John  Hopkins.  British Wildlife Oct 2019  Volume 32 No 1
https://www.britishwildlife.com/back-issues/british-wildlife-311-october-2019

This is a review of insect decline.  The research was based on 73 studies of insect declines each covering a time period of 10 years or more.

Conclusions :-

• The rate of insect declined  is twice that reported for mammals.
• About one third of insects  are threatened with extinction in the countries studied.
• Rates of decline  in aquatic species are higher than for terrestial species.
• Many insect communities are  shifting  towards species poor collections of generalists and pollution tolerant species are dominating many fresh waters.
• Not only specialists with  narrow ecological requirements are being lost but also some once common generalists.
• The rate of decline for the UK was 60%, higher than the global rate of 41% and the 44% decline in Europe as a whole.

As well as pollination, insects also control pests, play a key role in decomposition , nutrient cycling, and soil aeration and are food for many other animals.

The largest driver of change was loss and conversion of habitats to intensive  agriculture or development.  The second most important factor is identified as pollution by synthetic pesticides and  fertilisers, significantly more influential than climate change.