The enchanting light shows of adult fireflies make ignoring the rest of their life cycle easy. But therein lie the threats to them
SUMMER MAY OFFICIALLY BEGIN on June 21, but in much of southern Ontario, a person could argue that it starts with the year’s first flash of a firefly. Where I live — in the countryside north of Cobourg, atop the eastern limits of the Oak Ridges Moraine — that is usually around June 1.
Signalling intentions: Airborne male fireflies flash to attract a mate. Females on the ground flash back in response

For weeks, I peer into the backyard after dark, and then one day the light show begins. First a few, then dozens of lights in the air; stationary flashes, wobbling strobes and the odd speeding streamer. They appear here, then there, usually no more than a metre or two off the ground — tiny LEDs signalling against a blackout curtain in a dark theatre.

Whatever the pattern, the message is always the same: “I’m available.” These airborne flashers are all male fireflies attempting to catch the attention of a female in the vegetation below who will respond with a come-hither blink of her own.

The fireflies are most bountiful on parts of my property where spring runoff soaks the ground and uncut meadow mingles with stands of green ash and Scotch pine. Using bioluminescent chemical beacons in built into their abdomens, the insects repeat the spectacle each night at dusk and after sunset, often for hours on end, until petering out in mid-July.

Some nights, I am spellbound and, clearly, not alone. Around the world, fireflies have inspired artists and cultural traditions for centuries. In Japan, they symbolize sacred spirits and passionate love, while among Indigenous cultures of Peru, their flashes conjure up eyes of ghosts. More recently, tourists have been flocking to locations where firefly displays are particularly strong or distinctive. In Thailand, Malaysia and several U.S. Appalachian states, certain species that flash in sync draw tens of thousands of observers annually.

Yet for all of their popularity, fireflies still hold many secrets. My first discovery was that the light show marking the start of summer actually signifies an end. What people consider to be firefly season is a short, climactic dance, the insect equivalent of the exploding supernova of a dying star. It is triggered when fireflies — which are not flies at all but beetles — enter their all-too-brief adulthood. After spending one to two years on or under the ground as larvae and then undergoing metamorphosis in a short pupal stage, the adults push their way up from the soil, light up and take flight with the purpose of finding a partner and reproducing before dying.

There are over 2,200 species of fireflies in the beetle family of Lampyridae worldwide. About two dozen live in Ontario

MOST PEOPLE ASSOCIATE FIREFLIES solely with the adults they see in summer — something that Sara Lewis feels needs to be corrected. “Evolutionarily, ecologically, that [long] larval stage is a key part of the life cycle that people don’t even know about,” says the biology professor emerita at Tufts University outside Boston who co-chairs the Firefly Specialist Group at the International Union for the Conservation of Nature (IUCN).

Explaining why the larval stage matters begins with some basic firefly facts. Worldwide, there are over 2,200 species of fireflies in the beetle family of Lampyridae, divided into three primary groups: those that flash as adults (sometimes also called lightning bugs); non-flashing adults that are active during the day and signal to prospective mates by emitting pheromones; and glow-worms, in which only females, who are flightless, produce light.

Almost all North American fireflies fall into the first two groups. About two dozen species live in Ontario, and close to two-thirds of them flash, according to Stephen Luk, co-author of the identification guide The Fireflies of Ontario. Despite the relatively few varieties, Ontario fireflies are quite distinctive. Among the most common non-flashing species, for example, are winter fireflies, which are active in summer, fall and winter and are the bane of maple sap collectors because these insects tend to fall into the sap-collection buckets, contaminating the harvest, says Luk. Several other species are found only in specific rare or geographically limited habitats, such as boggy wetlands, tallgrass prairie or stands of Carolinian forest in Ontario’s extreme southwest.

What the North American species have in common is that their larval stages are entirely terrestrial with very limited dispersal. The larvae remain underground or in surface organic litter (a few types also feed in water) within a few metres of where they hatch. There they eat worms, snails, slugs and other soft-bodied invertebrates. The adults are generally weak flyers, females especially (if they fly at all), so they rarely relocate very far. All of which means that the fate of every population depends on the sustained health of its immediate habitat.

Moisture and fertile soil, which assure a food supply, are key. Any large-scale disturbance — urbanization, industrialization or intensive agriculture — can wipe out local populations. Because fireflies are prone to desiccation, water loss due to drought, irrigation extraction or overgrazing can be just as harmful.

