Do Insect Pollinators Prefer Symmetrical Flowers?

Do Insect Pollinators Prefer Symmetrical Flowers?

André Amaral

1 APS337 fieldcourse 140104523

Abstract

Pollinators use a wide range of cues to choose between the flowers they will visit to get

nectar or pollen. Some of those cues are considered honest signals, such as symmetry,

which can be a measurement of fitness because of the challenge of developing

symmetrically, overcoming parasites and ecological stress. In this study, it was

investigated the preference of insect pollinators, more specifically honeybees and flies, for

symmetry and the rewards provided by flowers with different levels of symmetry. It was

found a relationship between flower symmetry and visitation of both groups, as well as a

general trend relating positively symmetry and relative sucrose content. Further

investigation showed that bees actively choose symmetry, while flies might use other cues

correlated to it.


Introduction

Symmetry is often regarded as an indicator of fitness. It is hypothesized that genetic

variations which increase an individual’s ability to resist developmental accidents, (caused

by environmental or genetic stress, for example) are positively related to their capacity of

maintaining the symmetry of morphological characters. The higher genetic quality would

also be reflected in reproductive success, i.e. fitness, because it would be directly related

to survival, be sexually selected for, or developmental stability could also affect

reproductive characters development (Giurfa et al., 1999; Møller & Eriksson, 1995;

Rodríguez et al., 2004; Van Valen, 1962).

Flowers present bilateral or radial symmetry and, if that is indeed related to fitness,

then insect pollinators visiting flowers will receive better rewards by selecting the most

symmetrical ones. Although these assumptions seem fairly plausible, many studies have

found conflicting results for the relationship fitness-symmetry, when investigating insect

visitation, pollen or seed production by the plants (e.g., Frey & Bukoski, 2014; Weeks &

Frey, 2007; Giurfa et al., 1996; Møller, 1995) and the topic remains controversial.

The present study aims to collect further evidence to answer three questions about

the relationship between symmetry of flowers and insect pollination. These are (1) Do

insects prefer symmetrical flowers? If so, then (2) Do symmetrical flowers provide better

rewards? And (3) Are there other factors correlated to symmetry that those insects may be

choosing for? To answer these questions the present study used chamomile flowers

(Matricaria sp.) and their relationship with honeybees (Hymenoptera: Apidae) and flies

(Diptera), their most frequent visitors.

The first question was approached through a set of observations regarding to the

frequency of visitations by those insects to flowers with different degrees of symmetry.

Measurements of sucrose content in flowers of different degrees of symmetry were taken

as means to address the second question. For the third question an experiment was set

whereas a group of flowers with initially the same degree of symmetry was manipulated

and separated in three groups with distinctly different degrees, followed by the observation

of the behaviour of those two groups of insects towards the manipulated flowers.


Material and Methods

Study Site

The study was conducted within the boundaries of Quinta de São Pedro in Sobreda,

Portugal, a Study Center with 4 hectares of mixed vegetation types, including a lower-lying

grassy area, densely occupied by wild carrots (Daucus spp.) and several Asteraceae

species, where the present study took place (Fig. 1).

Figure 1. Map of the study site at Quinta de São Pedro, Sobreda, Portugal. (Quinta Sao Pedro,

2015)

Observing Pollinators Preference

In order to distinguish different degrees of symmetry, there were established 5 categories

of symmetry based on the natural variation found in the site, ranging from 1 (most

symmetrical and undamaged flowers) to 5 (very asymmetrical and damaged flowers, with

just few white petals remaining) (Fig. 2). The next step was the location of ten patches of

flowers with a good representation of each category. The patches are usually formed by

one or two plants.


Figure 2. The

5 categories of Chamomile flowers established in the present study, ordered by decreasing levels of

symmetry from 1 to 5.

To observe the possible preference of the insect pollinators for symmetry, each of

the five group members observed two different patches for 30 minutes each patch, during

an afternoon (from about 12:00 to 15:25), in a total of 300 minutes of observation. It was

recorded the category of the flower visited, as well as the group to which the insect

belongs (the most relevant ones in this study were flies and bees; other groups were too

little represented).

The data were compared using a chi-squared test goodness-of-fit for the number of

visits against categories of symmetry, in order to test the null hypothesis that observed

values are not significantly different from random ones.

Measuring Nectar Content

To assess if pollinators would have better reward by choosing symmetrical it was

necessary to measure the nectar of flowers. Sucrose content was extracted by squeezing

the yellow florets of two flowers dissolved in 0.25 ml of water, and read in a pocket

refractometer. It was taken 5 samples for each category and the values were given in

relative sucrose concentration.

A one-way ANOVA was then used to verify if the values obtained varied

significantly from each other, according to the sample size.

Eliminating Confounding Factors

Because there could be other factors correlated to symmetry affecting insect choice, an

experiment was set in a way that symmetry would be the only variable in a patch. Five

patches had all but category 2 flowers removed. Three treatments were submitted to the

remaining flowers in the same proportion. The first treatment was the removal of the white

petals of half of a flower (Fig. 3.1). The second treatment was to remove alternated petals,


maintaining the symmetry but removing the same area of petals than treatment one (Fig.

3.2). Since the first two treatments have altered the surface area of the flowers, a third

treatment was subjected to control for that factor; it consisted of trimming, with scissors, a

third of the length of all the white petals around a flower, maintaining symmetry and

matching the surface area of the others (Fig. 3.3); also this could indicate a possible

preference for petal size. After treating the patches, it was taken 110 minutes of

observation for each of the five patches, also recording the insect group visiting, in a total

of 550 minutes. The data was subjected to chi-squared tests to confront the null

hypothesis of random visitation.

