross-pollination between
different cultivars was found to markedly increase
pollination success (Lo et al. 2007, S. Somsri unpubl.
data). Herkogamy, one of the mechanisms to promote
out-crossing pollination, was also described in some
cultivars (Honsho et al. 2007b). Herkogamy is also
observed in most but not all semi-wild durian trees in our
study. When no pollination occurs, almost all unfertilized
fruit of semi-wild durian dropped within 20 d, as also
previously observed in commercial cultivars (Honsho
et al. 2004a). Specifically, non-pollinated flowers of ‘Mon
Thong’ and ‘Chanee’ abscised within 8 d (Lo et al.
2007). No mature fruit has been observed in autogamy
pollination in D. zibethinus, although some clones are selfcompatible. Pollen transfer without animal vectors was
suggested to be impossible since durian pollen is sticky
and is not released at dehiscence. In addition, its pollen is
clumped and still adheres to the pollen sac for at least 6 h
(Honsho et al. 2007a).
Fruit bats are clearly effective pollinators of this semiwild durian, although visits by bats are sporadic (Gould
1978, Start 1974). A further investigation on pollen load
per fruit bat visit is recommended. Since D. zibethinus
has big-bang flowering (i.e. a large number of flowers
available for a short period), it attracts a number of visitors
including nectarivorous bats. Several species of fruit bat
are reported to visit D. zibethinus flowers (Brown 1997,
Gould 1977, 1978). Eonycteris speleae appears to be the
major pollinator since it is a true nectarivore and also
the most common nectarivorous species in Thailand, and
can travel at least 38 km per night (Start & Marshall
1976). Previous studies indicated a varying proportion of
D. zibethinus pollen found in faeces of E. spelaea (Soepadmo
& Eow 1977, Start & Marshall 1976) and a recent
study indicated that pollen of Durio contribute 39–42%
of the diet of captured E. spelaea in March and April
(n = 30–41 in each month) (Bumrungsri et al. unpubl.
data). Although a previous study indicated that stingless
bees and bees are responsible for the majority of flower
visits (Boonkird 1992, W. Thawiphon unpubl. data),
Pollination ecology of durian in southern Thailand
our pollination experiments did not show that they are
pollinators of D. zibethinus. A recent study on stigma
pollen load found that both bees and stingless bees visit
D. zibethinus flowers primarily for collecting pollen and
did not touch and transfer pollen onto stigmata (W.
Thawiphon unpubl. data, S. Bumrungsri pers. obs.) which
mostly exsert beyond the anthers. During D. zibethinus
flowering, 90% of pollen loads collected from stingless
bees returning to their nests is pollen of this species (W.
Thawiphon unpubl. data).
Since pollination success in durian depends on
fruit bats, protecting fruit bat populations, especially
nectarivorous ones, is vital for securing the future of the
durian crop, otherwise no natural pollination will occur.
In one case, open pollination carried out in Chantaburi
Horticulture Research Center was reported to yield very
low fruit set (0–1.4%) (Honsho et al. 2004a). This may
indicate that pollinator activity is minimal, probably
resulting from a relatively small population of fruit bats.
In such areas, most fruit farmers treat fruit bats as a
nuisance, so they are netted and killed and when a
colony is found, it is eradicated (S. Bumrungsri, pers.
obs.). An alternative explanation for pollination failure
is resource-density dependence in pollinator activity.
Since D. zibethinus has a big-bang flowering system, and
flowering is synchronous between trees, bats may ignore
a small number of flowers in experimental inflorescences
(i.e. two to five flowers, Honsho et al. 2004a) compared
with a large number of opened flowers available in
other inflorescences. Field observations based on scouting
cameras also suggests that a very small number of
visits occurred when there were few opened flowers
in inflorescences. As the nectarivorous bats, especially
E. spelaea are also the principal pollinators of other
chiropterophilous plants, e.g. Parkia speciosa Hassk. and
P. timoriana (DC.) Merr. (Bumrungsri et al. 2008),
Oroxylum indicum Vent. (Srithongchuay et al. 2008, Start
& Marshall 1976), the protection of bat populations and
their habitats is ecologically and economically important.
A recent economic assessment in the area of 8756 km2 in
southern Thailand surrounding four caves with colonies
of E. spelaea indicated that the economic value in 2007
of the pollination services of fruit bats to D. zibethinus and
P. speciosa was at least 13 million dollars annually (K.
Petchmunee unpubl. data).
ACKNOWLEDGEMENTS
Thanks are due to Dr Chris Wilcock from Aberdeen
University, who commented on the original research
proposal, to Drs S. Sotthibundhu and C. Satasuk for
valuable discussion especially during the beginning
of the project, and to the 4th year students of the
Biology Department of Prince of Songkla University (PSU)
91
between 2003 to 2006, and Masters students both from
PSU and Khon Kaen University for help in the field. We
are grateful to the owners of the durian orchards for
permitting us to conduct research and for their hospitality
during field work, to Prof. P. Poonsawad for training in
tree-climbing techniques. Thank are due to the Ministry
of University Affairs and the Thailand Research Fund for
providing us with an opportunity and financial support to
conduct this research. Additional financial support was
also made by Prince of Songkla University, the Carnegie
Trust for the Universities of Scotland, and The British
Council.
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