A volunteer at the Entomology Society Krefeld shows a collection of insects on May 9 at the group's home in Krefeld, Germany. (Photo: AFP)

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.

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

Each bottle contains the amount caught by a single insect trap over a set period, and each box represents a collection of such catches over nearly three decades.

"Since 1982, the traps we manufacture ourselves have been standardized and controlled, all of the same size and the same material, and they are collected at the same rate in 63 locations that are still identical," says Sorg.

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 in the region has plummeted by 76 percent.

To demonstrate the rapid decline, a lab technician holds up two bottles: One from 1994 contains 1,400 grams of trapped insects, the newest one just 300 grams.

"We only became aware of the seriousness of this decline in 2011, and every year since then we have seen it get worse," says Sorg, the man who sounded the alarm.

At the time, the news didn't make major waves outside ecological circles.

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

"Then our German colleagues from Krefeld got in touch and said, 'We have the data. We've witnessed a strong decline. We are very concerned. Could you analyze the data?'. "That's how it all started."

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 realize 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."