Currently, the only way to shoot down an harverster with a single interceptor is to load ur stingray with aerial lasers (not because it has good damage, but simply because it does not run out of ammo), since stingray can take quite a beating as well. Alien plating helps here to ensure stingray survival.
Keep in mind that UFOs should get a lot nastier when they actually get to use PBW (when it's working, that is), because according to console aliens use sparrowhawk missiles so far.