Fortunately, firefly larvae have a trait that aids in their detection and can help prevent inadvertent destruction of their habitat: all are bioluminescent. “In the fall and spring, you can sometimes see them if you’re digging in your garden,” says Lewis. Unlike flashing adults, which are believed to have “co-opted” the flash for use in courtship, the glow of larvae is a signal to potential predators that they are generally toxic and best avoided.

THE RECENT WORLDWIDE attention on declining insect populations has raised questions about how fireflies are faring. Answers are elusive. “We certainly have examples of sites being extirpated because of development or some other degradation, and we keep hearing anecdotal reports about fireflies declining,” says Candace Fallon, senior endangered species conservation biologist and firefly program lead at the Xerces Society for Invertebrate Conservation in Portland, Oregon. “But we don’t have any baseline or monitoring data to back that up in North America.”

The primary threats are well understood. In 2020, the IUCN Firefly Specialist Group teamed up to evaluate global threats to fire- flies and concluded that the top three are habitat loss, light pollution (see sidebar) and pesticide use. Next, North American members of that

Night Light Blight

For people, low ambient light is a romantic sweetener. For fireflies that flash signals to find mates, it is not just a buzzkill — it is an existential threat. That is why experts rank the increasing spread of artificial light at night as the second-biggest risk to nocturnal fireflies and are urging any and all means to reduce it.

“It makes sense, right?” says biologist Sara Lewis. “It’s going to obscure their courtship signals and reduce mating success.”

Scientists theorize that firefly species that flash long after sunset are more susceptible to disruption by artificial light, as those that flash at dusk are likely more accustomed to contending with ambient light. However, light pollution is detrimental to all firefly species that produce light as adults. “If adults aren’t finding each other, that has huge repercussions for future generations of fireflies,” says the Xerces Society’s Candace Fallon.

specialist group gathered data that they used the following year to assess 132 species of North American fireflies for the IUCN Red List of Endangered Species, the benchmark for estimating species’ risk of extinction.

Of the total, the researchers categorized 20 species, none of which are found in Canada, as threatened or near threatened. Approximately a third were found to have healthy populations, and 53 percent were deemed “data deficient” — highlighting the need for more surveying, monitoring and life history studies. However, the Wild Species 2020 report that was published last fall by a collaboration of all provincial and territorial governments in Canada, and of the federal government, lists three of Canada’s 31 native firefly species as vulnerable. All three are found in the Windsor area or along the Lake Erie shore. Building on the Red List work, the Xerces Society recently launched the Firefly Atlas, an online resource to educate the public and organize data collection to improve knowledge about North American fireflies, particularly those that are threatened or data deficient. The website includes surveying guidelines and species profiles targeted at land managers, naturalist groups, protected area stewards and community scientists.

The tools are intended to help overcome some significant obstacles in surveying fireflies. “You have the challenge of being in the dark while you’re with your study subjects,” says Fallon. “You have to know that X individual produced X flash pattern, and [try to] follow it to make sure that you’re not counting the same individual over and over each time it flashes.” Captured individuals can be examined for defining morphological details, but even that can be tricky, adds Luk. Many species have similar physical characteristics and certain groups of fireflies are almost impossible to tell apart.

Hearing this, I ask Fallon and Luk whether they can tell me if I’m seeing one or multiple species in my yard at night. “Fireflies have really strict partitioning,” says Fallon. “Each species of night-active flashing fire- flies is active during a specific time of night, and they have specific flash patterns. If you’re out in a rural area and you’re seeing a lot of fireflies, especially from dusk through to complete darkness, you’re probably see- ing a few different species.”

Different species also fly at different times of the season. Sometimes, the colour of their flash can provide a clue. According to Luk, the three main flashing firefly genera in Ontario —Photuris, Photinus and Pyractomena — cover a spectrum from neon green to yellowish to orange. “A bit like a stoplight,” he says.

I resolve to pay attention to these details when firefly season comes around. In my mind’s eye, I see mostly neon green, but the flash patterns are varied and seem random. Finding order in those signals will be like unlocking a secret code of summer.

This article was originally published in the Summer 2023 issue of ON Nature magazine. Photos courtesy of (top); (middle);