I don't think bacteria can do it on their own. I'm no biologist so I don't know which insects are the most essential, but I'm sure even the biologists would have a hard time understanding the potential effects of removing species here and there.
You said 99% of all insects, that's pretty extreme and would definitely kill us all. I don't know what less extreme actions might do.
Fire ants are probably pretty important to their native ecosystems, I think the main problem with them is that they have spread to all kinds of places thanks to being accidentally transported by humans and now have rather negative impacts on the environments they have spread to. I'm sure you could remove them from the places they have been introduced to without negative consequences, although doing such a thing is probably impossible.