Actually it’s not true that there are far fewer insects. Or more accurately, We just don’t know whether insect numbers are in decline.
“More or less” on Radio 4 here https://www.bbc.co.uk/programmes/m0004f6h
looked at the very claims referred to in the linked article.
To summarise the main points as I recall them
- There are actually very few studies and most of those are narrow in scope (geography or types of insects)
- Some of the studies merely compare count at two different dates – one or both could have been particularly high or particularly low. You need more than two data points to identify a trend.
- You also need to rule out or adjust confounding factors. Changes to the environment around a study site (eg different crops, different animals, new buildings..) can all change the count at that location without changing the true number of insects.