"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 18, 2022

Bdallophytum oxylepis

The ecological roles played by parasites can often get overlooked because they are largely hidden from sight, but their presence can have a cascading effect on the rest of the ecosystem. Bdallophytum oxylepis is a parasitic plant that is only found in Mexico, and it parasitises the roots of Bursera trees.

Left and Centre: Trigona fulviventris bees on the flowers of Bdallophytum oxylepis, Right: Arrows indicating the pollen baskets on the legs of T. fulviventris bees. Photos from Figure 4 of the paper

Unlike other flowering plants, this parasite does not photosynthesize - indeed, the plant itself is entirely embedded in the host plant's tissue, with its flowers being the only parts that protrude from the host plant, emerging out of the ground like some kind of exotic mushroom. While the flowers of many other angiosperm plants are brightly coloured, smell sweet and are often filled with nectar, the flowers of Bdallophytum are mostly dark or dull red, do not secrete any nectar, and it smells absolutely dreadful - at least to human noses. This is a common trait among many parasitic plants which often use carrion-feeding insects as pollinators.

Recently, a group of researchers in Mexico conducted a study at a patch of seasonally dry, tropical forest in San Fernando to figure out what animal(s) might be responsible for pollinating this parasite's flowers. Their study took place in 2018 and 2019 during the month of May, in the brief period between the dry and rainy seasons when the parasite's flowers bloom. 

Using a combination of direct observations during the day and camera traps during the night, they watched for any animals that might visit those stinky flowers. They also caught some of the insects that visited the flowers during the day, fixed them in ethanol, and spun them down in a centrifuge to count the number of pollen grains that they ended up carrying after visiting the parasite's flowers. Additionally, they also collected some of the flowers after they have been visited by said insect to count the number of pollen that the visitor had left behind on the stigma

Based on the researchers' observations, insects visited the flowers of B. oxylepis mostly during the day, with midday being peak hour for pollinator traffic. And despite the smell which might have led one to infer that the flower's main visitors would be carrion-loving flies, the researchers discovered that this parasitic flower's main pollinator is in fact a species of stingless bee - Trigona fulviventris, which regularly visited the flowers of B. oxylepis. While the flowers were also visited by ants and the occasional fruit flies, neither of them turned up nearly as often as the stingless bees. Nor do they end up being useful as pollinators since they didn't pick up nor deposit any pollen onto the flower's reproductive parts.

When the stingless bees landed on the parasite's flowers, they helped themselves to more than just its pollen. They treated the flower like an all-you-can-eat buffet, munching on various parts of the flower itself, all while busy shoving pollen into their pockets. But in return for munching on the flowers and hogging all the pollen, each time they visited B. oxylepis, they brought with them a big pollen deposit, plastering the flower's stigma with hundreds of pollen grains. When the researchers examined what type of pollen the bees were carrying, 21 out of 23 bees they looked at only had pollen that came from B. oxylepis. And while T. fulviventris is known to visit a wide range of different flowering plants, it seems the one they like to visit the most in May is this little parasitic flower.

There are a few reasons why this parasite's stinky flowers might be this bee's favourite - T. fulviventris build their hives on the ground near the roots and buttress of trees, and the flowers of B. oxylepis also emerge at ground level. This means that the bees don't have to expend as much energy to reach their flowers. Additionally, B. oxylepis also bloom in May, right at the end of the dry season when the flowers of most other plants are depleted and the newer flowers are yet to sprout. So this parasite is a life-saver for these bees, providing them with the food that they need to survive what would otherwise be a very lean month.

Protecting pollinators means more than just the catchy slogan of "save the bees!" - you need to save the plants they are dependent upon as well, whatever they might be. And sometimes it might just be an obscure parasite that most people would not have even heard of, with flowers that briefly bloom only once a year.

Reference:
Rios‐Carrasco, S., de Jesús‐Celestino, L., Ortega‐González, P. F., Mandujano, M. C., Hernández‐Najarro, F., & Vázquez‐Santana, S. (2022). The pollination of the gynomonoecious Bdallophytum oxylepis (Cytinaceae, Malvales). Plant Species Biology 37: 66-77.

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