Identifying Contributions of Wasps, Diptera, and Bees to the Pollination of Parsonsia alboflavescens
Bonnie
Lumi
3
Mema ,
Tatyana
2,3
*
Livshultz
Biology Department, 2 BEES Department, Drexel University, Philadelphia, PA; 3 Academy of Natural Sciences, Philadelphia, PA ;
* tl534@drexel.edu
Results (cont.)
Abstract
Methods (cont.)
Premise: The Pollination Syndrome Hypothesis proposes that one can predict the pollinators
of a flower from its traits because different pollinators are attracted to different characteristics
and exert selection pressure on their floral hosts. Parsonsia alboflavescens (Apocynaceae –
milkweed family), a Southeast Asian vine, has traits indicative of the wasp pollination
syndrome: exposed nectar and greenish flowers. Wasps, bees, Diptera, and Lepidoptera were all
observed as frequent floral visitors in the field in Taiwan.
Methods: To determine which visitors are legitimate pollinators and nectar thieves, virgin
flowers were enclosed with a single insect: 12 wasp, 8 fly, 1 bee, and 7 control enclosures. The
insect was allowed to nectar for 1-4 hours and then both the insect and flowers collected.
Flowers were scored for pollen grains and clumps.
Key Results: Stigmas from wasp enclosures had an average of 98 +/- 98 pollen grains,
compared to 13 +/- 30 for Diptera, 7 +/- 6 for bee, and 19+/- 38 for control enclosures. Pollen
clumps were present on 15 wasp-exposed stigmas, none of the fly- or bee-exposed stigmas and
1 control stigma. Wasp-exposed stigmas are significantly more likely to be pollinated than
control stigmas (chi-square=8.933, p=0.0028). However only 6 of 12 wasp enclosures averaged
more pollen grains per stigma than control enclosures.
Conclusions: Wasps pollinate P. alboflavescens, as predicted by its pollination syndrome, but
flies appear to be nectar thieves. There is insufficient replication of enclosures with bees to draw
conclusions. Further enclosure experiments (using large flight cages for Lepidoptera) are needed
to determine if bees and Lepidoptera are pollinators.
Field Methods: May 24, 2014-May 27, 2014, at Kenting National Park, Taiwan. Enclosures
with virgin flowers, buds opened inside the enclosure (Fig. 2B), protected from insects. Caught
and introduced one insect to the Virgin Flower Enclosure (VFE). Observed insect begin to
nectar on flowers (Fig. 2A). Left the insect in the enclosure for 1-4 hours. Collected insect and
gynostegia (anthers and stigma) of open flowers in enclosure.
Lab Methods: Imaged the gynostegium (Fig. 2C) using a Leica M165 C, separated the anthers
from the stigma using a blade, recorded number of pollen clumps on stigma, placed pollen
from anther in dye (Calberla’s Fluid) on a labeled slide and held the coverslip in place using
clear Revlon nail polish, repeated the process for the pollen from the stigma but on a separate
labeled slide, waited several days for the dye to properly set into the pollen grains, imaged the
slides (Fig. 2D) using a Leica M165 C, counted the pollen grains using ImageJ based upon
grain size (number of pixels) and circularity of the grains. Recorded the data collected on paper
and later transferred the data into an Excel spreadsheet. Four flowers from each enclosure were
scored for the number of pollen clumps and pollen grains on the stigma.
In nature, which insects are effective pollinators of P. alboflavescens and which are
nectar thieves?
B
A
Wasps
33%
Lep.
8%
Bees
Wasps
Lep.
Diptera
Diptera
59%
July 4-6, 2011
Lep.
0%
35
Diptera
2%
30
25
0 Pollen Clumps
1 Pollen Clump
2 Pollen Clumps
3 Pollen Clumps
20
15
10
5
0
Control
Bees
29%
Fig. 1. Frequency of nectaring visits. Five minute intermittent bouts of continuous
observation during daylight hours. A. 2011, 330 min. total, 520 visits, whole plant
observation. B. 2012, 320 min. total, 1000 visits, focal inflorescence observation.
Bees, wasps, Lepidoptera and Diptera were all observed visiting Parsonsia alboflavescens.
Methods
Wasp
Small Bee
B
Diptera
250
Wasps
69%
August 16-21, 2012
A
1 mm
Fig. 3. Pollen Clumps on Stigma of Flowers Collected.
Wasps are the only visitors whose flowers actually had pollen clumps on the stigma. One
control flower had a pollen clump.
