Theological “wisdom” makes me roll my eyes » « Wicked: For Good

Are insect populations declining or not?

I am so confused…but then science is often confusing. I was reading this article in Science magazine that went against my impressions and biases.

For years, scientists have been warning of a precipitous drop in insect numbers worldwide, driven largely by deforestation, pesticide use, and other human activities. But the first study to survey insect populations on a continental scale—based on radar data typically used to study weather patterns—finds no evidence of widespread decline, at least over a recent 10-year period. Instead, the research—published this week in Global Change Biology—suggests bug numbers tend to be sensitive to the severity of winter weather, with warmer winters posing a problem.

What, no decline? But I’ve seen a dramatic decline here in western Minnesota! Could I be wrong? Maybe. My perspective is narrow and local, and I’ve been looking at a small number of species, just spiders, that I’ve assumed would be a good proxy for overall insect number. I could be totally off, misled by a local variation that fit what I expected to see.

So I read the source paper. First surprise: the title doesn’t say there is no evidence of decline, but rather “Systematic Continental Scale Monitoring by Weather Surveillance Radar Shows Fewer Insects Above Warming Landscapes in the United States“. So there is evidence of decline in areas that show signs of warming. The abstract complicates matters further.

Anthropogenic change is predicted to result in widespread declines in insect abundance, but assessing long-term trends is challenging due to the scarcity of systematically collected time series measurements across large spatial scales. We develop a novel continental-scale dataset using a nationwide network of radars in the United States to generate a 10-year time series of daily aerial insect density and assess temporal trends. We do not find evidence of a continental-scale net decline in insect density over the 10-year period included in this study; instead we find a mosaic of increasing and declining trends at the landscape scale. This spatial variation in density trends is associated with climatic drivers, where areas with warmer winters experience greater declines in insect density and areas with cooling winter trends see increases in density. Winter warming has a stronger negative effect on density at higher latitudes. After assessing temporal trends, we also use the 10-year dataset and atmospheric variables to model insect aerial abundance, finding that on a typical summer day approximately a hundred trillion (1014) flying insects are present in the airspace, representing millions of tons of aerial biomass. Our results provide the first continental-scale quantification of insect density and its response to anthropogenic warming and demonstrate the utility of weather surveillance radar to provide large-scale monitoring of insect abundance.

Right away, I have reservations. If my observations are insufficient because I’m looking at too few species in one locale, this study is using one technique with low resolution on a continent wide scale and one could argue that it could be equally insufficient and misleading. It is data, though, and should be part of any analysis of the problem. Let’s not pretend that their sampling method doesn’t incorporate its own systematic biases. It’s only going to detect flying insects that exhibit swarming behavior, and they’re only looking at daytime numbers. It’s a correlational study that associates declines with only temperatures, but I’d suggest that those other factors (deforestation, pesticide use, and other human activities) are so ubiquitous and difficult to measure discretely that they’d disappear in the analysis.

Also, their own data does show evidence of a decline…in latitudes above 40°.

Temporal pattern of change in insect density as a function of change in winter temperature. (a) 10-year trend in day-flying insect density as a function of the change in local mean winter temperature, colored by site latitude. (b) Temporal trend as a function of winter temperature at latitudes ≤ 40°. (c) Temporal trend as a function of winter temperature at latitudes > 40°. Fitted lines are derived from a least-squares linear regression on percentage change in insect density. Linear model with change in mean winter temperature, interaction with latitude, and longitude explains 18% of variation in insect declines.

They also see some interesting variations, like the effect of land development on the sensitivity of populations to change.

Temporal pattern of change in insect density as a function of developed land cover. (a) 10-year trend in day-flying insect density as a function of the fraction developed land cover in the landscape, colored by the change in mean winter temperature. Line is given by LM. (b) Change in mean winter temperature as a function of the fraction developed land cover. Line is given by LM, correlation coefficient = 0.37 p <  0.0001. (c) Change in mean winter temperature as a function of the fraction grassland in the landscape. Line is given by LM, correlation coefficient = −0.54, p < 0.0001.

Insect populations are actually increasing over developed areas? I’d like to know the baselines on that — this is a study over a short timescale of ten years, and who knows, minor fluctuations over areas where the population has already been decimated by development might appear as a larger percentage change. I also wonder if we might be seeing the effect of adaptation or invasive species on those areas.

I’d also be concerned that native grasslands are hurting.

They do argue that anthropogenic stressors are having a serious effect.

