What did hobbyists in Krefeld find in their insect traps?

Hobbyist insect collectors in Krefeld, Germany, found that the weight of flying insects caught in traps in protected nature reserves had fallen by about three-quarters over 27 years. The Krefeld Entomological Society, founded in 1905, keeps meticulous records and specimens of the area's insects. It has around 50 members, many of them hobbyists who have become world experts on their favourite insects.

In 2013 the society reported a huge reduction in the biomass of insects caught in malaise traps in 63 nature reserves in Germany. A reanalysis published in 2017 suggested that, in 1989 to 2016, there had been a seasonal decline of 76% and a mid-summer decline of 82% in flying insect biomass over 27 years. The decline was apparent regardless of habitat type and could not be explained by changes in weather, land use and habitat characteristics. You will see the figure quoted both as 75% and as 76%. Both point to the same 2017 paper in PLOS One: its analysis estimates a seasonal decline of 76%, its title speaks of "more than 75 percent", and some accounts, such as the encyclopedia article on the society, simply say 75 percent.

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. They could not link the decline to climate change or pesticides, but suggested intensive farming was involved.

The paper became a sensation: according to The Economist it was the third most frequently cited scientific study in the media in 2017, and it pushed the governments of Germany and the Netherlands into setting up programmes to protect insect diversity.