"it could nuke the planet eliminating all insect life"

We'd have to spend a while building a supply of big cobalt-salted bombs because the nuclear arsenals we've actually had have come nowhere near to the level required to wipe out all people let alone all insects.

This is just entirely untrue. We have plenty of nuclear warheads, enough to nuke the thousand most populated cities several times over. All humanity would be dead or dying. those not killed immediately have to deal with fallout and new weather patterns caused by huge amounts of dust blocking the sky.

It is conceivable a few might live, but it is also entirely possible the survivors would be too irradiated to sustain a breeding populations. Vast plant populations would die off, only those not directly blasted and able to survive a few years of artificial global winter survive. This presumes the dust is also non-reflective, because if it lowers the Earth's albedo enough it could trigger an ice age. If that happens then pretty much no individuals survive the triple threat of nuclear blasting, perpetual winter and loss of vegetation, unless they have some uncommon radiation resistance and incredibly low nutrition requirements. In this last paragraph was I talking about humans or insects? Doesn't matter; the cost of nuclear war is too high for us to talk about it lightly, because it is an awesome power that can destroy us if used lightly.

I think even with nuclear weapons we'd have a hard time eliminating ALL the microfauna out there. Of course the recovery time would be measured in hundreds of millions of years at that point.

I suggest you take a look on what a big asteroid impact can do. (You can start on the one 65M years ago, it is a good example.)

We are on the path, but we still didn't create something that Nature couldn't outdo. And then there are those interstellar (e.g. supernovas) and pan-galactic (e.g. gamma ray bursts) killers...