Flies

If dirty items are left on a lot, it can attract Flies.

If too many flies build up on a lot, Sims can be eaten alive by them.

Oddly, in the 1st Sims game, if the move_objects on cheat is enabled then picking up the flies in Build or Buy mode depicts them as the "SSRI Virtual Reality Set" and so they can be sold for 3400 simeolons.