Moth populations

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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Well, the topic can be twisted as we want, but the windshield effect is there. At least for me in northern Italy. And speed limits have not changed much - and the difference when I was a kid and nowadays is tangible. I was the one who had to wipe.
And the heavy lobbying to hide all this is documented
So I’d be cautious thinking of business as usual.

Once people hear an idea repeated often enough, such as “insect populations are declining globally”, they will, at first glance, interpret almost everything as supporting evidence for that idea. Only after deeper thought will other possible explanations come to mind. But most people will stop after the first glance, because nothing obviously contradicted the big idea, and deeper thought is hard work, to be avoided if possible. (Or, deeper thought is a limited resource, to be used only on the highest priorities.)

For example, the windshield effect. There are multiple independent observations from people who have noticed that fewer insects get splattered on their windshields now than in the past. At first glance, since insect populations are declining globally, the windshield effect is either obviously supporting evidence for the global decline of insect population, or the windshield effect is obviously an effect of the decline in global insect population (an important but often-neglected distinction, if you’re trying to understand what it all means). But one possible alternate explanation is that this is just evolution in action. Cars (and windshields) were invented about 100 years ago, and became common about 70 years ago. Insect populations being subjected to this new source of mortality have been evolving to avoid it, and this explains the reduced need to clean splattered insects off your windshields. You can see evidence for this when driving at night: your headlights reflect off of moths, and you can see them make a sharp upward turn as soon as they come into the beam of the headlight, when in the past they wouldn’t, and would end up splattered. (Note: this is just an example of an alternate explanation, one I’m not convinced is true. If you think on it a bit more, you may be able to come up with another possible explanation or two, for the windshield effect and for almost any other isolated piece of anecdotal evidence.)

For moth populations, the U.K. long-term studies are the best I’ve seen. I’m much more familiar with monitoring bird populations in North America, but there are a lot of parallels. The main thing is that one or two years of data are somewhat useless on their own. There are large changes in local and regional populations from year to year. The populations plotted over time don’t look like straight lines, they look like crazy zigzags which don’t tell you anything until you have 10 or 15 years of data, preferably 30 to 50 years. This is the sort of thing that’s meant when people say that anecdotes aren’t useful data. I expect the same thing is true of moth populations.

For example, the Monarch population where I live quintupled over the past two years, but this tells me nothing about the long-term prospects for Monarchs, because I live in Canada and the eastern North American population’s long-term prospects are almost entirely dominated by the conditions in their tiny wintering areas in Mexico. The low population here two years ago was almost entirely due to a severe winter storm there, and the quintupling of the population is simply recovery from that. Any long-term population trend requires data over a much wider spatial area (Monarchs have populations in Australia and Indonesia) and over a much longer time-span (decades at the least). And the specific circumstances affecting Monarchs don’t even apply to closely related butterfly species, much less to moths as a group.

In North American birds, we’ve found about 30% of species increasing, 30% stable, and 40% declining, and the total population declining slowly. Some few species have declined incredibly rapidly, while some few others have increased enormously (e.g. goose populations in N. America went up by ~400% between 1970 and 2010). The species going up in N. America are mostly those which are recovering from the DDT-caused crash in the 1950s and 60s and from past over-hunting. The species going down are mostly the ones which depend on non-agricultural grassland and those which migrate to South America. (I don’t know what’s happening down there, but something important is.) Overall, the total biomass of birds in N. America is declining slowly, i.e. <1% per year from 1970-2018. Compared to the year-to-year fluctuations, this is all but undetectable without really massive amounts of data. On the other hand, like all percentage-based changes it compounds quickly. The media inevitably quotes the most heart-attack-inducing number they can find, which in this case is “3 billion birds have disappeared”, while downplaying (or rather, failing to emphasize with equal force) that it happened over a 50 year period, and failing to mention the total population size. I don’t mean that birds aren’t declining, or that it isn’t important, but I’m trying to maintain a reasonably correct perspective, along with some healthy uncertainty about the true situation. Otherwise I might start thinking all birds will be extinct by the end of the century, which is nonsense.

For moths, the long-term U.K. moth records seem to show something similar: some species going up, others down, still more relatively stable, and an overall steady decline in biomass as wild lands get converted to agriculture to supply the increasing human population. Since this is not the kind of thing which grabs your attention, this is not what you’ll hear about most often in the news or in gossip. Real answers are always along the lines of “Well, it’s more complicated than that (and here’s how:)”, except when they aren’t (sigh).

Incidentally, I received a moth trap from Britain for Christmas. The bird observatories there have long been catching and recording moths in addition to their other activities, and I’m planning to experiment with doing the same at Long Point Bird Observatory. If that works out, then we might try getting the other bird observatories in Canada to do the same thing, so that in 20 years or so we have enough data to do a report like the one from the U.K. for Canadian moths.


Absolutely, I agree anecdotal and indigenous knowledge can provide excellent starting points for the collection of quantitative data to provide a scientific view.


