Oil Bee Pollination: A Breakthrough in Orchid Study

Orchids are known for their diverse and intricate floral structures that often mimic the appearance or scent of their pollinators. However, some orchids have evolved a more unusual strategy to attract their vectors: offering oil as a reward.

Oil bee pollination is a type of pollination mutualism that involves oil-collecting bees and oil-offering flowers. The bees collect the lipid-rich droplets from the flowers with specialized hairs and use them for larval food or nest lining. This interaction is mostly found in South America and South Africa, where hundreds of plant genera and bee species are involved.

However, a recent study published in the journal National Science Review has discovered a new case of oil bee pollination in tropical Asia, involving male-bee-pollinated orchids and Ctenoplectra bees.

The study, conducted by a team of researchers from China, Thailand and Singapore, found that 33 of 41 orchid species sampled in the genera Dendrobium and Galeola produce fatty oil in the minute hairs on their labella, the lower lip of the corolla. These orchids are mainly visited by male bees of Ctenoplectra cornuta, which collect the oil with their hind legs and thorax.

This is surprising because female bees are usually the ones that collect oil from flowers, as they need it for their offspring or nest construction. Male bees, on the other hand, usually seek nectar or other floral secretions as energy sources. The observation of males collecting floral oil from orchids has not been reported before.

The researchers suggest that the male bees use the oil to attract females during mating. They observed that male bees deliver the floral secretion they collected from the orchids to the females during copulation. This may enhance the reproductive success of both the bees and the orchids, as the females may prefer males with more oil or better quality oil, and the orchids may benefit from more frequent visits by male bees.

The discovery of this new oil-flower/oil-bee pollination mutualism sheds light on the diversity and evolution of plant-pollinator interactions in Asia. It also reveals that some orchids have developed a novel way to deceive their pollinators by offering oil instead of nectar or pollen, which are more common rewards.

The study also provides some data on the morphology, chemistry and distribution of the oil-offering orchids and their pollinators. The researchers found that:

• The oil-producing hairs on the labella are glandular and secrete lipids that stain red with Sudan IV dye.
• The lipid composition of the oil varies among different orchid species, but generally contains fatty acids, alcohols, esters and hydrocarbons.
• The oil-offering orchids are mainly distributed in Southeast Asia, especially in Thailand (17 species), Myanmar (14 species) and Yunnan Province of China (13 species).
• The male Ctenoplectra cornuta bees have characteristic hind legs with long spines and curved claws that help them collect and transport the oil. They also have pollinaria attached to their thorax after visiting the orchid flowers.
• The male bees visit an average of 3.6 ± 1.2 flowers per minute and spend an average of 16.4 ± 5.6 seconds per flower. They transfer an average of 1.8 ± 0.7 pollinaria per visit.

The study also raises questions about the origin and distribution of oil-offering orchids in Asia, as well as the ecological and genetic factors that influence their pollination by male bees. Further research is needed to explore these aspects and to understand the implications of this mutualism for orchid conservation and cultivation.

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