Awesome idea, really. It even gives the PCs an incentive for why a chaotic evil drow, lawful good elven wizard, chaotic neutral troll, and a flumph have for working together (just an example, here): the insects are a much bigger threat.
Instead of ONE BBEG, you should do something similar to what I did in an old "horde" campaign - have there be several "verminlords", or whatever, and stage it up so that the PCs can take one down at early levels, all the while knowing the BIG ONE is out there. Or, all of these Verminlords could serve some dark, evil power from another plane, and the campaign ends when said god is forced away from our realm.
I'd also take a look at templates. They're a great way to inject variety into a game, while keeping a thread of continuity. You could make a "winged insect" template, a "mantis" template, a "shelled insect" template, and maybe an "arachnid" template, as well. You could then apply them to pretty much every fight the PCs encounter... only, a fight with a "shelled insect" green dragon is going to be a helluva lot different than the "shelled insect" ankheg they fight!
A cult of humanoids (not all of whom are clerics) working from the shadows in the cities would be cool, too. They also make great villains for low-level adventures, since these adventures could consist of a few 1st level warriors, led by a 2nd level cleric, with a few giant fire beetles and the like thrown in (as well as swarms!) to set the mood of the campaign.
Something I found in a "horde" campaign that works well is this: start the campaign off following a major war between the nations. When the swarm begins, the nations are wary of one another, and aren't able to effectively put their differences aside to push back the bugs. Then, as things get bigger and the nations ally, they find there are still too many bugs to fight, and are pushed back into their seperate cities.
The latter half of the game in the campaign could consist of isolated cities, doing their best to defend themselves against the hordes of bugs. You could seed the campaign with news of "the City of Elverhaven fell last night", all the while re-inforcing the notion to the PCs that THE END OF THE WORLD IS NIGH unless they do something.
Check out a lot of zombie movies, too. I think the zombie movies are a great source of inspiration, simply because in most of those movies, an individual zombie is mostly harmless - it's only when they attack in numbers that they're a problem. These movies also give a lot of inspiration on just how you could approach running the game.
As for the bug city idea, I like the idea. Personally, I'd have the PCs be part of an elite team, sent in to "open the gates" or trigger some sort of bug pheromone that will weaken the bugs so that when the armies of humanity break in, they'll have a fighting chance. And, of course, once the armies are in the bug city, you can have a lot of fighting going on in the background while the PCs fight the evil king bug.
Another idea I used in an undead horde campaign to emphasize how nations were falling was to create small teams of low-level characters, and every now and then, the Players would play these pregen characters in a special episode I had created. I would add in reward notes for each character, saying "if you betray the party, your main PC will receive an additional 1,000 XP". The general idea was to run high-fatality adventures every now and then without harming the main PC group - it also let the players have some input in what happened in distant towns and villages. Plus, it made them realize the hordes were attacking EVERYWHERE, which made the PCs that much more eager to find a fix.
but i'm an entomologist 
