Global warming is apparently leading to glow-worm waning in the UK. Today the Guardian has an articlei on its website under the title “How the speed of climate change is unbalancing the insect world”. This deals with a lot of issues, and inevitably it’s always bad news. Good insects are going to struggle with climate change, while nasty ones are going to thrive. It’s always the way, isn’t it? However, one section, which particularly interests me, concentrates on glow-worms in the UK. And this is what it solemnly tells us:

Further south, in the UK, glowworm numbers have collapsed by three-quarters since 2001, research has found, with the climate crisis considered the primary culprit. The larvae of the insects feed on snails that thrive in damp conditions, but a string of hot and dry summers has left the glowworms critically short of prey.

No link is offered up to support the claim that climate change is the primary culprit for the decline in UK glow-worm numbers, though a team of researchers from Sweden and Spain are mentioned (though not, curiously, researchers from the UK).

But here’s the thing. On 22nd February 2020, the Guardian ran a story under the headline “Why the lights are going out for fireflies”ii. And it told a rather different story. It started by discussing declining firefly populations in the US, then went on to say that the best data is from the UK, where citizen scientists have tracked the UK’s only firefly, the common glow-worm, Lampyris noctiluca, since the 1970s.

Then it linked to a reportiii entitled “A Global Perspective on Firefly Extinction Threats” which, as even the Guardian had to acknowledge, makes it abundantly clear that climate change is far from being the main threat to glow-worms. Indeed, as the report’s abstract states:

We conducted a survey of experts from diverse geographic regions to identify the most prominent perceived threats to firefly population and species persistence. Habitat loss, light pollution, and pesticide use were regarded as the most serious threats…

And the body of the report goes further:

More than half of the 49 respondents assigned the highest possible threat score (5) to habitat loss, whereas nearly one-third did so for light pollution, and one-fifth did so for pesticide use. However, their threat scores differed considerably across geographic regions… with additional threats such as water pollution and tourism ranked as important concerns in some regions.

Rather than quote extensively from the report, I simply note that habitat loss was noted as occurring for a variety of reasons, though other than two brief references to drought, the reasons are all man-made, but nothing to do with climate change. In Europe, reference is made to urbanization, industrialization, and agricultural intensification, plus increased use of pesticides, herbicides, and fertilizer. In England specifically, mention is made of road construction, ditch filling, timber stockpiling (and drought; I’m nothing if not fair). Italy sees blame put on agricultural intensification, while in Spain it’s down to the abandonment of small orchards and irrigated agricultural plots in which Nyctophila reichii, Lampyris iberica, and Lamprohiza paulinoi often occur. Once abandoned, these cultivated areas become more xeric and less suitable for snails, which constitute the main prey for certain fireflies, apparently. In Malaysia it’s conversion of riverbank mangroves to agriculture, aquaculture, and urbanization. Throughout south east Asia, large areas of riverbank mangroves have been cleared for oil palm plantations, shrimp farms, or flood mitigation, making these sections unsuitable for the growth and development of Pteroptyx firefly larvae and their snail prey

On and on it goes. Globally, increasing human populations along coastlines have caused extensive habitat loss and fragmentation, threatening both mangrove fireflies and other species inhabiting coastal marshes. In the western United States and Texas, several fireflies are restricted to habitats adjoining permanent water sources, including rivers, streams, lakes, ponds, springs, and irrigated fields. Groundwater pumping to meet urban and agricultural water demands has substantially reduced surface water flow and lowered groundwater tables, and this might be exacerbated by climate-change induced drought (though that’s far from being the primary issue).

Apart from habitat loss, light pollution, and pesticides, other threats were mentioned – water pollution, tourism, overharvesting, invasive species, and finally climate change. And what do the authors have to say about climate change?

…the effect of anthropogenic climate disruption on firefly populations remains unknown, the restricted ranges and specialized habitat requirements of certain fireflies suggest that they are likely to be threatened by drought and sea level rise.

In other words, it’s a potential threat, and an additional problem, but it’s well down the list, and certainly isn’t, as today’s Guardian article tells us, “the primary culprit”.

And yet, despite all that, less than two weeks after the Guardian article summarising the report that said climate change was almost immaterial, on 5th March 2020 a new articleiv appeared at the Guardian under the heading “Glowing, glowing, gone: plunge in glow-worm numbers revealed – Exclusive: study shows a 75% fall in 18 years in England, with climate a clear factor”. The story this time was broadly the same as the story today:

Glow-worm numbers have plunged by three-quarters since 2001, research in England has revealed, with the climate crisis a clear factor.