Although we do not observe continental scale declines, the spatial patterns of abundance trends identified in this study can pinpoint potential stressors or drivers of insect declines. Declines in aerial insect density were stronger in regions that experienced increasing winter temperatures. During overwintering, warming can decrease fitness by releasing organisms from cold-induced dormancy, thereby increasing metabolic rates, and depleting energy reserves. Winter warming may also result in increased mortality due to phenological mismatches with resources, and may extend the activity period for natural enemies and reduce pathogen die-off during the winter season. Negative effects of winter warming on insect abundance in temperate regions have been shown in local surveys of beetles, butterflies, and arthropods generally, indicating that winter is a particularly sensitive season for temperate ectotherms.

Sensitivity to winter warming varies across populations and is likely more common in cooler climates where thermal seasonality is strong. Our results show a negative effect of winter warming at high latitudes, with no effect at latitudes below 40°. This latitudinal interaction between winter warming and aerial insect density aligns with theory suggesting that climate warming will have the strongest effect on cool-adapted arthropods. For example, metabolic costs are greater at high latitudes, affecting organisms’ cold tolerance and resulting in greater risks of energy depletion if winters become warmer under global change. Experimental warming has shown that high elevation gall wasp species experience greater decreases in survival and fecundity than those from lower latitudes. These stronger responses from high latitude insects to winter warming are particularly concerning because the magnitude of warming under climate change also increases with latitude.

I definitely live in an area with harsh winters, which would explain how I have a strong impression of declines on the basis of local observations. I don’t understand, though, how the work in this paper can be used to minimize the changes in insect populations. I’m also a little concerned that it’s being used to endorse a hands-off analysis of relatively coarse radar data over expecting entomologists to get their hands dirty and get up close with the organisms.

Theological “wisdom” makes me roll my eyes » « Wicked: For Good

Comments

  1. drsteve says

    My spider senses are tingling regarding each figures’ panel a. It looks like outiers in the negative and positiive directions respectively could be having an outsized efffect on the regression fit.

  2. imback says

    Be careful. The powers that be will suggest that to keep the insect population stable we only need to reduce the arachnid population that preys on them.

  3. Robbo says

    very anecdotally, driving around minnesota in the 80’s during summertime, insects would splat all over the windshield and the radiator grill was often a graveyard of insect husks.

    today that doesn’t happen.

    so it does seem plausible that insect populations are dropping.

    (or perhaps cars are much more aerodynamic, so insects are getting deflected rather than splatted.)

    (mosquito populations don’t seem too affected tho…)

  4. raven says

    I would be concerned that by using weather radar, they are only looking at a small subset of species. Most insects don’t fly in multi millions insect swarms.

    Radar is mostly looking at:

    Google Search:
    Specific examples of insects detected by radar

    Spotted lanternfly: In September 2025, huge swarms of these invasive insects were picked up by radar in the Mid-Atlantic states (including Washington, D.C., Maryland, and Virginia), appearing as “rain clouds” on weather maps, despite clear skies.

    Mayflies: Massive hatches of mayflies, which gather to mate in large swarms, have been detected by radar in areas like Wisconsin.

    Grasshoppers: Large swarms of grasshoppers, such as a massive swarm that appeared over Las Vegas in 2019, have been detected by radar.

    Butterflies: In 2017, a 70-mile-wide swarm of butterflies was located on radar in Colorado.

    Beetles: In 2015, swarms of beetles were detected by radar in Texas.

    To make this even less comprehensive, a lot of those events look like they only happened once or very sporadically. Which means you aren’t going to get multi-year data sets from them.

  5. larpar says

    Robbo @#3
    I haven’t had to clean the bugs out of my truck radiator for at least a decade.
    (Rural Iowa.)

  7. Becky Smith says

    Same here in North Georgia. The front end of my car has been virtually bug free this past summer. This is not normal. Unfortunately, I don’t have previous data comparing bug splats to miles driven, so it’s just an observation at this point.

  8. says

    I’m at 50°, on Vancouver Island, off the West coast. Our winters, checking back over 50 year averages, are about the same as usual; wet and mild, but extremely variable. But — and this is anecdotal only — I am seeing a severe reduction in insect and spider populations. And in our native berries, which rely on insect pollinators. Other local residents have mentioned this, as well.
    One good thing; I don’t think I saw more than a half-dozen mosquitoes all summer. And I do a lot of highway driving, but can’t remember the last time there was a bug splat on my windshield.
    But we’re told to watch out for ticks that are moving north.

