When Simon Leather was a student in the 1970s, he took a summer job as a postman and delivered mail to the villages of Kirk Hammerton and Green Hammerton in North Yorkshire. He recalls his early morning walks through its lanes, past the porches of houses on his round. At virtually every home, he saw the same picture: windows plastered with tiger moths that had been attracted by lights the previous night and were still clinging to the glass. “It was quite a sight,” says Leather, who is now a professor of entomology at Harper Adams University in Shropshire.

But it is not a vision that he has experienced in recent years. Those tiger moths have almost disappeared. “You hardly see any, although there used to be thousands in summer and that was just a couple of villages.”

It is an intriguing story and it is likely to be repeated over the next few weeks. The start of summer is the time of year when the nation’s insects should make their presence known by coating countryside windows with their fluttering presence, and splattering themselves on car windscreens. But they are spectacularly failing to do so. Instead they are making themselves newsworthy through their absence. Britain’s insects, it seems, are disappearing.

This point was underlined last week when tweets from the naturalist and TV presenter Chris Packham went viral after he commented on the absence of insects during a weekend at his home in the New Forest. Packham said he had not seen a single butterfly in his garden, and added that he sleeps with his windows open but rarely finds craneflies or moths in his room in the morning. By contrast, they were commonplace when he was a boy. “Our generation is presiding over an ecological apocalypse and we’ve somehow or other normalised it,” he later said.

Certainly, the statistics are grim. Native ladybird populations are crashing; three quarters of butterfly species – such as the painted lady and the Glanville fritillary – have dropped significantly in numbers; while bees, of which there are more than 250 species in the UK, are also suffering major plunges in populations, with great yellow bumblebees, solitary potter flower bees and other species declining steeply in recent years. Other threatened insects include the New Forest cicada, the tansy beetle and the oil beetle.

As for moths, some of the most beautiful visitors to our homes and gardens, the picture is particularly alarming. Apart from the tiger moth, which was once widespread in the UK, the V-moth (Marcaria wauaria) recorded a 99% fall in numbers between 1968 and 2007 and is now threatened with extinction, a fate that has already befallen the orange upperwing, the bordered gothic and the Brighton wainscot in recent years.

A great yellow bumblebee. Its numbers have declined steeply in recent years. Photograph: Alamy

An insect Armageddon is under way, say many entomologists, the result of a multiple whammy of environmental impacts: pollution, habitat changes, overuse of pesticides, and global warming. And it is a decline that could have crucial consequences. Our creepy crawlies may have unsettling looks but they lie at the foot of a wildlife food chain that makes them vitally important to the makeup and nature of the countryside. They are “the little things that run the world” according to the distinguished Harvard biologist Edward O Wilson, who once observed: “If all humankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed 10,000 years ago. If insects were to vanish, the environment would collapse into chaos.”

The best illustration of the ecological importance of insects is provided by our birdlife. Without insects, hundred of species face starvation and some ornithologists believe this lack of food is already causing serious declines in bird numbers, a point stressed by the naturalist and wildlife author Michael McCarthy. “Britain’s farmland birds have more than halved in number since 1970,” he points out. “Some declines have been catastrophic: the spotted flycatcher, a specialist predator of aerial insects, has both declined by more than 95%, while the red-backed shrike, which feeds on big beetles, became extinct in Britain in the 1990s.”

Further confirmation of the link between insect and bird numbers was provided last week with the publication of a study by Aberdeen University researchers which showed that the plunge in numbers of cuckoos in some areas of England was closely linked to declines in tiger moth caterpillars on which cuckoos feed.

“There is now a lot of correlational evidence to show that when certain insects do badly, very often the birds that feed on them get into trouble as well,” said David Gibbon, of the RSPB.

However, insects also play invaluable roles in other parts of the environment – for example as pollinators of our orchards and fruit fields. And again, scientists are worried. “People think that it is just bees that pollinate orchards, but there are huge numbers of flies that also pollinate – and they are all also threatened,” said Leather.

In addition, flies, beetles and wasps are predators and decomposers who control pests and who generally clean up the countryside. “Just think of the work of the dung beetle,” added Leather. “If they go, the land would be covered with the excrement of cows, sheep and other animals.

The tansy beetle, which is ‘nationally rare’, is the subject of a major conservation programme in Yorkshire. Photograph: Alamy

But perhaps the most alarming indication of the ecological apocalypse we face was provided a few months ago by researchers who published a startling paper in the journal Plos One. Their work was based on the efforts of dozens of amateur entomologists in Germany who began employing strictly standardised ways of collecting insects in 1989. They used special tents called malaise traps to capture thousands of samples of insects in flight over dozens of different nature reserves.

Then the weight of the insects caught in each sample was measured and analysed – revealing a remarkable pattern. The annual average weight of insects found in the traps fell by 76% over the 27-year period of their research. Most alarming, however, was the discovery that the decrease was even higher – 82% – in summer, a time when insect numbers should reach their peak.

Such figures give strong numerical support to the veracity of anecdotes about splattered car windscreens and moth-plastered patio windows becoming a thing of the past. Equally stark is the fact that although meteorological patterns fluctuated to some degree during the years of the study, it was clear that weather was not the cause of the declines.

But perhaps the most alarming aspect of the research was the realisation that these grim drops in insect numbers were occurring in nature reserves – in other words, in areas where the landscape was highly protected and should be the most friendly of habitats for insects. Conditions elsewhere were likely to be a lot worse, the scientists warned.

“Insects make up about two-thirds of all life on Earth [but] there has been some kind of horrific decline,” Professor Dave Goulson of Sussex University, said at the time. “We appear to be making vast tracts of land inhospitable to most forms of life, and are currently on course for ecological armageddon. If we lose the insects, then everything is going to collapse.”

