German researchers have reported a rapid drop in the number of flying insects at dozens of natural reserves over the past three decades.

The study did not pinpoint a reason for the decline, however many of the reserves involved are surrounded by farms and it is believed pesticides could be to blame.

According to the report, which was published in the journal PLOS One, between 1989 and 2016 the biomass of flying insects captured in 63 different locations in Germany - including grasslands, swamps and sand dunes - decreased by a seasonal average of 76%.

All of the natural reserves in the study are protected areas.

Researchers are concerned because insects are important pollinators and also a key part of the food chain, serving as meals for birds and other small creatures.

"As entire ecosystems are dependent on insects for food and as pollinators, it places the decline of insect eating birds and mammals in a new context," said the report's lead researcher Hans de Kroon of the Radboud Univeristy in the Netherlands.

"We need to do less of the things that we know have a negative impact, such as the use of pesticides," he added.

While it is well documented that butterflies and bees have been disappearing in Europe and North America, the study is the first to document that flying insects in general have decreased by more than three-quarters across Germany since 1989.