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: butterfly bush varieties established in the research garden at the Cherry Farm Entomology Laboratories.
|Variety||Number of Plants||Color|
|'Royal Red'||2||very deep pink|
|'Petite Plum'||2||deep pink-purple|
|'Black Knight'||2||very deep purple|
|'Empire Blue'||1||deep blue|
|'Nanho Blue'||1||pale blue|
|'Nanho Purple'||2||pale blue-purple|
Flower color analysis. (image: black swallowtail) 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. 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. (figure: A.Three dimensional color qualification scale. L* axis is white-black, a*axis is red-green, and b* axis is yellow-blue. B.Color space of Buddleia varieties examined.)
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. (image: silver spotted skipper) 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 Yellow butterflyper 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.
(image: silver spotted skipper) 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.
Figure: A.Distribution of Buddleia flower colors on the L* and a* axes. Distribution of Buddleia varieties flower colors on the L* and b* axes. 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. 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, while only the white varieties fell outside of the main cluster on the b* [yellow-blue] axis. Although they are available, we did not have any yellow-flowered varieties in the research garden in 1995.
Nectar qualities. (image: long-tailed skipper) 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. Black butterfly on a red flowerThe amount of sugar in the nectar was similar across all 13 varieties examined. With the exception of 'Opera', 'Dubonet', and 'Nanho Purple', all varieties had two to three times more sucrose than either glucose or fructose (Figures). 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, August, and September (Figures). Season-long, the greatest amount of butterfly activity was observed on 'Charming Summer', 'Royal Red', 'Pink Delight', and 'Petite Plum'.
(image: bumblebee hawk) 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. Yellow butterflyHowever, 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.