The fact that insect biomass has been declining at a steady rate for almost three decades strongly suggests some profound influences must be at work. Most entomologists believe habitat change lies at the heart of the problem. “There have been massive alterations to the way we use the land and it is hard not to believe these are closely involved in what we are seeing,” said Leather.

As he points out, intensively farmed wheat and cornfields support virtually no insect life, and this means that as intensive agriculture spreads there are fewer and fewer islands of natural habitat left to support them.

A garden tiger moth caterpillar. Changes in habitat caused by intensive farming have been blamed for the decline of insect populations. Photograph: H Lansdown/Alamy

And then there is the issue of urban spread. Housing schemes continue to encroach on our woods and heaths so that streets and buildings generate light pollution that leads nocturnal insects astray and interrupts their mating. “That is the reason we see most changes to insect life in south-east England, for that is where we see the greatest spread of cities and towns,” said Leather.

In addition to habitat changes, there are the dangers posed by pesticides, in particular neonicotinoid pesticides, which have already been blamed for recent crashes in bee populations. These chemicals are water soluble and so leach out of fields after they are applied to crops.

According to research quoted in the journal Science last year, these pesticides have since been found in high concentrations in nectar and pollen in wildflowers near treated fields. Though still not at levels sufficiently high to kill insects directly, they do affect their abilities to navigate and communicate.

In the face of this mixture of ecological woes, it is perhaps not surprising that insects in Britain are faring so badly. Whether or not they face an ecological apocalypse is a different matter, for not every expert shares a sense of doom. Professor Helen Roy of the Centre for Ecology and Hydrology, for example, sees cause for hope. She told the Observer that there were too many success stories – tales of insects that were recovering in numbers and thriving – to feel a sense of despair. “Obviously, many species are suffering, but I am an optimist and I just don’t think it is right to call this an apocalypse,” she said.

Roy pointed to explosions in the number of ladybirds and painted lady butterflies that have occurred in the past as evidence. “There are huge variations in numbers of a particular insect species in a year and huge variation in the places you see them.” She also pointed to one study of pollinators that showed while 32% became less widespread between 1980 and 2014, 16% became more widespread. “It is not all doom and gloom,” Roy added.

This view was supported by David Gibbons of the RSPB who agreed that not every investigation about insect numbers revealed a tale of irrevocable decline – though he added that he still believed the overall picture was worrying. “It is hard not to see a link between some of the bird number declines and drops in insect populations we are experiencing. There are very close correlations in many cases. But proving there is a causative link – in establishing the one effect is leading to the other – is much more difficult.”

We appear to be making tracts of land inhospitable to most forms of life. If we lose insects, it all collapses

An illustration of the problem is provided by one of the few cases where a causative link between insect loss and bird-number declines has been established: the grey partridge, Gibbons said. “During the 70s and 80s, pesticides were killing off plants on which sawflies and other insects fed. Grey partridge chicks feed on these insects and so this process led to a decline in their numbers – and that has since become dramatic.” In fact, the grey partridge’s drop in numbers has brought its population to less than 5% of its figure last century.

The crucial point is that researchers were able to show that these twin declines were connnected by manipulating herbicide levels in places where chicks were being reared. When herbicide levels went up, insect levels went down and so did bird numbers. “That manipulation provided the causative link,” said Gibbons. “It was possible to change insect numbers and so see the impact. However, such research is difficult to carry out and is very rare.”

And of course, threats to our birdlife are only one aspect of the dangers posed by losses of insects in the UK. As entomologists point out, they also keep our soil fertile, degrade waste, pollinate our orchards and control pests such as the aphid.

“We cannot afford to lose them and that’s what makes this issue so urgent and so important,” Leather concluded. “That’s worth keeping in mind as the summer evenings begin – and we see hardly any insects.”

Ladybirds



Many of Britain’s native species of ladybirds are suffering serious declines in numbers, thanks to the arrival of the harlequin ladybird. It has been declared the UK’s fastest invading species, after reaching almost every corner of the country in just a decade. It preys on native ladybirds and is believed to have caused the decline of at least seven species, including the popular two-spot ladybird, which – when last assessed in 2012 – had slumped by 44% in numbers.

Moths



More than 2,500 moth species have been recorded in Great Britain, of which around 900 are called larger moths. In the report The State of Britain’s Larger Moths 2013, it was revealed that larger moths had declined by 28% between 1968 and 2007. This was most noticeable in southern Britain where there was a 40% decline. By contrast, numbers showed no significant change in northern Britain, where disappearing species are balanced by moths spreading north because of climate change.

Bees



Seventy of the 100 crop species that provide 90% of food worldwide are pollinated by bees. In the UK, there are more than 250 species of bee: 25 species of bumble bee, 224 species of solitary bee and one honey bee species. According to a government report in 2014, figures there has been an overall decline in wild and honey bees over the past 50 years. The figures also revealed evidence that there has been parallel declines in the plants that rely on them for pollination.

Butterflies



The State of the UK’s Butterflies report – produced in 2015 by Butterfly Conservation – provided further evidence of “the serious, long-term and ongoing decline of UK butterflies”. Overall, 76% of the UK’s resident and regular migrant butterfly species had declined in either abundance or occurrence (or both) over the past four decades, it was found. “This is of great concern not just for butterflies but for other wildlife species and the overall state of the environment,” the report noted.

Beetles



These insects eat large volumes of slugs and aphids and large numbers of weed seeds, thus helping to stop fields being overrun by unwanted plants and pests. However, a study, published in the Journal of Applied Ecology in 2012 - which looked at 68 beetle species at 11 locations around the British Isles over 15 years - found that three-quarters of those examined had declined in number over the periodOf these, half had fallen at rate equivalent to 30% per decade.