Butterflies: 50% reduction in 50 years. Beetles: 75% decline since 1985. Ladybugs: 50% reduction in 20 years. Across the border in Germany: a 75% decline in flying insects since 1989.
It doesn't take much of a scientist to see the trend. I've been hiking the forests here multiple times per week for 20 years. You can clearly see how some groups are disappearing whilst others remain.
There's several root causes but the primary one is a lack of native plant diversity. Since agriculture over here is done in close proximity to forests, the soil becomes acid due to the nitrogen disposition. This benefits the growth of a handful of plants at the expense of all others. In some of our forests, the soil is now so acid that snails no longer develop a case and bird eggs collapse.
In this plant monoculture, insects depending on a specific host plant disappear. Outside (protected) forests, things aren't much better. Even the tiniest of strips of grass that would normally produce wildflowers, are aggressively mowed down. People's gardens are designed to be as hostile to insects as is possible.
Secondary reasons are invasive species and light pollution.
It's a sad state of affairs and one we should be deeply ashamed of. We're not talking about some iconic predator requiring hundreds of acres of wild forest just to survive. We're talking about insects that require little space, healthy soil, a flower, and for it to be left alone. We can't even offer that.