An annual decline of 5.2% in flying insect biomass found in nature reserves in Germany – about 75% loss in 26 years.
Several studies report what appears to be a substantial decline in insect populations. Some of the insects most affected include bees, butterflies, moths, beetles, dragonflies and damselflies. Anecdotal evidence has been offered of much greater apparent abundance of insects in the 20th century; recollections of the windscreen phenomenon are an example.
Possible causes of the decline have been identified as habitat destruction, including intensive agriculture, the use of pesticides (particularly insecticides), urbanization, and industralization; introduced species; and climate change. Not all insect orders
are affected in the same way; many groups are the subject of limited
research, and comparative figures from earlier decades are often not
available.
In 2018 the German government initiated an "Action Programme for Insect Protection", and in 2019 a group of 27 British entomologists and ecologists
wrote an open letter calling on the research establishment in the UK
"to enable intensive investigation of the real threat of ecological
disruption caused by insect declines without delay".
History
A 1902 illustration of a Rocky Mountain locust.
These insects were seen in swarms estimated at over 10 trillion members
as late as 1875. Soon after, their population rapidly declined, with
the last recorded sighting in 1902, and the species formally declared
extinct in 2014.
The fossil record concerning insects stretches back hundreds of
millions of years. It suggests there are ongoing background levels of
both new species appearing and extinctions.
Very occasionally, the record also appears to show mass extinctions of
insects, understood to be caused by natural phenomena such as volcanic
activity or meteor impact. The Permian–Triassic extinction event saw the greatest level of insect extinction, and the Cretaceous–Paleogene
the second highest. Insect diversity has recovered after mass
extinctions, as a result of periods in which new species originate with
increased frequency, although the recovery can take millions of years.
Concern about a human-caused Holocene extinction
has been growing since the late 20th century, although much of the
early concern was not focused on insects. In a report on the world's invertebrates, the Zoological Society of London suggested in 2012 that insect populations were in decline globally, affecting pollination and food supplies for other animals.
It estimated that about 20 percent of all invertebrate species were
threatened with extinction, and that species with the least mobility and
smallest ranges were most at risk.
Studies finding insect decline have been available for
decades—one study tracked a decline from 1840 to 2013—but it was the
2017 re-publication of the German nature reserves study that saw the issue receive widespread attention in the media. The press reported the decline with alarming headlines, including "Insect Apocalypse". Ecologist Dave Goulson told The Guardian
in 2017: "We appear to be making vast tracts of land inhospitable to
most forms of life, and are currently on course for ecological Armageddon." For many studies, factors such as abundance, biomass, and species richness are often found to be declining for some, but not all locations; some species are in decline while others are not. The insects studied have mostly been butterflies and moths, bees, beetles, dragonflies, damselflies and stoneflies.
Every species is affected in different ways by changes in the
environment, and it cannot be inferred that there is a consistent
decrease across different insect groups. When conditions change, some
species adapt easily to the change while others struggle to survive.
Causes and consequences
Suggested causes
The decline has been attributed to habitat destruction caused by intensive farming and urbanisation, pesticide use, introduced species, climate change, and artificial lighting. The use of increased quantities of insecticides and herbicides
on crops have affected not only non-target insect species, but also the
plants on which they feed. Climate change and the introduction of
exotic species that compete with the indigenous ones put the native species under stress, and as a result they are more likely to succumb to pathogens and parasites. While some species such as flies and cockroaches might increase as a result, the total biomass of insects is estimated to be decreasing by about 2.5% per year.
Effects
Insect
population decline affects ecosystems, other animal populations, and
humanity. Insects are at "the structural and functional base of many of
the world's ecosystems." A 2019 global review warned that, if not mitigated by decisive action, the decline would have a catastrophic impact on the planet's ecosystems. Birds and larger mammals that eat insects can be directly affected by the decline. Declining insect populations can reduce the ecosystem services provided by beneficial bugs, such as pollination of agricultural crops, and biological waste disposal. According to the Zoological Society of London, in addition to such loss of instrumental value, the decline also represents a loss of the declining species' intrinsic value.
