Relationship of Butterfly Visitation with
Nectar Qualities and Flower Color in Butterfly Bush, Buddleia Davidii
Department of Entomology, Clemson Univeristy
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Designing gardens to attract butterflies is one of the fastest growing
areas of wildlife gardening. As evidence of this, both "how-to"
articles and descriptions of established gardens have appeared in the popular
press (newspapers, magazines, and books) and on a variety of television
shows (gardening, handy-man). Many gardening centers, extension groups,
and nurseries have begun to provide flyers containing information about
the best types of plants for butterfly gardens. Some seed producers (i.e.,
Park Seed Co., Inc.) have had consultants suggest which of their materials
are "butterfly friendly" and indicate that in their catalogue
with a butterfly symbol.
Although considerable knowledge exists concerning the types of plants
that are important as nectar sources for adult butterflies, many of these
plants are available in a wide range of varieties. There have been few detailed
studies conducted examining whether all varieties of a given plant species
are equally as good at attracting butterflies. As an example, most butterfly
gardening guides suggest using butterfly bush ( Buddleia davidii
) as a perennial 'anchor' plant. However, there are 10 to 15 commercially
available cultivars with flower colors ranging from white to an extremely
dark purple. There is also considerable variation in individual flower size,
inflorescence size, and flowering cycles. This range of variability also
can be found in several other commonly recommended butterfly garden perennials
such as Lantana and Verbena , and annuals such as Cosmos
and Zinnia .
The object of this project in 1995 was to quantify flower color and nectar
qualities in butterfly bush and to determine if those parameters were related
to butterfly visitation patterns. The longterm objectives are to obtain
this kind of information for all of the major plants used in butterfly gardens in the region. This information will allow persons planning
butterfly gardens to select cultivars that will be the most effective at
attracting butterflies. It also will aid nurseries to emphasize production
of stocks of those plants that are most suitable for use in butterfly gardens.
Materials and Methods
This project was conducted at the research garden located at the Cherry
Farm Entomology Laboratories of Clemson University (Clemson, South Carolina).
Butterfly bush cultivars currently established in this garden are listed
in Table 1.
Flower color analysis. Visible flower colors were assessed
using a Munsell Colorimeter located in Dr. J.R. Aspland's color analysis
laboratory (School of Textiles, Clemson Univeristy). This equiptment provides
a quantitative measure of reflected light and so avoids variation based
on human color perception. Using this system, color is measured on a 3-dimensional
scale having a black-white (L*), red-green (a*), and yellow-blue (b*) axes
system (Fig 1A). Measurements were taken
using the daylight settings. Inflorescence were removed from the plants
and placed into vials of distilled water and taken within 30 minutes to
the color laboratory for analyses. Three inflorescence were examined from
each plant on two dates. Flowers from the tip (youngest), middle region,
and base (oldest) of each inflorescence were analyzed separately to determine
if color varied with flower age.
Nectar analyses. Nectar qualities examined were: amount of nectar
produced; percent sugar in the nectar, and; the relative amounts of sucrose,
glucose, and fructose. In order to ensure that nectar was present for analysis,
nectar feeders were excluded from three inflorescence per plant by enclosing them in fine-mesh
bags for 24 hours prior to sampling. A microsyringe (10µl [microliter])
was inserted into the corolla of five flowers on each inflorescence, the
nectar was extracted, and the amount of nectar from the five flowers was
recorded to the nearest 0.1µl. Sugar concentration of each sample
was determined immediately after removal from the flowers using a pocket
sugar refractometer. Temperature, relative humidity, and rainfall were recorded
in the garden.
Identification of sugar composition of the nectars was accomplished
as follows. Nectar was extracted from flowers as described above. After
sample preparation, they were injected into an HPLC carbohydrate column
and analyzed using a refractive index. Quantities of each sugar, expressed
as percent mass, was determined by comparison to fructose, glucose and sucrose
standards. Samples for sugar identifications were gathered and prepared
by Tze-Wah Leung and HPLC analyses were conducted by Anil Ranwala in Dr.
