Street lamps keep insects from pollinating plants at night

Plants benefit from nocturnal pollination, but they’re getting less of it due to light pollution.

Bees and butterflies get most of the credit, but recent research suggests that less glamorous pollinators—moths, beetles, flies, and other bugs active at night—are also important. That means that when insects swarm around lights at night, nearby plants may be missing out. The result is less pollination and reduced fruit development compared to unlit areas. We spoke with author Eva Knop to learn more.

Beetle actively moving on the cabbage thistle (Cirsium oleraceum) during night. Credit: Eva Knop

ResearchGate: How did you evaluate the impact of light pollution on nighttime pollination?

Eva Knop: We set up a large experiment using LED street lamps in Swiss Alpine foothills, where levels of light emission are still relatively low. We installed lamps in seven meadows, which had never previously been exposed to artificial lighting, while using another seven meadows as controls. We sampled nocturnal interactions between plants and flower visitors and analyzed how artificial light at night affects the structure of the interaction network.

To test whether the negative impact of artificial on nighttime pollinators translates into a lower pollination function, we conducted a fruit set experiment using the Cabbage thistle, Cirsium oleraceum. This plant was common to all our sites and was one of the most frequently visited species during day and night.

RG: What did you find?

Knop: We show that in artificially illuminated plant-pollinator communities, nocturnal visits to plants were reduced by 62 percent compared to dark areas. Furthermore, this reduction in nocturnal visits to plants lead to an overall reduction of fruit. This is particularly interesting as the Cabbage thistle was frequently visited by daytime pollinators. So it seems that daytime pollinators cannot compensate the loss of nocturnal pollinators.

RG: Why do you think nighttime pollinators pollinate less when there’s light present?

Knop: A light source usually attracts or deters nocturnal insects. Both distract insects from visiting a flower. Physiological reactions to artificial light at night might also have played a role.

RG: Why should people be concerned about reduced pollination in general?

Knop: Pollination is a key ecosystem service with most wild plants depending on it, and more than 30 percent of crop plants depending on it for maximum yield. Also, the demand for animal-pollinated crop plants is currently increasing worldwide. Together with the increase of human population over the next years, demand for this key ecosystem service will be even higher in near future.

RG: Why is nighttime pollination specifically important?

Knop: This question should be further explored. At present, answers are just hypothetical. One possibility is that nocturnal pollination increases the resilience of the plant-pollinator communities, as nocturnal pollinators are likely to respond to environmental factors in a different way.



Featured image courtesy of Lukas D. Schuler, Dark-Sky Switzerland and xirrus GmbH, Zurich, Switzerland.