Moth populations

I’ve been reading a lot about decreases in insect populations, which seems like a fairly vague statement. There are a huge number of insect species. I know people ‘trap’ moths, and have done so for a number of years. I was wondering if those folks have a sense that they are getting less at their light traps. Obviously this would be a subjective assessment, but the trajectory might lead to something (if there is one), especially if the person uses the same site for a long time.
I have no idea how to quantify such data, but I would be interested to know if people have noticed any change.

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Ohio has had concerted survey efforts via standardized transects thanks to the many volunteers at the Ohio Lepidopterists. Though they focused only on butterflies (aka the popular moths), their findings weren’t inspiring. See: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0216270

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https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185809 this is a study from Munich, Germany, done accross many years. Those findings are concerning.

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Hello @boguo, welcome to the forum!

They certainly are! Thanks for the link.

Welcome to the forum, and thank you for the link. I’m not a hard science person, but the variability in capture years and positions raises some questions for me. And unless I’m missing something (probably) the species caught are not listed.

To my mind, this seems like a thorough study. Very concerning.

Of course this is “casual” evidence (Gulf Coast United States), but some of the local enthusiasts on moths/caterpillars in general have remarked on the lower numbers of them to be found this year over the last 3-5 years.

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Insects often have very fluctuated numbers, this year there’re 100 and second there’s 2, but next one 100 again. Also it depepnds on ecotope changes, e.g.Parnassius apollo population in Moskow region completely dissappeared mostly just because of vanishing its host plant species Sedum from places where it was found before.
That’s also well illustrated by the graphic from that German study.

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Thank you all for a warm welcome!

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of course, @boguo, we hope to see more of you on the forum :)

And another welcome to @michael64!

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Thanks. That’s kind of what I was looking for - more anecdotal evidence from the people doing the collecting. They would be the ones best able to give an overall sense of what is possibly going on.

And Welcome!!

Butterfly monitoring in the UK:
https://www.ukbms.org/docs/reports/2018/UKBMS%20Butterfly%20Annual%20Report%202018%20Low%20res.pdf

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Thank you for that. On first reading, very interesting!

I feel the ‘insect apocalypse’ narrative has been blown out of proportion.

Yes, biodiversity is inevitably declining as a result of human impacts. But that German study linked above, plus another one from Puerto Rico, and a review published last year by authors from an Australian institution, have been used to justify conclusions that step far outside the limits of their studies.

First we’re talking biomass, i.e. tipping out traps and weighing the bugs inside them. We’ll never know which species are going up or down from that data, or if the species themselves are invasive/native. Most of the sites in the German study were only sampled once, and the Puerto Rico data came from sites sampled multiple times but with decades between sampling events. This means we have no idea what the trend is. Were these sites sampled in trough or peak years?

The review published last year was host to a litany of basic errors in methods and interpretations. The lead author made very strong comments to the media which were not supported from the results of his work at all. Even the title (Worldwide decline of the entomofauna: A review of its drivers) is misleading. Practically all of the studies included were from North America or Europe, places which don’t contain the majority of insect diversity or abundance. Frustratingly, this review has become the “must-cite” source for introducing the topic of insect declines now, and will probably be uncritically parroted in most new papers.

Anyway, to answer your question more specifically, a rigorous, standardised, long-term moth sampling study from the UK (in which sampling occurred every year for 50 years) showed moth biomass to actually increase 2 fold between the first (1967–1976) and last decades (2008–2017) of monitoring. Study here: https://www.nature.com/articles/s41559-019-1028-6

The key thing to note from this study is the extreme variation in abundance between years.

Again, this is biomass so we can’t draw reliable conclusions about whats happening with different species, but at least this study gives a solid look at biomass. We need more proper monitoring of insects around the world if we want to uncover whats really going on. And we need higher resolution data - which species are declining, and where?

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Thank you. I do take all these studies with a grain of salt. I’ve been around science long enough to know that some things are just too difficult to quantify. Even medical research involving standardized matching and double blinding of groups produces dodgy results.
That’s partly why I was looking for anecdotal data - it’s possible to get a sense of what is going on with populations if you have enough observers. In my immediate area, American Robins have been hard to find in the past two years after 10 of abundance. Obviously, I can’t make a statement that robin populations are decreasing, but if enough observers around the city make the same observations, then a trend might be discernible.
Thank you for the link, and I’ll look at it later.

Just to be clear, I consider anecdotal evidence to be a form of Indigenous Knowledge (possibly opening up a new topic). I am not Indigenous, but know that Indigenous peoples have a long term knowledge of the lands they live on. If many people who have been hunting seals for decades say that they are more scarce now, then those observations should be considered. Not as quantitative science, but as a starting point for quantitative science (if possibe). I can tell you more about the river I go to every day than you probably want to know, but when it floods in the fall, that is a unique event, perhaps worthy of examination.

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I did a quick search on this topic and found this reference from Wikiwand: https://www.wikiwand.com/en/Decline_in_insect_populations. It reviews the studies noted thus far as well as a number of others. I agree that caution is needed in evaluating conclusions, but like you, I’m not a scientist, rather someone interested in science. Anecdotal evidence is often discounted, for sometimes obvious reasons, but if anecdotes are numerous, independent (i.e., not just repeating ‘common knowledge’) and consistent, then, it seems to me, they become a form of evidence worth paying attention to. The windshield splash test mentioned earlier is an example, as are comments from people finding fewer bugs at street lights or porch lights or realizing that we generally face fewer insects than many historical accounts describe. So, I really hope that insects are not in serious decline, but am concerned that they could be.

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From what professional enthomologists say fluctations happen all the time, some take years, but others need decades. Insects need environments, that’s what defining their future as group.

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And their environments are under threat, hence they are under threat…

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