Like Moths to a Flame – Rethinking Outdoor Lighting in Your Garden

by Emma Murphy, Master Gardener

A few weeks ago, the incomparable Lorraine Johnson spoke at my local horticultural society. She summarized four key actions gardeners could do to help our pollinators, and the last one stuck out for me — reduce/reconsider outdoor lighting (see her full list of actions at the bottom of the blog).

I thought, what does outdoor lighting around my house or garden have to do with pollinators? I know that the presence of lighting (or even more importantly, light frequency) is disruptive for migrating birds and nesting sea turtles, but for pollinators in my garden?

However, what I’ve learned in my research is that artificial lighting at night (aka ALAN) poses a hazard to nocturnal pollinators and prevents proper navigation, reproduction, and their ability to find food.

This really cool graphic (from Kenilworth Park and Aquatic Gardens in Washington DC) shows some examples of day and night pollinators.

Why is it a problem?

Use of ALAN has rapidly spread around the globe over the past few decades. This 2021 Nature Communications journal article states increasing evidence that ALAN adversely affects the behaviour, physiology, and survival of animals and plants, ultimately leading to a significant decline in their abundance and diversity.

This 2020 Biological Conservation journal article says that although habitat loss, pesticide use, invasive species, climate change all play a role in insect decline, ALAN is another important—but often overlooked—bringer of the “insect apocalypse”.

A Nature journal article back in 2017 also sounded the alarm, showing that in ALAN plant–pollinator communities, nocturnal visits to plants were reduced by 62% compared to dark areas.

This UK Royal Horticultural Society article lists some of the effects of ALAN, not just on pollinators but all wildlife:

Nocturnal insects (including many moths) who navigate using natural light sources (like the moon) are disoriented by ALAN (although research is now indicating that ALAN disrupts circadian rhythms in both nocturnal and diurnal animals).
Security lights appear to temporarily blind some animals and may even attract them (for example, frogs – I have seen this in my backyard near my pond).
Birds are disturbed from sleep by sudden lighting and can begin singing before dawn (robins especially seem sensitive to light). Birds that start migration flights at night can become disoriented.
In ALAN areas, shorter periods of nighttime darkness means less time for foraging/hunting for crepuscular (dawn/dusk) or nocturnal species.
ALAN is thought to be partly to blame for the decline of fireflies/glow worms; the females emit low, greenish light to attract mates and even low level ‘skyglow’ from distant light sources such as floodlit playing fields or towns will lessen their breeding success.

The type (frequency) of light seems to affect species differently. For example, research indicates that LEDs seem to attract more moths and flies, but fewer beetles than sodium lamps. And LEDs with cool white light (blue end of the spectrum) attract more insects than warm white ones. As a general rule insects are more sensitive or attracted to short-wavelength (UV, blue and green) than long-wavelength (orange, red and infra-red) light.

So what can gardeners do?

While ALAN has become a trendy part of our outdoor living spaces, consider whether you really need it, or modify it to minimize its impact. In my back garden I definitely need some path lighting for visiting guests, but I make sure it’s shielded lighting that is on a motion sensor so it’s only on for a short time. The International Dark-Sky Association has some great information on keeping our skies dark, including using shielded light fixtures that minimize glare, light trespass, and skyglow.

Besides, a garden can be just as magical a place enjoyed in moonlight or simply with the aid of a flashlight! And if you minimize ALAN you might just see more fireflies/glow worms in your garden, like I do (hint: having a pond or water feature also attracts them).