Wing Loading and Flower Preference of Migrating Monarchs in South Texas
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Christal Hall, Margaret Hall, Layla Parast, William Rubink Science Academy Abstract | Introduction | Methods | Results | Discussion | Acknowledgements | Literature Cited | Research Projects The purpose of this experiment is to determine if there is a correlation among the migrating monarchs’ flower color preference and the monarchs’ wing loading and to determine the monarchs’ flower color preference and preference for native and non-native flowers in South Texas. Sixty-seven butterflies were observed three times per day in fifteen-minute intervals. Students recorded the nectaring behavior of the monarchs with native plants, orange-flowered butterfly weed (Asclepias tuberosa) and scarlet sage (Salvia coccinia) and non-native plants, yellow chrysanthemum and blue-flowered mistflower (Eupatorium ordoratum). The observed flower color preference of the sixty-seven butterflies provided statistically significant evidence that monarchs prefer orange butterfly weed flowers to the yellow, red, and blue flowers of other plants. The collected data also provides evidence that fat butterflies are attracted to orange flowers on butterfly weed, while skinny butterflies are attracted to red flowers of scarlet sage. The monarchs preferred native flowers significantly more than non-native flowers. Introduction As monarchs migrate south, the ground cover vegetation seasonally changes with changes in climate. Migrating monarchs do not likely become acclimated to flowers native to a specific area, but they can develop a color preference according to their nutrition and nectaring resources available (Zahl, 1973). The purpose of our study was to determine if there is a correlation among the migrating monarchs’ flower color preference and the monarchs’ wing loading, which is the mass of the butterfly in relation to its wing length, and to determine the monarchs’ preference for native and non-native flowers in South Texas. Wing loading is being considered as a correlating variable because it describes the relative size (fat or skinny) of the monarchs. Based on observing that some migrating monarchs nectar from primarily one flower color, we hypothesize that there is such a correlation between flower color preference and wing loading in migrating monarchs. The fatter monarchs with a higher wing loading may be choosy about the color of the flowers from which they nectar because they are not very hungry. The hungrier monarchs are skinnier and may be less choosy about their nectaring sources. We recorded the number of times the monarchs nectared on variously colored native and non-native flowers and measured their wing loading before nectaring. Plants were collected from a local nature center; observations were done in a classroom with an experimental PVC screen enclosure. The monarchs were reared in the classroom. The questions we investigated and the hypotheses for each follow: Question 1. Is there a color preference difference in migrating monarchs? H0 Migrating monarchs have no flower color preference. Question 2. Is there a correlation between flower color preference and wing loading in migrating monarchs? H0 There is no correlation between flower color preference
and wing loading in migrating monarchs. Question 3. Do migrating monarchs prefer non-native or native flower types? H0 Migrating monarchs have no preference between native and
non-native flower types. The experimental apparatus was a mesh rectangular PVC screen enclosure of 26" by 37" inches, which was set up at the Science Academy classroom. Inside the experimental habitat there were two native species, butterfly weed (Asclepias tuberosa) and scarlet sage (Salvia coccinia) and two non-native plants, mistflower (Eupatorium ordoratum) and yellow chrysanthemum. The orange butterfly weed flowers are in umbels; their corolla lobes measure approximately 12 mm long. The scarlet sage flowers are in whorled clusters at different levels forming a raceme 2-5 inches long. The flowers are dark red with 5 petals united in a blossom; each flower has 2 stamens and 1 pistil, (Miles, 1999). The mistflower corolla is light blue-violet; the heads are in corymbs with a discoid shape. The chrysanthemum flowers vary greatly in size. The flower is tubular in shape, made up of disc flowers surrounded by a whorl of bracts. The enclosure had one of each plant with many flowers placed in the corners with the flowers facing the center of the enclosure. The monarchs were obtained from the Monarchs in the Classroom Program at the University of Minnesota. Two separate shipments of eggs were sent and raised in plastic containers on butterfly weed. The chrysalids were consecutively hung in the center of the PCV screen enclosure. A total of 67 butterflies emerged from the chrysalids over a three-week period. The newly hatched butterflies were marked for their gender, and wing loading measurements (mass divided by wing length) were calculated. The lower hind wing was marked with a red sharpie marker, denoting females and a blue sharpie marker for males. A chronological number was marked on their wings to keep track of the hatching order. Observation and scanning of feeding behavior was done at 72° F to 80° F. Artificial white light was placed near the enclosure to control the room temperature at night and keep the butterflies warm, due to cold environmental room air-conditioning. The 67 butterflies (33 females and 34 males) were allowed to nectar on the plants during daylight conditions. The plants were randomly moved from corner to corner throughout the chamber for a three-week period. Plant rotation was done every third day in the early morning. Students scanned and recorded the nectaring butterflies for 15- minute periods, four times daily for three weeks. Students also tracked which butterflies had high wing loading (.13g/cm to.15g/cm) and low wing loading (.13 g/cm to .09 g/cm). We used a Chi-square test to analyze the butterflies’ flower color preference and native or non-native flower preference. We also used a Chi-square test to analyze the correlation between wing loading and flower color preference. In the flower color preference test, flower color was the independent variable and the dependent variable was the number of butterfly visits to each flower color. We rejected the null hypothesis, which stated migrating monarchs have no flower color preference (X2= 43.5, df = 3, p< .005). The preference is for the orange flowers of Asclepias tuberosa. The two proportion Z-test reflects there is statistically significant evidence that the butterflies prefer orange flowers to yellow, red, and blue flowers. The monarchs visited orange flowers 37 times, yellow flowers 21 times, red flowers 5 times, and blue flowers 4 times (Figure 1).