People can and will argue in whatever way serves their worldview, but I’m far more interested in quantitative evidence collected over a long period of time. The situation is far more complex than “I personally don’t see as many bugs these days as I used to which means insects are declining globally.” My caution is exactly the reason why I’m suspicious of arguments put across without evidence to accompany them.

As a side note, whether or not chemical companies are hiding evidence that their products are causing bee declines is very different to whether or not insects as a group are declining globally. Putting the probable corporate wrongdoing aside, honey bees are not native to most parts of the globe, and in these areas they are essentially cows with wings, they’re managed livestock. They can easily outcompete native bees and other native insects. So their very presence in some areas may be contributing to the decline of native insects. They’re also not the only insects that pollinate crops or wild plants, and in fact, they’re probably not the most important group of pollinators for wild biodiversity. People tend to assume bee populations are a mirror to how nature is doing in general.

Theres so much nuance and complexity tied up in this topic and current reporting is simply not doing a good job of unravelling it and explaining it (and this is certainly not only the fault of the media when you have study authors and press releases embellishing their findings).


Well, as an engineer I would rule out thinking that an insect (speed… 5-7kph, if we exclude dragonflies?) has evolved in 30 years to avoid an object travelling at 110 kph.
I’d be more ready to believe that car have a significantly improved Cx, but that happens with vehicles like buses as well. In any case the windshield effect is the most readily available hint - that has been a starting point in the Krefeld investigations I read about here , where long terms analysis is also mentioned, if you’ve some time to read it through I think is a good arcticle.

well, I’d avoid extremes and I’m no expert to talk about birds I just base on my irrelevant experience that in my area in Italy I cannot see the sparrows, partridges, and the like (I know it’s not statistics but again clue) I used to see when I was a child.
I’d be interested reading some studies focusing on insectivore birds. That would be interesting, but I have not found any.

By the way, Jeremy, from the link you have added I extrapolate that insectivore birds are the worst faring with -59%, what’s your view on that? I don’t think it’s helping to think business as usual in insect world.

Obviously as it always happen in the insect world i’m ready to think there can be exceptionn to a general trend (which I find quite reasonable, as functional extinction generate new niches where other insect can fit into), but I am trying to get a reasonable opinion which applies to the area I live in, taking in mind other points like the concept of functional extinction mentioned in the article of nytimes linked above
After all there must be a reason behind Council Directive 2009/147/EC and the Habitats Directive in EU
I am quite wary of the shifting baseline syndrome descrived in the article above
and of the brain tendency to deny problems which imply cost, future effects and uncertainty (you can listen to this podcast entry to understand what I mean)

of course. Totally agree. but I think in the end (I’m no scholar) but you have to build an opinion, and between the three options (situation improving/stable/worsening) I personally feel that worsening, all information collected so far, is the most reasonable opinion, based on the global studies available

Thanks Jeremy, your comment was a very good read.

see my reply to Jeremy.
I know very well that bees are just the cows of the insect world, and alloctone as a consequence, (btw I am for wild pollinators so figure it) but they are also linked to money interests, which is pushing some other people to analyze the impact of other industry practices on them and take action to protect their own money- that’s why I mentioned them

Honestly, I think no one would lift a finger to analyze the impact or fight neonicotinoid use just to protect bugs who do not directly generate revenue. That’s the way it is, especially outside Europe (and some small areas of India, I reckon) where unrefrained heavy lobbying by big industry is part of the way things work.
That’s why bees are the only reference for me to understand in some detail what’s up in the world of insects.

have you read the Krefeld study mentioned above?


Re the UK butterfly monitoring: It helps to know 1976, the year it began, was an exceptionally hot summer, a good year for adult butterflies, so the starting point for the index was higher numbers than in a more typical year.

Re splattering: I should think cars becoming more aerodynamic has some effect. Is that what Edolis means by improved Cx?


yes sorry for the Cx lingo. I obviously mentioned the windshield effect as it is the one at everybody’s reach - that was the trigger for the Krefeld study too (if I remember properly, just for fun, there was also a “no longer flies in your mouth as you go around by bicycle in Denmark”), which obviously has a different approach.
Hope that now we do not start talking of it as if it were the only proof of insect decline :grinning:,
The links to the article of the New York Times Magazine and the link to the published study above are more ponderated info for anyone interested.


I have looked at that study, and reiterate my concerns about it. There is only one year that (2014) that has a decent number of traps. There appear to be no data as to how long each trapping site has been operating. There is no species list given. The trapping areas are quite far apart.
I’m not a trained scientist, but these sorts of data raise do not seem to be conclusive.
Again, if I am missing something, please let me know.

I see, but mine is probably a matter of cautionary approach. The study might not have been perfect but are there any (imperfect) studies pointing with the same coverage in the opposite direction?
It’s a bit like the difference in approach on food safety between US and EU. You can deem it safe until not proven otherwise or the opposite. I do not think there is a way of coming out of this, everybody just have to gather elements and take their own call. We are talking of studies spanning decades, with low or no funding, I do not expect perfection - but that’s just my view :slightly_smiling_face:


Fair enough. I also prefer to take a cautionary approach, partly because I know how difficult it is to model such a difficult topic. In 1979, I heard
David Suzuki at the University of Manitoba, talking about climate change. He essentially said that we should change what we are doing - if it turns out to be false, what have we actually lost? I feel the same way about biodiversity.


