What are the sirens for? (Patreon)

Sometimes as an author, you write a cliff without knowing what it is, or why, it just fits nicely.

I'm attempting to crib largely off of Superminion. This particular mini-arc in his story was based around an infestation of mutated mosquitos (vampires)

What kind of bug or nasty force can we have that would help worldbuilding, while also putting a section of the city into lockdown?

Tiny replicators tunnelled in?
Mosquitos, directly lifted from Gogglesbear?
Some other insect?

I just don't know the specifics. Suggestions welcome. I've got about 20 hours to think on it before I gotta start on it tomorrow.