Nearly 2,000 species of fireflies flit, crawl and sparkle across the planet. Some of these lightning bugs are doing fine. Others are not.

A survey of 49 of the world’s firefly experts, published Monday in the journal BioScience, has identified the most serious threats to the animals. Habitat loss, in almost all of the regions surveyed, is a problem. Other threats include artificial light, which disturbs their mating rituals; pesticides, which can harm the insects or their invertebrate prey; and water pollution, for species that have an aquatic stage.

The report is not a census of the world’s firefly population. But it is “the very first time that we’ve gathered information — this is based on expert opinion — about what the most prominent threats are to the fireflies in different parts of the world,” said study author Sara M. Lewis, a biologist at Tufts University.

“For the last decade or more, people have been anecdotally reporting that they’re not seeing fireflies where they used to,” Lewis said. “Good census data over the past few decades” exists for some species, such as Malaysia’s synchronous fireflies and the common European glowworm, Lewis said. “We know that those populations are, in fact, declining.”

Elsewhere, however, firefly literature remains “kind of obscure,” she said, and the research community is relatively small.

This poll of firefly experts was the “next best thing” to traveling back in time to count firefly populations, said University of Florida entomologist Marc Branham, who was not a member of the research team. He has been told many anecdotes of missing fireflies. And often, he said, they’re believable. Fields once full of flashing insects “are so obvious, in a sort of a sad sense,” when the light vanishes, he said.

“One of the things we’ve kind of taken for granted is that fireflies will always be here,” said naturalist Ben Pfeiffer, founder of the nonprofit Firefly Conservation & Research organization and one of the firefly experts who was surveyed. “And we’ve been terribly wrong about that.”

In 2018, the International Union for Conservation of Nature created the Firefly Specialist Group, co-chaired by Lewis, to determine whether certain firefly species should be listed as threatened or endangered. “That’s something we’ve never seen happen for a firefly species,” Fitchburg State University biologist Christopher Cratsley said. Cratsley was not a member of the study team.

The survey, Lewis said, represents a first step in that process. She cautioned that “we don’t know what the relative importance of these threats to fireflies are. We only know the ranking of what firefly experts believe.”

A contrast in firefly health is evident in the eastern United States. There, Photinus pyralis — also known as the big dipper firefly, for the dipping J-shape path the beetle makes as it flies — remains a common sight at dusk. “It’s a very weedy species. It’s a habitat generalist,” Lewis said. These fireflies swoop over rural meadows and the streetside gardens of the District. “We’re lucky that we have some fireflies that are probably going to be just fine.”

Due east of the nation’s capital, however, the situation is dire for the Bethany Beach firefly. That insect, which produces bright green double-flashes, lives only in Delaware’s coastal freshwater wetlands. Residential development has imperiled the species, and in May the Center for Biological Diversity and the Xerces Society for Invertebrate Conservation petitioned the Interior Department to add the firefly to the Endangered Species List.

Artificial light at night can confuse the fireflies and glowworms that use bioluminescence for mating rituals. In the United Kingdom, female glowworms climb up to perch at the tips of vegetation and glow to attract males. “A number of different studies have shown that artificial light in a glowworm habitat actually prevents the males from finding the females,” Lewis said.

Background illumination can also mess up the animals’ sense of timing. “I’ve seen fireflies in New York City that begin courting at like 4 in the afternoon in the summertime, which is not the right time,” Lewis said.

In countries such as Japan, Malaysia and the United States — particularly where there are synchronous firefly displays, like the Smoky Mountains — firefly tourism attracts about 200,000 visitors per year, Lewis estimated. Well-meaning tourists may not realize they are endangering the animals they wish to appreciate. “If you have a lot of people who are tromping through the firefly’s habitat, they’re stepping on larvae” or flightless females, she said.

Some places have taken precautions against trampling feet and have developed firefly sanctuaries with elevated footpaths. A recently enlarged firefly preserve in New Canaan, Conn., is the first of its kind in North America, Cratsley said, at least as far as he was aware.

“The land trust was immediately adjacent to a large mansion — a beautiful home,” he said, of his visit in summer 2019. “But you could go from being surrounded by fireflies to a complete dead zone, of nothing, in that manicured lawn.”

The firefly experts encouraged people to join monitoring groups such as Firefly Watch, a citizen-science project run by Mass Audubon that has partnered with Cratsley, Lewis and other researchers.

“If people are willing to spend five or 10 minutes each week out in their backyard figuring out what kind of fireflies they have and then counting their flashes,” Lewis said, “we think we could begin to gather the kind of long-term data that we need to figure out what species are in trouble.”

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