Mysteries of the humble ribwort plantain

 

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Figure 1 A Plantago lanceolata flower head, photo by Laura Russo.

Even if you’re not a botanist, you’re probably familiar with the long, thin leaves of ribwort plantain (Plantago lanceolata) poking up through a grassy yard. Growing up in the United States, I knew it well as a potential weapon. If you wrap the stem around the base of the mature seed head, it can be fired as a projectile at the back of your sibling’s head. (The harmless nature of the seed head bouncing off did not deter us from pretending it was a deadly weapon.) Late in the summer, the grass was full of fluffy plantain seed heads: ready ammunition for an eight year old.

The plantain is in the plant family Plantaginaceae, which includes the very familiar foxglove (Digitalis purpurea), which grows in gardens all over Dublin. If you squint, maybe you can see the familial relation between Digitalis and Plantago (or maybe not), but the Plantaginaceae also includes things like speedwell (Veronica sp), which looks pretty different.

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Figure 2 Foxglove in our garden, photo by Laura Russo.
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Figure 3 Speedwell in the grass, photo by Laura Russo

The humble plantain is also at the heart of a massive research network investigating global patterns in population dynamics. Headed by Trinity’s own Yvonne Buckley, this network of researchers around the globe is collecting data on the “environmental and biological drivers of population persistence and extinction”, making Plantago lanceolata like the white lab rat, Drosophila melanogaster, or Arabidopsis thaliana of ecology.

In the States, I was also fond of plantain because of its interactions with bumblebees. Bumblebees, especially the common eastern bumblebee (Bombus impatiens), love to gather pollen from its anthers, but the stem is too thin to hold their weight. It was fun to sit and watch the bee land on the flower and send it plummeting to the ground. Undeterred, they would busily collect pollen from it on the ground and then fly away, allowing the stem to pop back up. In a meadow of flowering plantain, the stems seemed like they were constantly bobbing up and down with bee visits.

After moving to Ireland, I wish I’d done more than idly observe this interaction. Here, pollinator visitation to Plantago lanceolata is rare. And I can quantify that! After spending 12.5 hours (in 150 5-min segments) watching over 600 plantain flowers across the summer, I observed only 14 visitors. That’s just over one visitor per hour of observation. Here in Ireland, those visitors were syrphid flies (Scaeva pyrastri, Melanostoma mellinum, Eupeodes corolla, Episyrphus balteatus, Sphaerophoria sp., Platycheirus angustatus), and two bumblebee species (Bombus pascuorum, Bombus pratorum (aka the tiniest bumblebee)). In the largest dataset on plant-pollinator interactions in Ireland (spanning from 2009-2015 and including more than 200 plant and insect species), the plantain is visited by some of the same syrphid flies as I observed, plus Platycheirus clypeatus, Melanostoma scalare, Helophilus pendulus, Eupeodes luniger, Cheilosia sp, and Bombus lucorum (Fig. 4).

This is a surprisingly high diversity (> 15 visitor species) for a plant that only rarely receives visits! If you were to observe this plant for only a short period of time, you might conclude that it is a specialist, when in fact it is a generalist. It interacts with many pollinator species, just in small numbers.

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Figure 4 Interactions of Plantago lanceolata compiled from Jane Stout’s dataset on plant-insect interactions. The x-axis is the week of the summer, starting at the beginning of May and continuing through August. The y-axis is the number of interactions observed.

The plantain is generally considered to be wind-pollinated, but in Brisbane honeybees have been observed to methodically and deliberately collect pollen from it, and it has been shown to be in their pollen baskets (meaning they were likely taking it back to the hive to provision offspring) (Clifford 1962). I have plenty of data to show that bees collect pollen from species that are putatively wind-pollinated (Russo and Danforth 2017), but the contrast between the visitation of this plant in its native and introduced range is very interesting.

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Figure 5 Distribution of flower visitors over time to Plantago lanceolata…the distribution seems relatively even across the day, including a visitor at five pm.

 

I have no data for the American plantain and its visitors, but I suspect the visitation rate is higher than in Dublin. Is this related to temperature, as Clifford (1962) suggests? Both the eastern US and Brisbane are much warmer than the UK or Ireland in summer. Or is it something else? Some have suggested that the visitation to the plantain is time dependent (eg Clifford 1962 suggests that because it is warmer earlier in the day in Brisbane, this drives pollinator visitation to the plantain before the wind blows away the pollen), yet the visitation in my study last year (in Ireland) was distributed relatively evenly across the day, with a peak at midday.

Is it possible that the plant has adaptations to attract insect pollinators in its introduced ranges? Is the quality of pollen different? Does visitation by insect pollinators increase seed set in its native range? In its invaded range? I think that this humble garden weed has a lot of fascinating questions for pioneering minds…especially with regard to its pollinators!

Dr Laura Russo is an EU Marie Skłodowska-Curie post-doc Fellow in the Plant-Animal Interactions group at Trinity College Dublin.

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