Step aside bees, the ants are pollinating

When you think pollination you typically don’t think ants, and with good reason. Ant pollinators are thought to be extremely rare.

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Credit: Nicola Delnevo

Researchers have now discovered, however, that flowering smokebush Conospermum plants in Australia’s southwest have evolved to enable ants to pollinate them as effectively as native bees.

“This is the first plant species in the world found to have adapted pollen traits that enables a mutually beneficial pollination relationship with ants,” says Nicola Delnevo from Western Australia’s Edith Cowan University.

Like many insects, ants love nectar – a plant secretion that attracts pollinators to help spread their pollen so they can germinate and reproduce.

But ants liberally secrete an antimicrobial fluid to ward off bacterial and mould infections, which, unfortunately, also kills pollen grain, Delnevo says.

“So ants have traditionally been considered to be a menace – nectar thieves whose aggression keeps other potential pollinating insects at bay,” he says.

Delnevo is working on a project to conserve the endangered native smokebush Conospermum undulatum, which first involves understanding who pollinates it and how effectively pollen is transferred between plants.

Nearly 90% of wild flowering plant species rely on animals to disperse their gametes for fruit and seed production – about 88% of those being insects. But environmental degradation is impacting this process.

“Populations are becoming more and more fragmented and isolated because of land clearing,” Delnevo explains. “And without pollination there is no future for this plant, as it won’t be able to reproduce.”

Observing C. undulatum shrubs in Swan Coastal Plain, a biodiversity hotspot also known as Kwongan, he noticed native ants – including sugar ants (Camponotus consobrinus), meat ants (Iridomyrmex purpureus) and bull ants (Myrmecia infima) – were frequent visitors, along with a native bee species (Leioproctus conospermi).

To investigate their role in pollination, he and his team collected fresh pollen from the flowers of several different smokebush species and other plants, along with different species of native ants and bees, and took them back to the lab to run germination assays.

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(A) White flowers of Conospermum undulatum in Banksia woodland. (B) Flowers of C. undulatum. (C–H) Insects visiting flowers of C. undulatum: Leioproctus conospermi; Camponotus molossus; Camponotus terebrans; Iridomyrmex purpureus; Myrmecia infima; Apis mellifera. Credit: Nicola Delnevo

Following ant contact, the smokebush germination rates were around 80% – no different to native bees – while it dropped to 10% in other plant species.

This showed that pollination was not occurring due to fewer ant secretions, as has been found in the other rare cases of ant pollination in dry regions, but that the smokebush species had evolved tolerance.

“We found evidence that Conospermum plants have adapted the biochemistry of their pollen grains to cope with the antimicrobial properties of the ants,” Delnevo says.

The ants also carried a high load of plant-relevant pollen grains, showing they are important contributors to smokebush seed dispersal.

That’s good news because honeybees – which are imported – are no good to smokebush plants as their flowers have evolved structures to optimise pollination by native bees.

It’s encouraging that they don’t need to put all their pollen in one basket, as it were.

“Our study demonstrates the importance of ant pollination in this threatened species and adds to the ecological roles that ants might play in the region,” Delnevo says.

“This highlights the complexity of ant-flower interactions and reinforces the fact that our understanding of these systems is still in its infancy.”

Details of the open-access study appear in the journal Annals of Botany.

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