In eastern Japan, the Genji firefly blinks roughly once every 4 seconds. In the west, the same species blinks roughly once every 2. A new genetic survey found that in Tokyo, which sits in the east, more than 70% of the firefly populations studied included insects whose maternal lineage traces back to western Honshu, Shikoku or Kyushu. Natural dispersal is unlikely to explain that. The simplest explanation, the researchers conclude, is people moving fireflies from place to place, often while trying to bring them back.

Same Firefly, Two Rhythms

The Genji firefly (Nipponoluciola cruciata) is known only from Japan. It lives on Honshu, Shikoku, Kyushu and nearby islands, and its larvae grow up underwater.

It also has a regional accent. Males in eastern Honshu flash on average about every 4 seconds. Males in western Honshu, Shikoku and Kyushu flash about every 2 seconds. The dividing line roughly follows the Fossa Magna, a great rift zone that cuts across the middle of Honshu. Where the two regions meet, researchers have also found an intermediate type.

The rhythm matters for finding a mate. In a study published in 2005, researchers exposed males from four wild populations to artificial lights blinking at different intervals. Males from Aomori and Sendai, both slow-flashing eastern populations, approached 4- and 5-second flashes significantly more than 2- and 3-second ones. Males from Otsu, a fast-flashing western population, did the opposite, while those from Inuyama, an intermediate-type population, showed no clear preference. The authors concluded that geographic differences in flash pattern can create some degree of reproductive separation between populations.

Earlier genetic studies had already sorted the species into three major lineages, one in East Honshu, one spanning West Honshu and Shikoku, and one in Kyushu.

164 Populations, and Tokyo Stands Apart

The new study, by a team from Meisei University, International Christian University, Kagoshima University and Saga University, was published in Conservation Genetics in August 2026. The corresponding author is Hirobumi Suzuki, a part-time lecturer at Meisei University.

The team read a stretch of mitochondrial DNA called COII in 1,109 fireflies from 164 populations across Japan, with especially dense sampling around Tokyo. Mitochondrial DNA is inherited through the mother, so it works as a marker of the maternal line. The sequences fell into 154 distinct types, known as haplotypes.

Nationwide, the three lineages stayed clearly separate, which suggests that fireflies rarely move between regions on their own.

Tokyo was different. More than 70% of the Tokyo populations analyzed contained haplotypes from lineages outside the region. West Honshu–Shikoku types turned up frequently, and Kyushu types appeared at some sites. The team found a few unusual populations elsewhere, but no other region surveyed showed mixing on a comparable scale. Haplotypes typical of western Japan had turned up in Tokyo in earlier studies, raising suspicion that transplanted fireflies were involved, but how far the mixing extended had not been measured against nationwide data.

On September 20, 2026, The Asahi Shimbun, one of Japan's largest newspapers, ran the story under a headline that translates as "Over 70% of Genji fireflies are 'outsiders.'" The article itself correctly described the figure as a share of surveyed sites, but the headline alone suggests that seven in ten Tokyo fireflies came from somewhere else. The figure counts populations, meaning places where fireflies live, not individual insects.

Released to Bring Them Back

The paper argues that occasional natural dispersal cannot easily explain how western lineages reached Tokyo. Tokyo lies in eastern Honshu, and other regions have kept their genetic structure intact. On top of that, the genetic connection between Tokyo and western Honshu–Shikoku is stronger than the one between regions that separated naturally. Putting that together with the known history of introductions, the authors conclude that repeated human-mediated translocation within Japan is the simplest explanation. They also discuss the possibility that some of the movement was unintentional.

Fireflies have been part of the Japanese summer for centuries. A 2022 study of firefly imagery, concentrating on Japanese prints made between 1600 and 1926, identified more than 90 artists who portrayed them, most often in evening scenes by the water with people catching them or watching them in cages. Across Japan, firefly revival campaigns have been run as environmental education and as a way to promote towns, with the insect treated as a symbol of restored nature, as a 2025 review of Japanese research on domestic invasive species points out.

In Tokyo, according to the research team, fireflies have been introduced alongside environmental restoration projects and events, and the geographic origin of the supply was sometimes unclear. The paper notes that citizens have done a great deal of conservation work on this species, including topping up existing populations and reintroducing fireflies where they had vanished.

Some did it carefully. In 2011, Tokyo's metropolitan Tama Zoological Park collected adult Genji fireflies from nearby habitats, took their eggs and raised the larvae. In March 2012, the zoo released grown larvae into a spring-fed channel on its grounds as a trial. It explained at the time that Genji fireflies differ by region in form, habits, flashing behavior and life cycle, so bringing them in from far away could erase local traits. That year, keepers saw fireflies flying and laying eggs in the channel.

What the Study Does Not Say

The mitochondrial data show which maternal lineages are present, not whether eastern and western fireflies have interbred. The team states that its results do not directly prove hybridization. A 2020 study by some of the same researchers, which looked at nuclear DNA, did find exceptional signs of mixing between the groups near the areas where they meet or where fireflies had been artificially transplanted.

Nor did the study measure flash intervals in Tokyo, so whether the rhythm there has changed is unknown. The paper raises the loss of local adaptation and outbreeding depression (weaker offspring from crosses between distant populations) as potential risks, without measuring either. The team notes that genome-level analysis is still needed.

When a Native Species Becomes an Outsider

"Invasive species" usually brings to mind animals from another country. Japan's government takes a somewhat wider view. Its action plan on invasive species, revised in March 2025 by the environment, agriculture and land ministries, says that even within a species' natural range, bringing in individuals from a different region can damage regional genetic lineages, and that careless introductions should be avoided even for species native to Japan. The Invasive Alien Species Act, by contrast, focuses on organisms brought in from overseas.

The researchers recommend not topping up populations with fireflies of unknown origin, using geographically close and genetically compatible sources when reintroduction is needed, and prioritizing the Tokyo populations that still mainly carry East Honshu lineages as sources for future restoration. For long-term monitoring, they say a low-cost DNA test called RFLP can help.

In the United States, five butterfly experts, one of them then a Smithsonian curator, wrote a statement published by the North American Butterfly Association (NABA) opposing the release of commercially raised butterflies at weddings and other events. They argued that butterflies released far from where they came from mix genetically with local populations of the same species in ways that should not happen, and warned that monarchs from California, a population that does not make the journey to Mexico, might leave offspring that cannot orient properly. Breeders, as Earth Island Journal has reported, counter that there is no evidence of harm to wild populations and that scientists overstate the risk. In both cases, the release comes from affection and the risk is hard to see.

The 2025 review, written by two researchers at Osaka University's Graduate School of Human Sciences, suggested that what defines domestic invasive species problems may be that they tend to stay out of sight. To the eye, a firefly carrying a western lineage looks like any other over a Tokyo stream. The difference shows up in its DNA.

Where you live, when people release butterflies, fish or fireflies to celebrate or to bring a river back to life, does anyone ask where those animals came from?

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