Are fireflies flickering out?

The magic of fireflies is irresistible, and mid-June through early July is their season. Even those who profess to hate "bugs" love to see fireflies as they transform the dusk landscape with their moving constellations of rhythmically flashing lights. Of course, fireflies are not doing this for our entertainment. It's all about procreation. The airborne flashing fireflies we see are the males advertising their search for a mate with a species-specific code analogous to Morse code. The flightless females observe from their perches in the grass or on branches and flash back to guide the male towards them. The back-and forth flashing may last up to an hour before the male homes in on the female to mate. 

These humble dark insects are actually beetles that display the product of an astonishing feat of evolution: communication with light flashes that are almost 100% efficient. They produce light but not heat through a chemical reaction within a special light organ in the lower abdomen. When the firefly chooses to flash, it introduces oxygen and ATP into a mixture of the compound luciferin and the enzyme luciferase contained in its light organ. The enzyme then oxidises luciferin to release cold, bright photons in a phenomenon known as bioluminescence. By comparison, compact fluorescent lamps waste about 80% of their energy as heat and LEDS about 20%.

Globally, there are about 2,000 firefly species, each with its unique flash pattern based on rhythm, color, and duration. In a less spectacular but equally important display, firefly larvae use bioluminescence to warn potential predators that they contain a foul-tasting, toxic substance. 

Local observations in Thetford, including meadow habitat on Tucker Hill Road and woods-edge habitat on Poor Farm Road, suggest that the firefly display isn't what it used to be. Such observations are based on memory and are certainly not scientific. However, they are borne out by the Firefly Watch dataset that amassed 24,683 citizen science surveys of firefly occurrence over the years 2008-2016. The data confirm that North American fireflies are in decline.

As mentioned above, fireflies are not flies, but beetles (family Lampyridae). Like all beetles they develop from an egg into a larva that pupates and undergoes metamorphosis into the adult. Completing this life cycle can take anything from a couple of months to as long as four years, depending on the species. Most of a firefly's life is spent as a larva that feeds voraciously to grow from a tiny hatchling to pupation size. Firefly larvae are fierce predators of soft-bodied invertebrates many times their size, like slugs, snails, and earthworms.

Typically they hunt their prey in moist or marshy ground. When they catch something, they use their jaws to inject it with a paralysing neurotoxin, followed by a digestive enzyme that liquefies the prey's internal structures for easy consumption. The larva undergoes pupation in a rotting log or underground, less commonly in crevices in tree bark. Adult fireflies emerge from the pupae in late spring to early summer. 

Firefly larvae won't make it to the adult stage if they don’t have any habitat. Indeed, a survey of firefly experts in 2020 ranked threats to fireflies in the following order: Habitat Loss, Light Pollution, Pesticide Use, Water Pollution, Drought, and Higher Temperatures. 

Humans are all too accustomed to mowing vast areas of lawns, draining wet areas, and removing dead and rotting trees. All these landscaping practices are detrimental to fireflies. Closely mown lawn grass offers no protection and is generally too hot and dry. Fireflies need meadows of tall grasses and diverse plants for resting and staying cool by day and for mating by night. Their larvae hunt in moist soils and pupate in rotting logs. 

Outdoor lighting at night is a serious issue. More and more, people install home security lights that are left on all night. This confuses fireflies and drowns out their flashing signals so they can't find a mate. No mate means no next generation of fireflies.

In addition, "bug zappers" and outdoor lights impose a huge toll on all nocturnal flying insects, including death by collision, dehydration, and exhaustion from flying around lights, increased predation, and disrupted foraging and reproduction. If you must have a light, invest in a motion-activated light that turns itself off, and use a yellow light bulb that attracts fewer insects, though this is not a cure-all. And get rid of that bug zapper that kills everything, including many beneficial insects, indiscriminately.

Pesticides decimate fireflies by direct exposure, in particular broad-spectrum products like neonicotenoids, which are applied to lawns in some areas, and pyrethroids, including Permethrin which is widely used against ticks and is applied by home pest controllers against mosquitoes. Pesticides applied to the soil and to landscaping also kill indirectly by starving firefly larvae of their prey, especially products such as slug baits that kill molluscs.    

That lovely green lawn indeed comes at a price to fireflies. In one study, urea fertilizers killed 27% of firefly larvae while ammonium fertilizers killed 56%. Even some herbicides (weedkillers) and fungicides are known to harm fireflies and other beetles.

We can't do much on a personal level about climate change-related drought and heat waves. But we can mitigate them by creating micro-refuges for fireflies and their soft-bodied prey. Protecting and enhancing cool, damp, vegetated areas with water features, shade, leaf litter, and dense native plantings can provide cool moist hiding places. Infrequent mowing, woody debris, and fallen leaves offer egg-laying and overwintering habitat. 

Let’s not lose these bioluminescent marvels. Embrace Diverse, Disorderly, and Dark-by-night landscapes for a firefly-welcoming habitat!