Yes I mentioned it in my first post in this topic:

I then elaborated:

The critical flaw: How can any conclusions be drawn on population trends from a study in which most sites were only sampled once? I genuinely don’t understand why people quote it to justify a belief in regional or global insect declines. Did you read it yourself? What did you think of the methods? Do they allow conclusions to be drawn on insect population changes?

Which species were they measuring? Are they native or introduced? They measured biomass so we can’t answer these questions, which in my opinion, are absolutely essential to draw the kinds of conclusions relating to insect declines that we actually care about.

If you look at the moth paper I linked you can see the extreme fluctuations in insect biomass over a long time (they sampled moths every year for 50 years and found a 2.2 fold net gain between first and last decade of trapping). The German study would have only measured one year of this and missed out on the whole picture.

Theres nothing inherently wrong with the methods of the German paper. But the framing is totally off. At best it shows a snapshot of insect biomass in the areas sampled, and this would be useful as a baseline for further biomass studies, but it certainly doesn’t provide anything more than that. I think we should be aiming for species-level data rather than biomass anyway, because biomass doesn’t tell us what we really want to know. I think the authors and the media have extrapolated way too much from the (rather limited) data.

I agree in principle, but we need to remember these changes will effect the people of different countries in very different ways. Developed countries have a high living standard and more resilient, diversified economies today partly because of the way they were allowed to burn fossil fuels and industrialise from the late 1700s and early 1800s. If reducing the use of carbon-based fuels is going to be an important part of mitigating climate change and addressing concerns around biodiversity, then what right do developed countries have to tell developing ones that they can’t achieve growth and prosperity in the same way that worked for them?


I really do not know how a mass capture and ID study at individual level spanning decades could be feasible. I have no clue if that’s possible, as I am no enthomologist, but having an idea how difficult can be identifying a single individual and all the other issues I think it’s a setup which is not likely to work.
And in any case I don’t think we can walk back to sampling before we were born.
Again, it’s a matter of cautionary approach and making the best of the resources available.
I haven’t got of course the sensitivity of a scientist, and maybe moth population is booming, but I was talking of insects in general.
I feel all that detail, when you have to get an idea on the overall trends on insect population, is like knowing the composition of water when you are struggling on your boat in a tempest (italian saying, I do not mean to offend anyone)

Well, I would expect the developed countries to take the lead given their technological prowess. Not to suggest “You go first!”. I feel this argument is similar to reiterating a bad behaviour because someone else is doing that too.
If you think on a “technological life stage” point of view it looks absolutely obvious that the first world should be the first to cut the tar too.
By the way, China I think is moving way faster than western countries are. Watching India, but they seem not to be moving in the opposite direction

You don’t need to catch and ID everything at a site, but you definitely need to repeat whatever you’ve done over multiple consecutive years. This is basic sampling design. On the other hand, if you want to make claims about regional or global insect declines you need evidence which actually shows that. I’m guessing you didn’t read the German study you linked. I think our conversation has run its course, thank you for your time.

The above is partly why I thought that gathering large numbers of anecdotal observations might be a good place to start. A lot of folks have been trapping in the same place for a while, and may have a sense of their catch cycles, and give an indication whether or not they think there is a problem.
One way to measure individual moth species populations in an area might be to use pheromone traps. Ideally these should attract only one species of moth, and could be set up in an area to sample over several years. A model for this might be the paper [G.L. Ayre, Turnock W. J. and Struble, D.L. (1982) Can, Ent. vol. 114, no. 11, 993 - 1001]. Although they were testing pheromone efficacy, they found that over 3 years E. messoria populations over an area of 64 km2 (traps every 1.6 km) “is not suitable for monitoring population levels.” E. ochrogaster populations were more stable and would need four traps in the same area to come within 20% of the mean population level. Pheromone trapping has improved since those early days, so might be a good way to indicate population fluctuations.

In my opinion, the existence of some sort of insect decline is obvious, at least in industrialized countries with intensive agriculture and dense population, like Germany. The question is how bad it is. I guess habitat loss and fragmentation play very important roles in this.
Also I’ve recently seen a lecture by Herbert Nickel, a cicada expert, according to him frequent mowing instead of the traditional extensive cattle ranching might play an important role in insect loss.


Does anyone study the ongoing effects of DDT and similar chemicals on insects in those places where it is still used? It seems their use would probably affect insect populations in those areas. After Silent Spring was published, more attention was focused on these issues, as well as a number of United States environmental laws were passed. Thus, there may have been a lot more people suddenly working on studying insects.

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The article I linked above is listing studies on neonics in US such as a study on agricultural land which concludes that the American landscape has become 48 times more toxic to insects since the 1990s, a shift largely fueled by the rising application of neonics.
Long but good read if you’ve got time/patience.

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