Evidence
Rothamsted Insect Survey, UK
The Rothamsted Insect Survey at Rothamsted Research, Harpenden,
England, began monitoring insect suction traps across the UK in 1964.
According to the group, these have produced "the most comprehensive
standardised long-term data on insects in the world".
The traps are "effectively upside-down Hoovers running 24/7,
continually sampling the air for migrating insects," according to James
Bell, the survey leader, in an interview in 2017 with the journal Science. Between 1970 and 2002, the insect biomass caught in the traps declined by over two-thirds in southern Scotland,
although it remained stable in England. The scientists speculate that
insect abundance was already lost in England by 1970 (figures in
Scotland were higher than in England when the survey began), or that aphids and other pests increased there in the absence of their insect predators.
Dirzo et al. 2014
Insects with population trends documented by the International Union for Conservation of Nature, for orders Collembola, Hymenoptera, Lepidoptera, Odonata, and Orthoptera.
A 2014 review by Rodolfo Dirzo and others in Science noted: "Of all insects with IUCN-documented
population trends [203 insect species in five orders], 33% are
declining, with strong variation among orders." In the UK, "30 to 60% of
species per order have declining ranges". Insect pollinators, "needed
for 75% of all the world's food crops", appear to be "strongly declining
globally in both abundance and diversity", which has been linked in
Northern Europe to the decline of plant species that rely on them. The
study referred to the human-caused loss of vertebrates and invertebrates as the "Anthropocene defaunation".
Krefeld study, Germany
Malaise traps in German nature reserves
In 2013 the Krefeld Entomological Society reported a "huge reduction in the biomass of insects" caught in malaise traps in 63 nature reserves in Germany (57 in Nordrhein-Westfalen, one in Rheinland-Pfalz and one in Brandenburg).
A reanalysis published in 2017 suggested that, in 1989–2016, there had
been a "seasonal decline of 76%, and mid-summer decline of 82%, in
flying insect biomass over the 27 years of study". The decline was
"apparent regardless of habitat type" and could not be explained by
"changes in weather, land use, and habitat characteristics". The authors
suggested that not only butterflies, moths and wild bees appear to be
in decline, as previous studies indicated, but "the flying insect
community as a whole".
According to The Economist, the study was the "third most frequently cited scientific study (of all kinds) in the media in 2017". The British entomologist Simon Leather
said that he hoped media reports, following the study, of an
"ecological Armageddon" had been exaggerated; he argued that the Krefeld
and other studies should be a wake-up call, and that more funding is
needed to support long-term studies.
The Krefeld study's authors were not able to link the decline to
climate change or pesticides, he wrote, but they suggested that
intensive farming was involved. While agreeing with their conclusions,
he cautioned that "the data are based on biomass, not species, and the
sites were not sampled continuously and are not globally
representative". As a result of the Krefeld and other studies, the German government established an "Action Programme for Insect Protection".
El Yunque National Forest, Puerto Rico
A 2018 study of the El Yunque National Forest in Puerto Rico reported a decline in arthropods, and in lizards, frogs, and birds (insect-eating species) based on measurements in 1976 and 2012. The American entomologist David Wagner called the study a "clarion call" and "one of the most disturbing articles" he had ever read.
The researchers reported "biomass losses between 98% and 78% for
ground-foraging and canopy-dwelling arthropods over a 36-year period,
with respective annual losses between 2.7% and 2.2%". The decline was
attributed to a rise in the average temperature; tropical insect species
cannot tolerate a wide range of temperatures. The lead author, Brad Lister, told The Economist
that the researchers were shocked by the results: "We couldn’t believe
the first results. I remember [in the 1970s] butterflies everywhere
after rain. On the first day back [in 2012], I saw hardly any."
Netherlands and Switzerland
In 2019 a study by Statistics Netherlands and the Vlinderstichting (Dutch Butterfly Conservation) of butterfly numbers in the Netherlands
from 1890 to 2017 reported an estimated decline of 84 percent. When
analysed by type of habitat, the decline was found to have stabilised in
grassland and woodland but had continued in recent decades in heathland. A report by the Swiss Academy of Natural Sciences
in April 2019 reported that 60 percent of the insects that had been
studied in Switzerland were at risk, mostly in farming and aquatic
areas; that there had been a 60 percent decline in insect-eating birds
since 1990 in rural areas; and that urgent action was needed to address
the causes.
