Chloraea membranacea, an orchid with a sweet smell and no reward

Floral features, pollination biology and breeding system of Chloraea membranacea Lindl. (Orchidaceae: Chloraeinae) has moved into free access, along with the rest of the December 2012 issue of Annals of Botany. This paper caught my eye because it’s a reminder of how clever orchids can be.

What’s going on here?

Insects are pollinating the flowers, but they’re doing it in a peculiar way. A lot of the time they have their abdomens in the flower and their head poking out. This partly because of the way this orchid has evolved to reproduce. Chloraea membranacea Lindl. is self-compatible, so it’s happy pollinating itself, or at least it would be if it could. It can’t so it has to rely on pollinators to visit. So this is why it has flowers.

This is hardly news, angiosperms have flowers and offer nectar to visitors to entice them in. But C. membranacea doesn’t.

Sanguinetti et al. found that C. membranacea gives off a sweet scent to attract insects, but does not offer any nectar as a reward. In fact landing on the flower is a bad idea as the plant slaps a heavy pollinarium on the back of the insect to carry to another flower, often another one of its own. Using deception, the orchid can get the benefits of pollinator visits with fewer of the costs, like having to produce nectar.

However, maybe bees aren’t always the helpless victims.

If you re-watch the video at around 40 seconds you’ll see a bee grooming itself. These pollinaria are hefty and can unbalance an insect. During observations of the plants some bees, Halictidae females, were seen transferring pollen from the pollinarium to their hindlegs. This is a valuable food, but it’s not often used from this orchid, because it’s packaged away in the pollinaria. If the bees are getting the pollen from the orchid then it might be inadvertently providing an award after all. Yet this might not be the start of a deliberate strategy, Sanguinetti et al. very clearly state:

The fact that some pollinarium-laden female bees were seen actively collecting pollen from the pollinaria should be interpreted with caution. All our evidence suggests that pollen collection is a by-product of grooming activities.

What we’re currently seeing is a snapshot of an ongoing process of refining and adaptation. So far all the Chloraeinae seem to be deceptive orchids, getting pollinated without offering a reward. Sanguinetti et al. say that pinning down how they evolved is likely to become easier. It looks like their sister clades all offer nectar, so that ancestors of these flowers may have initially offered a reward too, but found that with their looks and scent they didn’t need to go to the effort of producing nectar.

If you do ever reincarnate as a pollinator then it would be a very good idea to never trust an orchid.