Pollen Grains
Background
2
200
150
D
100
50
0
Control
A
Wasp
Small Bee
Diptera
Fig. 4. Average Pollen Count on VFE Stigma
The flowers that were visited by wasps had an average of 98 pollen grains per stigma, much
greater than the control average of 19. The flowers visited by small bees and Diptera had an
average of 7 and 13 pollen grains per stigma respectively.
Table 1. Chi-square Analysis
A
B
Control
Wasp
Small Bee
Diptera
C
C
D
Fig. 2. A. Virgin Flower Enclosure (VFE) with Diptera, B. P. alboflavescens flower, C. P.
alboflavescens gynostigium, D. Stigma pollen slide
800
700
600
500
400
300
200
100
0
Results
Number of Flowers
Research Question
900
Average Pollen per Stigma
1
1
Wiedel ,
Not
Pollinated
Pollinated
27
30
4
29
1
15
0
0
X2
8.9335
0.1475
1.0542
P-Value
0.0028
0.700969
0.30454
Table 1 shows the Chi-square statistics. According to the analysis, the P-value from the
comparison between wasps and control is less than 0.05 which means that the null hypothesis
that there is no difference in pollination between control flowers and those visited by wasps is
not supported. The P-values for small bees and Diptera are greater than 0.05, meaning that the
null hypothesis cannot be ruled out.
Number of Enclosures
Fig. 5. Average Pollen on Stigma per Enclosures
This figure shows the average number of pollen grains per stigma per enclosure that have an
average of greater than 10 pollen grains per stigma. Fifteen enclosures had an average of less than
10 pollen grains per stigma and were not graphed. Of the enclosures not graphed, five enclosures
were wasps, five were Diptera, one was bees and four were controls. Wasps had the greatest
average of pollen grains per stigma per enclosure.
Discussion and Conclusions
Figure 1 shows data taken from a previous experiment which recorded the insects that visited
the Parsonsia alboflavescens flower during July 4-6, 2011 and August 16-21, 2012.
Wasps are the best pollinators for P. alboflavescens. They were the only observed insect whose
flowers had pollen clumps on the stigma (Fig.3), which had a much greater average pollen count
on the stigma than the control flowers visited by small bees and Diptera (Fig. 4). The top five
greatest average pollen per stigma per enclosure were wasp enclosures (Fig. 5), further showing
that wasps are the best pollinators.
The Diptera appear to be non-pollinators, or nectar thieves. The flowers visited by Diptera
have no pollen clumps on the stigma (Fig. 3) and their average pollen count on the stigmas are
less than the average count on the control stigmas (Fig. 4). This makes Diptera ineffective
pollinators, especially when compared to wasps. The chi square analysis revealed that there was
no significant difference between the control flowers and the flowers visited by Diptera as the Pvalue for Diptera 0.30454, which is greater that 0.05 (Table 1)
More data are needed to determine if the small bees are actually nectar thieves or are
pollinators as only a few flowers visited by the small bees were collected, and only from one
enclosure.
The VFE control flower that had a clump of pollen on the stigma (Fig. 3) was most likely
visited by a pollinator that managed to enter and leave the enclosure without being noticed, most
likely through a hole that it chewed in the covering of the enclosure. This raised the average
pollen grain count (Fig. 4) for the control.
In conclusion, the analysis shows that wasps (p=0.0028) are pollinators while Diptera
(p=0.30454) are nectar thieves (Table 1). However, the variation amongst wasp enclosures
indicates that not all wasp species necessarily pollinate P. alboflavescens, as only 6 of the 12
wasp enclosures averaged more pollen grains than the control. Wasps collected from the
enclosures are being studied to determine if there are consistent differences between pollinating
and non-pollinating wasps. Further enclosure experiments (using large flight cages for
Lepidoptera) are needed to determine if bees and Lepidoptera are pollinators.
References
1. "Plant-insect Interactions in Terrestrial Ecosystems." Plant-animal Interactions: An Evolutionary Approach.
Ed. Carlos M. Herrera and Olle Pellmyr. Oxford: Blackwell Science, 2002. 77-79. Print.
2. Malhotra, Rumaan et al., “Identifying the Pollinators of Parsonsia alboflavescens (Apocynaceae - Milkweed
Family).” Academy of Natural Sciences of Drexel University. Poster Presentation.
Acknowledgements
We thank the following Taiwanese colleagues for their assistance in this research: Dr. Mei-ling Chan, Dr. Alec
Yang, National Museum of Natural Science; Dr. Shaw-Yhi Hwang, Wei-Ting Chen, M.S., and Millet Lee, M.S.,
Department of Entomology, National Chung-Hsing University; Dr. Michael G. Z. Song, National Chai-Yhi
University; Shu-wei Wu, M.S., Kenting Botanical Garden. This research was supported in part by a grant from the
Cottswald Foundation.