2019 Sánchez-Bayo and Wyckhuys review
A 2019 review by Francisco Sánchez-Bayo and Kris A. G. Wyckhuys in the journal Biological Conservation analysed 73 long-term insect surveys that had shown decline, most of them in the United States and Western Europe.
While noting population increases for certain species of insects in
particular areas, the authors reported an annual 2.5% loss of biomass.
They wrote that the review "revealed dramatic rates of decline that may
lead to the extinction of 40% of the world's insect species over the
next few decades", a conclusion that was challenged.
They did note the review's limitations, namely that the studies were
largely concentrated on popular insect groups (butterflies and moths,
bees, dragonflies and beetles); few had been done on groups as Diptera (flies), Orthoptera (which includes grasshoppers and crickets), and Hemiptera (such as aphids);
data from the past from which to calculate trends is largely
unavailable; and the data that does exist mostly relates to Western
Europe and North America, with the tropics and southern hemisphere
(major insect habitats) under-represented.
The methodology and strong language of the review were questioned.
The keywords used for a database search of the scientific literature
were [insect*] and [declin*] + [survey], which mostly returned studies
finding declines, not increases. Sánchez-Bayo responded that two thirds of the reviewed studies had come from outside the database search. David Wagner
wrote that many studies have shown "no significant changes in insect
numbers or endangerment", despite a reporting bias against
"non-significant findings". According to Wagner, the papers' greatest
mistake was to equate "40% geographic or population declines from
small countries with high human densities and about half or more of
their land in agriculture to 'the extinction of 40% of the world's
insect species over the next few decades'." He wrote that 40 percent
extinction would amount to the loss of around 2.8 million species, while
fewer than 100 insect species are known to have become extinct. While
it is true that insects are declining, he wrote, the review did not
provide evidence to support its conclusion.
Other criticism included that the authors attributed the decline to
particular threats based on the studies they reviewed, even when those
studies had simply suggested threats rather than clearly identifying
them. The British ecologist Georgina Mace
agreed that the review lacked detailed information needed to assess the
situation, but said it might underestimate the rate of insect decline
in the tropics.
Global assessment report on biodiversity and ecosystem services
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services
reported its assessment of global biodiversity in 2019. Its summary
for insect life was that "Global trends in insect populations are not
known but rapid declines have been well documented in some places. ...
Local declines of insect populations such as wild bees and butterflies
have often been reported, and insect abundance has declined very rapidly
in some places even without large-scale land-use change, but the global
extent of such declines is not known. ... The proportion of insect
species threatened with extinction is a key uncertainty, but available
evidence supports a tentative estimate of 10 per cent."
Anecdotal evidence
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Bug splats, New South Wales, 2009
Anecdotal evidence for insect decline has been offered by those who
recall apparently greater insect abundance in the 20th century.
Entomologist Simon Leather recalls that, in the 1970s, windows of Yorkshire
houses he visited on his early-morning paper round would be "plastered
with tiger moths" attracted by the house's lighting during the night. Tiger moths have now largely disappeared from the area. Another anecdote is recalled by environmentalist Michael McCarthy
concerning the vanishing of the "moth snowstorms", a relatively common
sight in the UK in the 1970s and earlier. Moth snowstorms occurred when
moths congregated with such density that they could appear like a
blizzard in the beam of automobile headlights.
The windshield phenomenon—car
windscreens covered in dead insects after even a short drive through a
rural area in Europe and North America—seems also largely to have
disappeared; in the 21st century, drivers find they can go an entire
summer without noticing it. John Rawlins, head of invertebrate zoology at the Carnegie Museum of Natural History, speculated in 2006 that more aerodynamic car design could explain the change. Entomologist Martin Sorg told Science in 2017: "I drive a Land Rover, with the aerodynamics of a refrigerator, and these days it stays clean." Rawlins added that land next to high-speed highways has become more manicured and therefore less attractive to insects. In 2004 the Royal Society for the Protection of Birds
organised a Big Bug Count, issuing "splatometers" to about 40,000
volunteers to help count the number of insects colliding with their number plates. They found an average of one insect per 5 miles (8 km), which was less than expected.
