Honey Bee Foraging Behavior on Lantana camara (Verbenaceae)

Macarena Arenas

University of Miami, Department of Biology

P.O. Box 249118, Coral Gables, Florida 33124, U.S.A.

Abstract

Many species of plants require animal visitors to ensure pollination and have evolved mechanisms to attract more or particular pollinators. It has been suggested that Lantana camara retains older flowers in an inflorescence to make it more visually attractive to pollinators because of increased inflorescence size. L. camara inflorescences contain from 2 to 24 flowers that open over many days. Flowers are yellow when they first open but turn pink in a 24-hour period. Both yellow and pink flowers may serve to attract pollinators and maximize platform size for optimal landing. To describe the relative abundance of flowers of different colors and their rewards, I estimated their distribution among flower heads in a patch and measured nectar volumes and sugar concentration on several days. I found day to day variability in the proportion of yellow and pink flowers in the patch. Yellow flowers were more rewarding than pink flowers. Although they offer similar volumes of nectar reward, yellow flowers have a higher sugar concentration than pink flowers. At this particular location, honey bees foraged on L. camara due to its proximity to an apiary. To determine whether honey bees can obtain nectar rewards from the long narrow corolla of L. camara, I collected dead bees and simulated foraging attempts. Although honey bees have a relatively short proboscis, they are able to reach nectar 46.7-100% of the time depending on the amount of nectar reward offered by the flower. To examine the pattern of bee foraging behavior, I observed the sequence of flowers bees visited and compared the flower heads visited to nearest neighbor flower heads. Honey bees visited more yellow than pink flowers, and visited larger inflorescences which also contained more yellow flowers than the nearest neighbor inflorescence. A previous study of butterflies foraging on L. camara showed that butterflies preferred larger inflorescences, even when containing only old flowers. Whether inflorescence size or color is more important to honey bees still needs to be studied.

Introduction

Several plant species retain old flowers in their inflorescences. In some species, older flowers are different in color from newly opened flowers. By retaining old flowers, plants may attract more pollinators to the larger display (Gori, 1989). Flower visitors may be attracted to larger inflorescences because they notice them more easily from long distances or because they actively select larger ones (Waser, 1983; Weiss, 1991, 1995). Since both old flowers and newly opened flowers have different colors, they may have different effects on their visitors. Also, the proportion of old and new flowers may influence insect visitation

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Lantana camara is a shrub of the family Verbenaceae (Barrows, 1996). The flowers of L. camara change color from yellow to peach to pink in a 24 hr period (Schemske, 1976; pers. observ.). Pink flowers stay in the floral head for a variable number of days. L. camara flowers open from the outermost to the center-most part of the inflorescence; the oldest flowers (pink) are those in the outer part of the inflorescence (Barrows, 1976; Schemske, 1976). Other studies of L. camara have not studied extensively the nectar volume and concentration in the flowers, but report that yellow flowers contain nectar reward, whereas pink flowers do not (Barrows, 1976; Weiss, 1991).

Objectives

• To determine the abundance and proportion of yellow and pink flowers in L. camara inflorescences.

• To determine nectar rewards offered by the flowers of different colors.

• To determine honey bee visitation to flowers of different colors and to inflorescences of different sizes.

• To determine whether honey bees can obtain nectar from L. camara flowers.

Methods

Inflorescence color and size: I counted the number of yellow and pink flowers on each floral head.

Nectar volume and concentration: I measured nectar volume and concentration in both yellow and pink flowers in bagged (unvisited) and unbagged (potentially visited) floral heads.

Insect visitation: I observed foraging honey bees and compared floral heads visited with nearest neighbor floral heads.

Honey bee access to nectar: I collected dead honey bees and simulated foraging behavior on yellow flowers that were pinned on a board in an upright position. By doing this I was able to determine how deep inside the corolla bees could reach. Also, I measured the length of the nectar inside the corolla. From these measurements, I could determine how likely honey bees are to receive nectar rewards.

Results

Inflorescence color and size: On most dates, the number of yellow flowers was less than the number of pink flowers in the L. camara patch.

Nectar volume and concentration: In general, the nectar volume in bagged and unbagged flowers did not differ greatly. The sugar concentration of nectar was higher in yellow flowers than in pink flowers. On one day, pink flowers had a higher nectar volume and a more diluted concentration.

Insect visitation: Honey bees visited disproportionately more yellow flowers than pink flowers. Honey bees also visited larger floral heads which contained more yellow flowers when compared to the nearest neighbor floral head.

Honey bee access to nectar: On average, honey bees reach 7.68 mm inside the corolla. Depending on the nectar volume, the honey bee has a 46.7-100% chance of obtaining nectar.

Discussion

The long corolla length of L. camara eliminates all the potential pollinators which have a short proboscis. Therefore, L. camara has been classified as a butterfly plant due to its corolla size and shape. However, I found that honey bees not only visit L. camara but are able to obtain nectar rewards 47-100% of the time.

Inflorescences varied greatly in the proportion of yellow and pink flowers in the population of L. camara that I studied. On some days, there were several flower heads containing only yellow flowers. On most days a large proportion of flower heads had less than 40% yellow flowers.

Other studies of L. camara report that yellow flowers contained nectar, whereas pink flowers offered no nectar (Barrows, 1976; Weiss, 1991). According to my observations, both yellow and pink L. camara flowers offered nectar reward. In general, pink flowers contained almost the same amount of nectar as yellow flowers. The sugar concentration in yellow flowers was higher than in pink flowers; therefore, yellow flowers were more rewarding than pink flowers.

It has been suggested that L. camara retains the older pink flowers in an inflorescence, to make it more visually attractive to pollinators either by increasing the inflorescence size or by maximizing the platform size for optimal landing by pollinators (Weiss, 1991). According to my observations, honey bees chose larger inflorescences compared to the nearest neighbor inflorescence and visited yellow flowers much more frequently than pink flowers in an inflorescence. Weiss (1991) found that butterflies similarly prefer yellow flowers. However, in an experiment where she manipulated inflorescences, she found that inflorescence size was more important to butterflies even when the larger inflorescence contained only old flowers (Weiss, 1991). In my study, naturally occurring larger inflorescences also contained more yellow flowers. It is clear that honey bees visited larger and more rewarding inflorescences. Whether inflorescence size or color is more important to honey bees still needs to be studied.

Acknowledgements

I am very grateful to Dr. Mindy Nelson for her helpful comments and advice in this research. I would also like to thank Dr. K. Waddington for providing the laboratory facilities necessary for this study, Dr. M. Gaines for his support to all of us in the research apprenticeship program, and R. Wright and M. Chiappone for their technical expertise in putting together this poster. This study was funded by the Howard Hughes Medical Institute and the National Institute of General Medical Sciences.

References

Barrows, Edward M. 1976. Nectar robbing and pollination of Lantana camara (Verbanaceae). Biotropica 8(2): 132 135.

Gori, David. 1989. Floral color change in Lupinus argenteus (Fabaceae): why should plants advertise the location of unrewarding flowers to pollinators? Evolution 43(4): 870-881.

Shemske, Douglas W. 1976. Pollinator specificity in Lantana camara (Verbenaceae). Biotropica 8(4): 260-264.

Waser, N. M. 1983. The adaptive nature of floral traits: ideas and evidence. In: Pollination Biology (Ed. by L. Real), Academic Press: Orlando Fl.

Weiss, Martha R. 1991. Floral colour changes as cues for pollinators. Nature 354: 227-229.

Weiss, Martha R. 1995. Floral color change: a widespread functional convergence. American Journal of Botany 82(2): 167-185.