The larvae feed on damp-loving snails, and increasingly hot and dry summers mean fewer prey and a greater risk of glow-worms becoming desiccated.

So, which is it? Climate change or not climate change? Well, there’s a problem with blaming hot, dry summers, at least so far as the data provided by the Met Office makes clear. They supply a UK summer rain chartv among other things, and I can’t see the decline in summer rainfall postulated by the Guardian. In fact, since the collapse in numbers is specifically said to have occurred sine 2001, on the contrary summer rainfall trends seem to have increased since them. The same is broadly true for the England-only data.

I knew very little about glow-worms before the Guardian’s article today piqued my interest. I am now slightly better informed, though still with a lot to learn. I note that others who know more than me, don’t blame climate change for the glow-worm decline. For instance, the Woodland Trustvi says this:

Whilst glow-worms remain fairly common, there is some concern about possible declines, and they have vanished from some sites. Possible threats include changes in land-use and habitat, use of pesticides, light pollution, and possibly parasites.

There are usually two sides (at least) to every story. Climate change may be playing a part in the decline of glow-worm numbers in the UK and firefly numbers globally. However, that part seems to be a lot less than the Guardian would have us believe.









  1. Disclosure: I copped out with the picture. It’s a firefly, not a UK glow-worm.


  2. “… a string of hot and dry summers has left the glowworms critically short of prey.”

    I’m puzzled.

    The Indie, Wednesday 19 June 2013:
    “Stand by for another DECADE of wet summers, say Met Office meteorologists”

    The UK Centre for Ecology & Hydrology – undated but the first entry from Wayback Machine is 2016
    “Summers getting wetter and soils are less acidic – UK Environmental Change Network findings

    The Met Office – undated (!!) but the first entry from Wayback Machine is 2020

    “Effects of climate change” see Table “Changes to the UK climate and weather events”


  3. Oh look, even the Met Office is at it, mixing weather and climate events. What actually is a “climate event”, one that persists for thirty years? Don’t think even the most loony are considering these, especially for the U.K. where different air streams compete and bring us so varied weather, day by day and in different years.


  4. Alan, thanks for pointing that out. The Met Office, of course, is more concerned about climate change than weather forecasting these days, but for such an organisation to confuse climate and weather is indeed worrying and disappointing. But not surprising.


  5. The last five years have been the hottest on record for glow-worms, which is yet another devastating indicator of climate disaster. While 2021 was not quite as hot as 2020, it ranks second in the list of all-time hottest years, and confirms the warming trend. Only one year in the top ten is from before 2010.

    A scientist said: “These trends are extremely worrying, and reinforce the need for urgent action by the government to curb carbon emissions by investing in whirligigs.”

    Records of Lampyris noctiluca on the NBN Atlas, ranked by number of records in a year:

    Link to map/data for 2020 below might not work. In case you’re wondering, the beasts are concentrated around the south and east of the UK, i.e. in the areas that are hot and dry in summer. They also prefer calcareous soils.

    Liked by 1 person

  6. Jit, thanks for commenting – I hoped you would, given that you were always bound to know far more than me about this sort of thing. The Guardian’s claims do look rather untenable (what’s new?).


  7. “Climate change and farming driving insect decline”

    “Insect numbers have plunged by half in some parts of the world due to climate change and intensive agriculture, a study has found.

    The combined pressures of global heating and farming are driving a “substantial decline” of insects across the globe, according to UK researchers.

    They say we must acknowledge the threats we pose to insects, before some species are lost forever.

    But preserving habitat for nature could help ensure vital insects thrive.

    Lead researcher, Dr Charlie Outhwaite of UCL, said losing insect populations could be harmful not only to the natural environment, but to “human health and food security, particularly with losses of pollinators”.

    “Our findings highlight the urgency of actions to preserve natural habitats, slow the expansion of high-intensity agriculture, and cut emissions to mitigate climate change,” she added…

    …In areas with high-intensity agriculture and substantial warming, insect numbers have plunged by 49% and the number of different species by 27%, compared with relatively untouched places that have so far avoided the most severe impacts of climate change, according to the research, published in Nature….”.

    I would like to know so much more about these findings. By lumping together untouched by humans and by climate change on the one hand and high-intensity agriculture/warming locations on the other, the picture is obscured. From that statement, we have no idea at al how much of the decline is driven by land use and how much (if any) by climate change.