  10. James Hammond says

    Insects are our friends and are a vital part of our ecosystems. Still, the phrase “millions of tons of aerial biomass” will haunt my day.

  11. nomdeplume says

    I have observed in my part of Australia, clear declines in numbers of insects, spiders and small birds. Not a coincidence i think.

  13. CompulsoryAccount7746, Sky Captain says

    Wikipedia – Windshield phenomenon

    The Guardian – A car ‘splatometer’ study finds huge insect die-off (2020)

    Two scientific studies of the number of insects splattered by cars […] A third study shows plummeting numbers of aquatic insects in streams.
    […]
    The survey of insects hitting car windscreens in rural Denmark used data collected every summer from 1997 to 2017 and found an 80% decline in abundance. It also found a parallel decline in the number of swallows and martins, birds that live on insects.

    The second survey, in the UK county of Kent in 2019, examined splats in a grid placed over car registration plates, known as a “splatometer”. This revealed 50% fewer impacts than in 2004. The research included vintage cars up to 70 years old to see if their less aerodynamic shape meant they killed more bugs, but it found that modern cars actually hit slightly more insects.
    […]
    Insect population collapses have been reported in Germany and Puerto Rico, and the first global scientific review [sci-hub link], published in February 2019, said widespread declines threatened to cause a “catastrophic collapse of nature’s ecosystems”.
    […]
    The Danish research […] used data from an average of 65 car journeys a year on the same stretch of road and at the same speed between 1997 and 2017. Møller took account of the time of day, temperature, wind speed and date of the journey and found an 80% decline in insect abundance over the 21-year period. Checks using insect nets and sticky traps showed the same trend.

    Møller said the causes were likely to be “a bit of everything”, but noted significant changes due to global heating. “In my 50 years, the temperature in April, May and June has increased by 1.5C [2.7F] on average in my study area,” he said. “The amount of rain has increased by 50%. We are talking about dramatic differences.”

    The stream research […] analysed weekly data from 1969 to 2010 on a stream in a German nature reserve, where the only major human impact is climate change.
    “Overall, water temperature increased by 1.88C and discharge patterns changed significantly. These changes were accompanied by an 81.6% decline in insect abundance,”
    […]
    Most scientific research to date has shown serious declines in the number of insects in the places studied. “There is no doubt about this,” said Møller. “What there is slight doubt about is the extent to which this occurs across geographical and temporal scales.” Long-term studies are rare and mostly from Europe and North America, with a few ranging from Australia to China and Brazil to South Africa, but hardly any elsewhere.

     
    Re: Robbo @3:
    Article says “modern cars actually hit slightly more insects.” and also “‘The most surprising thing was how rarely we actually found anything on the plate at all,’ […] despite the data showing that modern cars hit more”

  14. NitricAcid says

    @Susannah #9
    How precise is that 50 degrees? I can see the 50th parallel marker when I go for a walk from work.

  16. wsierichs says

    At certain times of the year on the Gulf of Mexico coast the air would be filled with what are popularly called lovebugs. On a long Interstate 10 drive you would have to clean the lovebugs off your windshield several times, and of course the front of your car would be plastered with them. I have not seen lovebugs in many years. I’m curious if the “Gulf of America” coast has the same situation. I don’t know where it is so I can’t take a drive to see if lovebugs are a problem there..

  17. BryantFinlay says

    I don’t think the windshield test is a reliable indicator of insect populations. I live in south-central PA and hardly ever have insects on my windshield, but the insect populations here are in good shape. No shortage of bees, moths, fireflies, and ladybugs. Yes, insect populations are declining worldwide, but there’s a lot of nuance. I do my part to help

  18. flange says

    I think there is enough anecdotal evidence here (including my own) of the windshield bug-splat benchmark decline to be…evidence of insect population decline.

  19. StevoR says

    ..the first study to survey insect populations on a continental scale—based on radar data typically used to study weather patterns—finds no evidence of widespread decline, at least over a recent 10-year period.

    So just a single study – suprised its the first – over a very short period of time albiet I know insect generation times are (usually) considerably shorter than ours.

    Sounds really inadequate and insufficient to draw any real, solid conclusions from and much more study needed I’d say.

