This is a continuation, and the conclusion, to the story that began in Spellsinger. The party of adventurers (which is what they are) have reached the main mammal city to warn the council of the invasion of a race of insects called the Plated Folk. Apparently the Plated Folk have access to Earthlike technology (which is why Clothahump wanted an engineer) and if they invade then it is going to be a bloodbath. However the council pretty much behaves like one would expect a council full of old fuddyduddies to behave, 'we have our defences, and they should hold up just fine, so thanks for the warning, but we have more important things to do (like taking bribes)'.
Anyway, they decide to travel to other places, such as the Ironclouders and the Weavers, to enlist their help, but the catch is that the Ironclouders and the Weavers pretty much hate each other, so as well as trying to enlist their help, they must also act as diplomats between these two sides. As can be expected in most books, they manage to broker a deal between the two sides, get to the gate in time, and fight off the invasion of the Plated Folk.
I find it interesting that insects are always portrayed as bad guys in novels, whether it involves talking animals, or just some fantasy or science-fiction universe, they are always the bad guys. The strange thing is that it is not as if insects are the most annoying creatures on Earth. In fact most of them are quite passive, even useful. Okay, ants and flies are annoying, but stick insects aren't, and bees, while dangerous if handled stupidly, actually provide quite a good service to our survival (and it goes beyond producing honey to actually pollinating flowers to enable them to reproduce). The only reason insects get aggressive is the same reason everything else gets aggressive, we annoy them. It is like us bursting into our neighbour's house and then pushing him and his family around. Understandably he is going to be very annoyed and either summon help, or deal with me himself. The same goes with insects.
I guess another reason is that insects are simply so alien when compared to our physiology. He can understand mammals, reptiles (though they are also portrayed as bad guys) and birds (who are generally portrayed as good). However when we get to fish (to a lesser extent) and insects we enter a much more alien world. Take flight for example, we understand how birds fly and they tend to fly in accordance with accepted laws of nature. However insects don't. In fact, most of the flying insects out there (such as bees) really shouldn't fly. Secondly is physiology: most animals seem to have a physiology that is similar to ours, but when we get to insects (and fish) once again the laws change drastically. Some of them don't even have mouths and absorb food through their bodies.
So, in answer to my initial question, I guess the reason why insects are always portrayed as nasty, evil, invaders is because to us they are probably the most alien animal life form that we know.