Reception
Responses
Chris D. Thomas
and other scientists warned of the need for "joined‐up thinking" in
responding to the decline, ideally backed up by more robust data than is
available so far. In particular, they warned that excessive focus on
reducing pesticide use could be counter productive. Pests already cause a
35 percent yield loss for crops, which can rise to 70 percent when
pesticides are not used, they wrote. If the crop shortfall is
compensated for by expanding agricultural land with deforestation and other habitat destruction, it could exacerbate insect decline.
In the UK, 27 ecologists and entomologists signed an open letter to The Guardian in March 2019, calling on the British research establishment to investigate the decline. Signatories included Simon Leather, Stuart Reynolds (former president of the Royal Entomological Society), John Krebs and John Lawton (both former presidents of the Natural Environment Research Council), Paul Brakefield, George McGavin, Michael Hassell, Dave Goulson, Richard Harrington (editor of the Royal Entomological Society's magazine, Antenna), Kathy Willis and Jeremy Thomas.
In April 2019, in response to the studies about insect decline, Carol Ann Duffy released several poems, by herself and others, to mark the end of her tenure as Britain's poet laureate and to coincide with protests that month by the environmentalist movement Extinction Rebellion. The poets included Fiona Benson, Imtiaz Dharker, Matthew Hollis, Michael Longley, Daljit Nagra, Alice Oswald, and Denise Riley. Duffy's contribution was "The Human Bee".
Conservation measures
Much of the world's efforts to retain biodiverity at national level is reported to the United Nations as part of the Convention on Biological Diversity.
Reports typically describe policies to prevent the loss of diversity
generally, such as habitat preservation, rather than specifying measures
to protect particular taxa. Pollinators are the main exception to this,
with several countries reporting efforts to reduce the decline of their
pollinating insects.
Following the 2017 Krefeld and other studies, the German environment ministry, the BMU, started an Action Programme for Insect Protection (Aktionsprogramm Insektenschutz).
Their goals include promoting insect habitats in the agricultural
landscape; and reducing pesticide use, light pollution, and pollutants
in soil and water.
In a 2019 paper, scientists Olivier Dangles and Jérôme Casas
listed 100 studies and other references suggesting that insects can help
meet the Sustainable Development Goals (SDG) adopted in 2015 by the United Nations.
They argued that the global policy-making community should continue its
transition from seeing insects as enemies, to the current view of
insects as "providers of ecosystem services", and should advance to a
view of insects as "solutions for SDGs" (such as using them as food and biological pest control).
The Entomological Society of America suggests that people maintain plant diversity in their gardens and leave "natural habitat, like leaf litter and dead wood".
The Xerces Society
is a US based environmental organization
that collaborates with both federal and state agencies, scientists,
educators, and citizens to promote invertebrate conservation, applied
research, advocacy, public outreach and education. Ongoing projects
include the rehabilitation of habitat for endangered species, public
education about the importance of native pollinators, and the
restoration and protection of watersheds. They have been doing a
Western Monarch Thanksgiving Count which includes observations from
volunteers for 22 years.
Phone apps such as iNaturalist can be used to photograph and identify specimens; these are used in programs such as the City Nature Challenge. Activities and projects may focus upon a particular type of insect, such as National Moth Week and monarch butterfly conservation in California.
Decline of insect studies
One reason that studies into the decline are limited is that entomology and taxonomy are themselves in decline.
At the 2019 Entomology Congress, leading entomologist Jürgen Gross said
that "We are ourselves an endangered species" while Wolfgang Wägele –
an expert in systematic zoology – said that "in the universities we have lost nearly all experts".
General biology courses in college give less attention to insects, and
the number of biologists specialising in entomology is decreasing as
specialties such as genetics expand. In addition, studies investigating the decline tend to be done by collecting insects and killing them in traps, which poses an ethical problem for conservationists.
