Moths pollinate clover flowers at night, after bees have gone home

Bees aren’t the only insects pollinating red clover. Moths do about a third of the flower visits after dark, new research suggests.

The findings, detailed in the July Biology Letters, come as a surprise, since almost all the credit for pollination of red clover has gone to bees. The discovery highlights what researchers may be missing during the night shift of plant pollination, including a previously unknown benefit the moth pollination bestows on the clover — a boost in seed production. 

This work may help deepen scientists’ understanding of the pollination services provided by nocturnal moths, says Daichi Funamoto, a pollination biologist at the University of Tokyo who was not involved with the new study.

For about a century, the general understanding of clover pollination has been that bees — and bees alone — are the key insect players. Clover is a “valuable agricultural plant and has received a lot of study,” says Jamie Alison, a pollinator ecologist at Aarhus University in Denmark. “Yet none of those studies have said anything about the possibility of moth pollination.”

Alison and his colleagues discovered moths’ pollination role while studying how plants and their insect pollinators respond to climate change by potentially moving uphill. To track pollinator visitation to grassland plants, the team set up 15 time-lapse cameras in the Swiss Alps. 

From June to August 2021, the cameras monitored 36 flowers of red clover (Trifolium pratense), an important crop used as forage for livestock. Such cameras are very useful for monitoring sites that are difficult to reach daily, Alison says. 

Nine of the cameras took images in a slice of the afternoon and again at night, while six of them continuously captured photos every five minutes. The technology provides substantial practical benefits.

“You can’t feasibly have someone stand there for 24 hours and record consistently what is visiting a flower,” Alison says. “Fortunately, you can do that with cameras.”

The method also allowed Alison and his colleagues to investigate nighttime visitors. In all, the team collected more than 164,000 photos of red clover flowers, with 44 of these images capturing visits by insect pollinators. Most of these nectar-seekers — some 61 percent — were bumblebees (Bombus). But a substantial proportion — 34 percent — were moths, mostly large yellow underwings (Noctua pronuba), visiting in the early morning hours. Butterflies and either a wasp or another bee species rounded out the other 5 percent of visits.

Moths are well-known as habitual pollinators of many other plants, but their role in clover pollination seems to have been overlooked, Alison says (SN: 6/27/17). He and his colleagues also investigated how many seeds the clover blossoms produced, finding that nighttime visits from moths added to seed yield.

It’s clear “the role of nocturnal moths as pollinators of crops has largely been neglected,” Funamoto says. “I think future studies will reveal many plant species that are thought to be dependent on pollination by diurnal insects are indeed pollinated by nocturnal moths, to some extent.”

Alison and his team are now looking to replicate their observations at different latitudes in Europe to confirm that N. pronuba moths pollinate red clover in other places. The researchers also would like to equip cameras with artificial intelligence–driven programs that are trained to identify and swiftly categorize the type of pollinator making a visit.

“The future isn’t just cameras,” Alison says, “but cameras should be a big part of it.”