Figure 3. Three different

treatments applied to Chamomile flowers in the present study to assess insect preference for

symmetry. (1.1) removal of white petals in half of a flower; (1.2) removal of alternating white petals

of a flower; (1.3) trimming of a third of all the white petals of a flower.

Results and Discussion

Observing Pollinators Preference

About the first question “Do insects prefer symmetrical flowers?” it was found that both honeybees (χ2 = 19.7, df = 4, P = 0.001) (Fig. 4), and flies (χ 2 = 16.6, df = 4, P = 0.002) (Fig. 5) have a significant preference for the more symmetric flowers, although in both group it was observed that the visitation to the most symmetrical category (1) was lower than to the second most (2) and not significantly different from the expected value, deviating from the general trend. A possible explanation for this is that category 1 flowers are mostly young ones and may not have their florets opened yet, so insects would tend to avoid those, but further investigation is needed in order to address this question properly.


Figure 4. Honeybees visitations to Chamomile flowers of different categories of symmetry (χ2 = 19.7, df = 4, P = 0.001) in 300 minutes of observations at Quinta de São Pedro Field Center, Portugal.

Figure 5. Flies visitations to Chamomile flowers of different categories of symmetry (χ 2 = 16.6, df = 4, P = 0.002) in 300 minutes of observations at Quinta de São Pedro Field Center, Portugal.

Flies (Diptera)

Honeybees (Apidae)


Measuring Nectar Content

The results had shown that flies and honeybees do have general preference for more symmetrical flowers, so the next step was to find out why.

Sucrose concentration measurements were taken from flowers of each category and the results show a general trend of increasing sucrose concentration towards the most symmetrical flowers (F = 3.79, P = 0.019), except for the second category (2) which have not shown significantly different concentration from the third or fourth categories (Fig. 6). This deviation could have been related to the more frequent visitations and, thus, a side experiment was set to test that. The experiment consisted in covering 10 flowers from category 2 with a fine mesh from early in the mourning until the evening (from 10:00 – 16:00) and comparing those with other 10 flowers that have been left uncovered during the day, then taking sucrose measurements in the same way described in methods section. The results were not significantly different and the question was left unexplained.

Figure 6. Measurement of relative sucrose concentration in Chamomile flowers of five different degrees of symmetry (F = 3.79, P = 0.019).

Eliminating Confounding Factors

Although some exceptions, the correlation between symmetry and insect preference still persisted and the last question “Are there other factors correlated to symmetry that those insects may be choosing for?” was addressed through the experiment manipulating the symmetry of flowers. The results obtained showed that honeybees have a clear preference for symmetry, choosing more frequently the two symmetrical treatments and rejecting the asymmetrical one (χ 2 = 28.9, df = 2, P = < 0.001) (Fig. 7). Contrastingly, flies have not shown significant preference for any treatment (χ 2 = 3.37, df = 2, P = 0.185) (Fig. 8), which suggests that they are using other criteria to select their foraging sites, correlated to symmetry in those flowers. Frey and Bukoski (2014) make notes about the behaviour


syrphid flies, which use cues such as colour, olfact, flower orientation, shape and patterns, while Møller (1995) found that bumblebees choose for symmetry and perhaps size. Further studies could investigate the reason behind those different behaviours. Also more research is needed in order to enlighten the genetic and developmental basis of symmetry.

Figure 7. Honeybees visitation to Chamomile flowers subjected to manipulation altering their symmetry (χ 2 = 28.9, df = 2, P = < 0.001). Treatment (1) had all petals of one side removed; (2) had alternated petals removed; (3) had one third of all petals trimmed. Total of 550 minutes of observations at Quinta de São Pedro Field Center, Portugal.

Figure 8. Flies visitation to Chamomile flowers subjected to manipulation altering their symmetry (χ

2 = 3.37, df = 2, P = 0.185) have not shown significant preference for any treatment group. Treatment (1) had all petals of one side removed; (2) had alternated petals removed; (3) had one third of all petals trimmed. Total of 550 minutes of observations at Quinta de São Pedro Field Center, Portugal.

Honeybees (Apidae)

Flies (Diptera)


Acknowledgements

I would like to thank all professors and staff members of the Portugal field course for their

dedication in teaching and helping us during the course, and especially for the opportunity

given to me to participate in it. It was also very fun.

I would also like to thank my group members and colleagues (even though they will

probably never read this), who were absolutely essential for everything I did in this course.

References

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GIURFA, M., EICHMANN, B., & MENZEL, R. (1996) Symmetry perception in an insect.

Nature. 382(6590), 458-461.

MØLLER, A. P. (1995) Bumblebee preference for symmetrical flowers. Proceedings of the

National Academy of Sciences. 92(6), 2288-2292.

MØLLER, A. P., & ERIKSSON, M. (1995) Pollinator preference for symmetrical flowers

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QUINTA SAO PEDRO. (2015) Field Study Center. [Online] Available from: http://

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flower-naïve bumblebees. Naturwissenschaften. 91(8), 374-377.

VAN VALEN, L. (1962) A study of fluctuating asymmetry. Evolution. 125-142.

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Do Insect Pollinators Prefer Symmetrical Flowers?