W.B. Miller's laboratory (Department of Horticulture, Clemson University).
Butterfly visitation rates. Data on butterfly visitation on the plants
in the study were determined by direct observations. Walk-through counts
of butterfly visits were made twice daily beginning at approximately 10
am and 2 pm on each Monday, Wednesday, and Friday. Observers recorded the
number and species of all butterflies on each plant.
Results and Discussion
Flower color. Statistical analyses showed that there were no significant
differences in flower color based on age (region on the inflorescence) or
time in the season (two samples approximately 4 weeks apart). Because there
was no variation within a given plant, and only slight variation between
plants of a given variety, color data is presented as a single figure for
each of the 13 varieties examined. Colors of the butterfly bush varieties
examined fell into three fairly distinct clusters, white, red, and lavenders-pinks-blues
(Fig. 1b). Examination on the individual
color axes indicated that the white and red varieties were the extremes
on the a* [red-green] and L* [black-white] axes (Fig.2a),
while only the white varieties fell outside of the main cluster on the b*
[yellow-blue] axis (Fig. 2b). Although
they are available, we did not have any yellow-flowered varieties in the
research garden in 1995.
Nectar qualities. The average amount of nectar recovered from five
individual flowers and the amount of sugar in each nectar is presented in
Figure 3. Nectar production fell into three broad groupings, with seven
varieties having greater than 1.0µl.four having between 0.75µl
and 1.0µ, and two below 0.5µ. The amount of sugar in the nectar was
similar across all 13 varieties examined (Fig
3). With the exception of 'Opera', 'Dubonet', and 'Nanho Purple',
all varieties had two to three times more sucrose than either glucose or
fructose (Fig 4). In those three varieties,
sucrose was only slightly more abundant than the other sugars.
Butterfly visitation. Four butterflies and one day-active moth were
chosen for use in examining activity in relation to flower color and nectar
qualities. These were the tiger swallowtail (Papilio glaucus ), spicebush
swallowtail (Papilio troilus ), silver spotted skipper (Urbanus
proteus ), longtail skipper (Epargyreus clarus ), and bumblebee
hawkmoth (Hemaris diffinis ). These were chosen because they are
common, relatively large and showy, and therefore easily observed and identified.
Patterns of butterfly bush visitation are presented for July (Fig. 5), August (Fig.
6), and September (Fig. 7). Season-long,
the greatest amount of butterfly activity was observed on 'Charming Summer',
'Royal Red', 'Pink Delight', and 'Petite Plum'.
In August, when butterfly numbers peaked, activity also increased on 'Lochinich',
'Black Knight', 'Empire Blue', 'Nanho Blue', and 'Nanho Purple'. Both 'Black
Knight' and 'Empire Blue' had increases in the number of flowers in August
that likely increased their attractiveness.
In butterfly bush, butterfly visitation appears to be based on both flower
color and nectar qualities. In general, visitation rates were greatest on
those varieties that produced the most nectar, and those having nectar containing
relatively high amounts of sucrose in relation to glucose and fructose.
However, of those varieties, visitation rates were low on those
having white or pale lavender flowers and greatest on those having red,
pink, or lavender-pink flowers. This is probably due to the fact that butterflies,
unlike many insects, can perceive red wavelength colors. Although many white
flowers have been found to reflect ultraviolet (UV), which is highly attractive
to many insects, preliminary UV analyses have indicated that butterfly bush
flowers do not.
The research garden at the Cherry Farm facility was begun in 1993 with
plant materials donated by Park Seed Co. (Greenwood, SC), Wayside Gardens
(Hodges, SC), McCorkle Nurseries, Inc. (Dearing, GA), and the South Carolina
Botanical Gardens. Support has been provided by the Enhancement of Research
in Ornamental Horticulture Program of the South Carolina Agricultural
Experiment Station, Clemson University, the Summer Research Scholars
Program of the South Carolina Governor's School for Science and Mathematics,
and the Minority Apprenticeship Program of CSREES.
Last Updated 8/3/98