This probably won't help any of you AT ALL, but I have discovered that Houseflies really, really like the Legendary Black Orchid (LBO). I started a new game and since it seems it will be a while before the story moves forward, I just planted the LBO in my Mayor's front yard and didn't do anything else with it. Lo and behold, I can harvest 5 or more Houseflies off of it each day, and I'm not playing that much.