[stinos]

I don't buy this as a cause of a global decline, though

The definition of habitat loss here is a bit poor because it doesn't mention the main contributor: farm land which over the past century got changed from something where insects could still survive to something which is more of a barren wasteland to them. And the thrid factor which goes hand in hand with that is pesticide use.

In many areas things have gone in the opposite direction.

Could be, but I'm pretty sure globally there are more areas where it is not going in that direction though. Or at least hasn't been going over the past century.

The Appalachian mountains were clear cut in the 1800s and now are back to forest. If this theory is correct, I would expect there to have been a massive increase in insect populations on the east coast.

Fir starters that's only one, quite localized example. Also the reasoning doesn't hold - ecosystems are complex systems. It's not because one specific area has been properly restored that insects would suddenly go there from surrounding areas, let alone that from the restored areas they then would suddenly have colonized neighboring areas (the entire east coast as you mention). Mainly because the speed at which they can colonize is limited and insects bound to forests don't necessarily all survive in other types of habitats.

[terminalshort]

I was only talking about the land use argument. Pesticides seem like a much more likely cause to me, especially because pesticide use on developed land can also cause contamination on undeveloped land.