Relationship of Butterfly Visitation with Nectar Qualities and Flower Color in Butterfly Bush, Buddleia Davidii
Joseph D. Culin Department of Entomology, Clemson Univeristy

Download the Adobe Acrobat version of this document.

Introduction

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.

Acknowledgment

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