    FWIW wikipedia has this page :

    https://en.wikipedia.org/wiki/Decline_in_insect_populations

    In the 2010s, reports emerged about the widespread decline in populations across multiple insect orders. The reported severity shocked many observers, even though there had been earlier findings of pollinator decline. There have also been anecdotal reports of greater insect abundance earlier in the 20th century. Many car drivers know this anecdotal evidence through the windscreen phenomenon, for example.

    …(Snip)..

    Studies concur that in areas where insects are declining, their abundance had been diminishing for decades. Yet, those trends had not been spotted earlier, as there has historically been much less interest in studying insects in comparison to mammals, birds and other vertebrates. One reason is the comparative lack of charismatic species of insects. In 2016, it was observed that while 30,000 insect species are known to inhabit Central Europe, there are practically no specialists in the region devoted to full-time monitoring.[4] This issue of insufficient research is even more acute in the developing countries. As of 2021, nearly all of the studies on regional insect population trends come from Europe and the United States, even though they account for less than 20% of insect species worldwide. In Africa, Asia and South America there are hardly any observations of insects that span several decades. Such studies would be required to draw conclusions about population trends on a large scale.

    Whilst a quick web search also finds :

    A new paper highlights over 500 interconnected factors contributing to the global decline of insect populations.
    Insects are vanishing at a concerning pace across the globe, and scientists are striving to understand why. While agricultural intensification is often cited as the primary cause, new research from Binghamton University, State University of New York, reveals a far more complex picture involving numerous, interconnected factors.

    Source : https://scitechdaily.com/the-great-insect-apocalypse-why-are-bugs-vanishing/

    Plus :

    One study in Germany found that flying insect biomass has declined by more than 75% over just a few decades. In the U.S., butterfly numbers have dropped dramatically, with some monarch butterfly populations declining by 95%. Nearly a quarter of the 4,000 native wild bee species in America are inching toward extinction. And these are just the species we’ve been able to accurately measure.

    This isn’t just a biodiversity issue. It’s a bona fide ecological crisis. Insects form the foundation of food webs. They pollinate our crops, feed birds and fish, break down organic matter and even help aerate soil. When insects suffer, the ripple effects can be seen in the populations of other species, the makeup of nearby meadows, and even the health of entire ecosystems. Without insects, food chains unravel, crops become more expensive, and the entire natural world grows quieter and less diverse.

    Source : https://environmentamerica.org/articles/should-you-be-worried-about-the-insect-apocalypse/

    Following the publication of Silent Spring I’d have expected a lot more studies and books on the insect decline or insect apocalypse” as its been dudbed . i’m disappointed and suprised if that really is the first study on that.

    We do know of many individual endangered insect species natch many on the Red List.

  21. drdrdrdrdralhazeneuler says

    Mathematically, this can’t be. They simultaneously show that the winter temperatures are increasing and that that warmer winters make insect populations decline. Therefore, by logic, there should be fewer insects. And then they claim that that isn’t true. It’s just not possible mathematically.

    Specifically, according to your country’s “Economic Research Service” (ERS), grassland (I take this as a subset of “pastures”) occupy at most about a quarter of the US: https://www.ers.usda.gov/about-ers/partnerships/strengthening-statistics-through-the-icars/land-use-and-land-cover-estimates-for-the-united-states

  22. seachange says

    The windshield test would select for mostly aerial arthropoda also. Any crawling bug would not deliberately be on a road, because a road is a manmade surface and there’s no food there. Any underground critter would also not be beneath a road, because metalled surfaces block water and the once living soil turns into regolith.

    I’ve been getting bitten by more fleas and mosquitoes than usual over the last ten years, and I am smack dab in the highly developed Los Angeles metro area. I have had to spray my socks and shoes with permethrin this year.

  23. Tethys says

    I notice the lack of both insects and birds here in urban MN, though we did experience a very warm and erratic winter last year.

    I’ve been wondering if having wildfire smoke for most of the summer in addition to some epic torrential downpours is disrupting reproduction.

  24. BryantFinlay says

    That German study is not at all representative of broader trends. There are numerous studies chronicling much lesser rates of decline and more nuance

  26. unclefrogy says

    I can’t help but think that agriculture and development play a large part in the insect populations.
    If not in total populations (biomass) but surely in distributions of species. Undisturbed ecosystems have a much larger biodiversity then an urban setting or a corn field. Just how can that be seen from weather radar data?
    As for windshield and grill data I would think that the design of vehicles would affect any results to some degree or other.
    What I get out of this is that there is something happening but just what will require more work to determine just what.

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