They're just doing the same job, but drastically increasing the manpower, equipment, and expenditures used though?
In the first place, it doesn't make much sense to put a selective, small force of elites in charge of protecting a large area like a city, and would be better off being sent for subjugations or defending a small important area?
Actually, why do the bugs always show up inside the town, where are they coming from, they should be stopped by the gates if they come from outside, and have uniform patrols to react and report an appearance quickly if they come up from underground. Not left to form entire nests like those spiders in the beginning.