Q&A: How artificial lights are dimming firefly survival rates

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While light bulbs make our nights brighter, they are dimming the social lives, and survival rates, of some of nature's natural light-makers: fireflies.

A 2021 report found that 1 in 3 North American species of fireflies may be at risk of extinction. A 2020 survey of nearly 50 of the world's leading firefly experts identified habitat loss and artificial light as the greatest threats to the fireflies' global populations.

This trend is not unique to fireflies. Studies show habitat destruction, climate change and pesticides are leading to declines in monarch butterfly populations across North America. A Department of Agriculture 15-year U.S. study found the number of bee species in a single forest had decreased by nearly 40%.

University of Virginia environmental sciences professor Kyle Haynes has spent years researching the impact of light pollution on different plant and animal species, including fireflies, at UVA's Blandy Experimental Farm. Haynes spoke with UVA Today and illuminated specific ways in which light is interfering with the insects' mating behavior and shared insights on how the bugs use light to communicate.

How do fireflies use light?

Each firefly species has a unique flashing pattern that they use to attract members of the same species for mating. They flash using different temporal sequences while dancing through the air in different patterns.

You can actually recognize individual species by their flashing patterns. There are firefly species in which the females eat the male of other firefly species. They do this by mimicking the flashing patterns of the females of a prey species to lure in and capture the males.

Of course, that use of lighting, bioluminescence, is what makes them especially vulnerable to the effects of light pollution.

How do artificial lights at night, or light pollution, impact fireflies?

Ariel Firebaugh, a former UVA doctoral student who is now the director of scientific engagement at Blandy, and I found that light pollution is definitely having an impact on firefly populations. The firefly species we studied tend to be drawn toward lights at nighttime and, once under them, stop flashing as if it's daytime even though nighttime is when they should be flashing for courtship or predation purposes.

Our data suggests they also stop mating. So, lights have a double impact. They work as a trap and draw the insects in and stop or decrease their reproduction.

What can individuals do to help avoid affecting nearby populations in this way?

You don't want to use any light brighter than is necessary once the sun goes down. Or, really, any light that isn't necessary.

Many insects are more strongly attracted to lighting with a lot of blue light on the emission spectrum compared to other colors. So, installing lights with less or no blue light might be one of the most effective things you can do.

It's also generally recommended to shield your lights or limit their shine. You can buy what are called full cutoff light fixtures, which are designed to direct the light downward rather than upward toward the sky. Motion sensor lights that are only triggered when someone walks by are also useful for reducing light pollution.

After years of working with fireflies, can you distinguish between species based on their flashing patterns?

Yes, a little bit. One of the main sets of species my students and I studied creates what looks like a yellow J-shaped pattern in the air. Ariel Firebaugh, who did the bulk of the fieldwork, is much more skilled at distinguishing species.

How is light pollution impacting other living species?

My students and I are studying the effects of light pollution on plants and the interactions between plants and insects that eat them.

One of the things we're researching is how light pollution affects native and invasive plant species. Past research suggests that while both types of plants are impacted by light, non-native plants may fare better in light-polluted areas, and we are trying to understand why.

Provided by University of Virginia