Slippery flowers help deter nectar-stealing ants

Though ants frequently form mutualisms with plants for the purposes of seed dispersal, they are only rarely involved in pollination. More often, when it comes to flowers, ants impact plants negatively, acting as nectar thieves and scaring away legitimate pollinators with their aggressive presence. It is therefore advantageous for plants offering nectar to develop mechanisms to discourage or prevent ants from visiting open flowers. Though there have been some documented cases of such adaptations, it’s not a phenomenon that’s been well-researched.

In a new article just out in Annals of Botany, lead author Kazuya Takeda and colleagues investigated a case of pendant flowers appearing to use a slippery coating on their perianths to prevent ants from reaching a nectar reward meant for the plants’ flying pollinators. The researchers noticed that ants rarely visit the flowers of Codonopsis lanceolata, despite the nectar being exposed and easily accessible. “During the observation, we found that ants slipped off and could not walk on their petals. [W]e hypothesized that this slipperiness might be a defence against nectar-thieving ants,” says Takeda, a Ph.D. student at Kyoto University. “Although some flowers, especially those in tropics, emit deterrent chemicals to ants, we still know little about how other flowers deter ants,” they add.

Using C. lanceolata and another slippery flower, Fritillaria koidzumiana, the authors conducted behavioural experiments to test whether the slipperiness prevents entry by ants, and how the presence of ants in the corolla affects pollination. They also used scanning electron microscopy (SEM) to study the petal surfaces and understand what gives them their slipperiness.

The SEM findings showed that the flowers’ slippery surfaces had a coating of wax crystals. When these were wiped with hexane, a non-polar solvent, the surfaces lost their slipperiness, significantly decreasing the number of ants that fell off while attempting to enter. What’s more, when artificial, non-slippery ‘bridges’ were added, ants entered the flowers more often.

To test the effect of ants on pollinators, the researchers actually tied individual live ants inside the corolla and observed the behaviour of visiting bees and hornets, as well as the pollination rates of ant-baited versus ant-free flowers. While the ants did indeed shorten pollinator visits, there was no significant difference in fruiting or seed set between the two groups.

Though this is the first study demonstrating that slippery flowers prevent floral access by nectar thieves, Takeda notes that there may be a cost to the plant in pollination efficiency. “What needs further experiments is whether slippery perianths affect the preference of pollinators. Previous studies report that pollinator bumblebees prefer less slippery flowers [for easier landing]. If such preference presents in our system, the slippery defence may have some cost on pollinator attraction. How plants deal with such a trade-off between floral defence and pollinator attraction may be an essential question.”