Frequency and behavior of Melipona stingless bees and orchid bees (Hymenoptera: Apidae) in relation to floral characteristics of vanilla in the Yucatán region of Mexico

Abstract

Vanilla planifolia is native to the Mexican tropics. Despite its worldwide economic importance as a source of vanilla for flavoring and other uses, almost all vanilla is produced by expensive hand-pollination, and minimal documentation exists for its natural pollination and floral visitors. There is a claim that vanilla is pollinated by Melipona stingless bees, but vanilla is more likely pollinated by orchid bees. Natural pollination has not been tested in the Yucatán region of Mexico, where both vanilla and potential native bee pollinators are endemic. We document for the first time the flowering process, nectar production and natural pollination of V. planiflora, using bagged flower experiments in a commercial planting. We also assessed the frequency and visitation rates of stingless bees and orchid bees on flowers. Our results showed low natural pollination rates of V. planifolia (~ 5%). Only small stingless bees (Trigona fulviventris and Nannotrigona perilampoides) were seen on flowers, but no legitimate visits were witnessed. We verified that there were abundant Euglossa and fewer Eulaema male orchid bees around the vanilla plants, but neither visited the flowers. The introduction of a colony of the stingless bee Melipona beecheii and the application of chemical lures to attract orchid bees failed to induce floral visitations. Melipona beecheii, and male orchid bees of Euglossa viridissima and E. dilemma may not be natural pollinators of vanilla, due to lack of attraction to flowers. It seems that the lack of nectar in V. planifolia flowers reduces the spectrum of potential pollinators. In addition, there may be a mismatch between the attractiveness of vanilla floral fragrances to the species of orchid bees registered in the studied area. Chemical studies with controlled experiments in different regions would be important to further elucidate the potential pollinators of vanilla in southern Mexico.

Fig 1. Process of vanilla flower anthesis documenting the separation of the top sepal (ts) and the superior margin of the labellum (lb) over time.

Fig 2. The different bee species found on the vanilla planting in the Yucatan are shown with their relative body size.

Specimens on the top row are stingless bees and on the bottom row are orchid bees. A colony of M. beecheii was introduced into the planting.

Fig 3. Box plots of the frequency of male orchid bees counted on lures of 1,4 dimethoxybenzene, eugenol and β-lonone placed in the vanilla plot.

Squares, diamonds, circles and triangles indicate the mean values at each time.

Fig 4. Box plot of the daily activity of M. beecheii workers measured by the number of bees entering and exiting a colony experimentally placed in the vanilla plot.

Triangles indicate mean values.

Fig 5. Box plots of the daily activity of Euglossa males as measured by the number of bees on 1,4 dimethoxybenzene and eugenol chemical lures placed in the vanilla plot during the application of the same chemical attractants to flowers.

Circles and triangles indicate the mean values at each time.