For almost 30 years they passed as quirky eccentrics, diligently setting up their insect traps in the Rhine countryside to collect tens of millions of bugs and creepy crawlers.

Now the group of German entomology enthusiasts can boast a world-class scientific treasure: evidence of what is described as one of Earth's worst extinction phases since the dinosaurs vanished.

Insects, which comprise two thirds of all terrestrial species, have been dying off at alarming rates, with disastrous impacts on food chains and habitats, researchers say.

The home of the Amateur Entomology Society of Krefeld on the Dutch border is a former school building where thick curtains block out the sunlight.

Inside in glass cabinets are stored thousands of butterflies, their wings bleached by time, along with exotic fist-sized beetles and dragonflies, brought back from around the world by amateur collectors.

Treasure trove

Traditionally "entomology was mainly about drying and collecting rare specimens," says the society's president Martin Sorg, wearing John Lennon-style glasses, a multi-pocket jacket and sandals.

He and an army of volunteers have over the years gathered as many as 80 million insects that are now floating in countless ethanol bottles.

The result is a treasure trove of quantitative data that dwarfs that of any funded university project, he says.

But if he is visibly proud of the society's research, the outcome terrifies him: in the test period, the total biomass of flying insects here has plummeted by 76 percent.

Concern about biodiversity loss focused mostly on large charismatic mammal species, and environmental monitoring such as that in Krefeld was considered a quaint Sunday hobby, largely ignored by the scientific community.

Also in 2011, just across the Dutch border, ecology professor Hans de Kroon was working on the decline of birds in the region.

He hypothesised that the birds suffered from a shortage of food, especially insects, but had no data to prove it.

'Point of no return'

In the search for the cause, the landscape around Krefeld provides some clues.

In the distance, industrial chimneys billow smoke.

On one side of the road lies a protected nature reserve. On the other, a sugar beet field is being sprayed with pesticides by an agricultural machine.

"You see, protected reserves are not so protected," says Sorg.

Across the border, Kroon says, "we must realise that here in western Europe our nature is getting smaller, the agriculture fields are very hostile to insects. There is no food, they get poisoned.

"And nature areas are also more and more isolated. Insects can't move from one place to another, it's too far away."

Although the exact cause for the die-off is not yet clear, he says, "the cause is anthropogenic, there's no doubt about it.

"It is our greatest fear that a point of no return will be reached, which will lead to a permanent loss of diversity."

'Path of extinction'

The Krefeld research played a central role in a meta-study published by Francisco Sanchez-Bayo and Kris Wyckhuys from the Australian universities of Sydney and Queensland.

In February, they published the first synthesis of 73 studies on entomological fauna around the world over the past 40 years, listing places from Costa Rica to southern France.

They calculated that over 40 percent of insect species are threatened with extinction, and each year about one percent is added to the list.

This is equivalent, they noted, to "the most massive extinction episode" since the dinosaurs disappeared.

The main drivers appeared to be habitat loss and land conversion to intensive agriculture and urbanisation, followed by pollution, mainly from pesticides and fertilisers, invasive species and climate change.

"The conclusion is clear," they wrote. "Unless we change our ways of producing food, insects as a whole will go down the path of extinction in a few decades."