In addition, wing loading is a direct measurement of the mass of the butterfly divided by the wing length of the butterfly. We rejected our null hypothesis, which stated that there is no correlation between flower color preference and the wing loading of migrating monarchs (X2= 8.0, df =3, p <.05). Skinny butterflies (wing loading = 0.09-0.13 g/cm) preferred red scarlet sage flowers while fat butterflies (wing loading = 0.13-0.15 g/cm) preferred orange butterfly weed flowers. The skinny butterflies visited red flowers 3 times and visited blue and yellow flowers 1 time each. The fat butterflies visited orange flowers 4 times and yellow flowers 1 time (Figure 2).
In the native and non-native flower preference test, we rejected our null hypothesis, which states that migrating monarchs have no preference for native or non-native flower categories (X2=4.3, df=1, p<.05). Given the choice of flowers on four different plants, migrating monarchs preferred to nectar from the orange flowers of a native plant, Asclepias tuberosa (Figure 3).
Discussion The observed flower color preference of the sixty-seven butterflies provided statistically significant evidence that monarchs prefer orange flower color of Asclepias tuberosa to yellow, red, and blue color. The collected data also provides evidence that fat butterflies are attracted to orange butterfly weed flowers, while skinny butterflies are either attracted to red scarlet sage flowers or are less choosy. The monarchs preferred native flower of Asclepias tuberosa significantly more than non-native flowers. Butterfly weed (Asclepias tuberosa) with its bright orange tubular flowers is a native plant that is an excellent nectar source for monarchs. The skinny monarchs might have preferred the scarlet sage because its nectar might have a higher sugar level or a greater quantity of nectar in the flowers. It would be a good idea to study the different levels of nectar in different types of flowers in a future experiment. The butterfly weed (Asclepias tuberosa) plants have many orange flowers and seem to provide greater amounts of nectar, which may enhance body growth. The butterfly weed is in the milkweed family, thus, it may also be popular with monarchs because it is a host plant for monarch larvae. Since this study was not conducted in a field, the experiment was limited by the numbers and types of plants collected. The butterflies only had a choice of two native and non-native nectar resources, all with differently colored flowers. This may have confounded color and natives in our results. The PVC enclosure had a confined space and limited number of native and non-native plants for foraging behavior. The foliage was in an open area and there was no camouflage provided in the screen enclosure as compared to a natural environment. All the monarchs used in the experiment were hatched from eggs in captivity and not from monarchs migrating through Texas. The experiment was conducted during the time of the year when monarchs migrate south to Mexico, but our results do not necessarily apply to these migrating monarchs. The sample size of the fat and skinny butterflies for wing loading was very small with a total of only ten butterfly visits to flowers measured. The experiment should be carefully repeated because of this low number. Perhaps a follow-up experiment could be conducted to answer numerous questions. Each native and non-native plant had different numbers of flowers and shapes. The two plants with the most flowers were the butterfly weed (Asclepias tuberosa) and yellow chrysanthemum. The scarlet sage (Salvia coccinia) and blue-flowered mistflower (Eupatorium ordoratum) had the least number of flowers. This data could change our chi square results of flower color preference and preference for native and non-native plants. The wing loading was only measured once throughout the experiment; therefore, the classification of fat and skinny monarchs could be a problem. Perhaps checking the wing loading every day and collecting daily data on individual plant food availability would have helped. Further studies could be conducted to determine wing loading and flower color preference in native and non-native plants with greater numbers of monarchs. Acknowledgments We would like to thank the Welsaco Nature Center, Weslaco, Texas and John Garcia, a native plant conservationist, for information on local plant conservation and the contribution of potted native plants. The Monarchs in the Classroom program at the University of Minnesota shipped the monarch eggs for use in the project. Special thanks go to Bethann Lavoie and Michelle Solensky, who are staff members of the Monarch Monitoring Project, for their expertise on monarchs. Dr. William Calvert was very helpful in his knowledge of native milkweed located in the South Texas area. All of the research was supported by the National Science Foundation (ESI-9731429). Literature Cited Monarchs in the Classroom world wide web site: www.monarchlab.umn.edu Miles, S. 1999. New students clarify monarch worries. Science News. 156:37. Zahl, P. A. 1973. Mystery of the monarch butterfly. National Geographic. 123:38. |
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