As an anecdotal data point, I'm trying to establish a robust, poly-culture, mostly chemical-free agricultural system on my hobby farm. We do seem to get more flying insects on my place than our neighbors, all of whom have grass lawns. Critters of all kinds, in fact - lots of frogs and toads as well, and box turtles. Obviously I haven't done a scientific study, but we see butterflies, lighting bugs, and so on; not as many as I saw growing up but they are there in some numbers.
If you look around suburbia, there really isn't much ecological space for anything to grow unless it can survive on grass. Even in a lot of rural areas, with the advent of heavy-chemical farming and bigger and bigger monocultures, we're taking a lot of living space away from mother nature.
I think it's up to city planners to include more green belts. Better for the environment, better for water drainage, better for humans, it's really win win win.
City planners would if they could, but they're not autocrats. They know that green space (and mixed zoning and high density and...) is a good thing, but they are captive to the whims of politicians and poorly-informed citizens. And property developers have more than enough money to buy policy and sway public opinion.
A close relative is a city planner, a job he went into as an idealist. He saw the economic, social, and ecological suffering created by terrible urban design, and he wanted to make a better world. After 30+ years, though, he's jaded and cynical.
He's seen historically active flood and landslide and earthquake areas quietly and repeatedly re-zoned for residential development. He's seethed at 40 years of opposition to even modest expansions of mass transit. Suburban sprawl, ghettoization of the poor, retail-only developments... At this point, although he still does his best to advance healthy policy, he's resigned to the fact that the developers always win in the end.
So please don't direct your ire at city planners. In many places, they're the only resistance to the boundless greed of property developers. As in all things, it's important to follow the money. The only people getting rich off the status quo are banks and developers.
One of our problems is the idea that a single cause is responsible for these things. It is not just pesticides or CO2 or some other single change. It is the combination of those changes (even the small ones) that is driving this huge environmental problem. The only way to get out of this is starting to think of the environment is a complex system that needs to be preserved, not trying to find a single culprit that we can manipulate.
I'd put monoculture and pesticides higher on my list of suspects.