    Unfortunately the BBC didn’t provide a link to the study to enable interested readers to look into it further. I wonder why not?


  8. Mark, I might look into this. Certainly for the UK it is 90 degrees wrong: intensive arable has produced an almost 100% decline in insect abundance and diversity, but the hottest places are the most diverse (compare brownfield sites in the Thames Gateway to, say, pristine sites in the Cairngorms).

    A cynic would say that they went looking for trouble, and they found it.

    Liked by 1 person

  9. “‘An ecological miracle’: Taiwan’s fireflies are flirting in the dark again
    Excited visitors take part in nightly tours after conservationists reintroduce species”

    “Taiwan is home to about 65 of the world’s estimated 2,200 species of firefly. In terms of density, it ranks behind only Jamaica and Costa Rica, with two species for every 1,000 square kilometre. But global populations are under threat from habitat destruction and pesticides, as well as water, air and light pollution.”

    Indeed. Though, of course, we also get this:

    “Climate change is also a concern, says Dr Wu Chia-Hsiung, an expert on fireflies from the National Taiwan University.”


  10. “Glowworms bred in captivity to be released in southern England
    More than 500 larvae already set free in Hampshire and Cornwall as part of project to revive declining species”

    A mildly interesting story, but for me this is the very interesting takeaway:

    “…But glowworms have had their lights dimmed by a cult of tidiness in the countryside, the loss of wild meadows and light pollution….”.


    “…They were everywhere, twinkling their way through eternity. With overgrazing and hedge-flaying and tidying up the countryside we’ve turned their lights out.”…”


    “…A 2021 scientific study found that the abundance of glowworms declined markedly with increased proximity to artificial lighting. Not only do night lights bamboozle the female-seeking males, but they can cause the females to dim their lights. The flightless female will also not move away from light pollution….”.

    Not a mention of climate change anywhere in the article.


  11. “Butterflies in major decline in Northern Ireland, says report”

    It may well be due to habitat loss, but the BBC report is singularly uninformative. Nevertheless, it manages to shoehorn references to climate change in to it, despite supplying no evidence whatsoever that the story is actually linked to climate change. It even ends with a heading “Climate change”, followed by five paragraphs which don’t mention climate change once!


  12. Amazingly, the Guardian article is arguably more balanced:

    “UK butterflies vanish from nearly half of the places they once flew – study
    Butterfly Conservation report reveals 42% decline in distribution of 58 native species since 1976”

    It even contains this amazing paragraph:

    There have been some successes, linked to climate change or concerted conservation action. Beneficiaries of global heating, which has facilitated their expansion further north through Britain, include strong-flying species such as the purple emperor, whose distribution is up by 58% and abundance by 110%, and the comma , whose distribution is up by 94% and abundance by 203%.

    The report itself can be found here:

    Click to access State%20of%20UK%20Butterflies%202022%20Report.pdf

    It contains almost three times as many references to habitat loss as it does to climate change, and the climate change references are much more equivocal, with some of them even saying that climate change has been beneficial to some species (as per the Guardian quote above).


  13. “Surrey: Hedge-planting scheme modified after glow-worm discovery”

    A council-run hedge-planting scheme has been modified after the discovery of a colony of glow-worms in Surrey.

    The species was found living within a small hedgerow in Norbury Park, near Dorking.

    Surrey County Council modified a proposed planting initiative on site after the find.

    It said the changes were to benefit the species and enhance the declining habitat to provide a better chance of survival.

    More than 3,800 native broadleaf trees were planted to provide a habitat corridor to allow the glow-worms to travel more freely in search of their food and encourage other wildlife species.

    Glow-worms, a declining species with no legal protection, thrive in a mosaic of habitats, including a combination of earth, tall grasses and scrub.

    To support this, gaps were left within the newly-planted hedgerows to encourage the growth of grasses and wildflowers, helping to attract snails which are the preferred food source of glow-worms.

    Marisa Heath, cabinet member for environment, said the council wanted to do all it could to grow the colony of the “rare” invertebrate.

    “By adapting our hedge-planting plans we are not only supporting Surrey County Council’s target to facilitate the planting of 1.2 million new trees by 2030, but also playing our part in helping to reverse the national decline of this rare species right here in Surrey,” she said….

    Habitat loss is the problem, not climate change. Interestingly, and encouragingly, the BBC article doesn’t mention climate change at all.


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