Bugs on the Door

The past few days there have been LOTS of box elder bugs around my front door.  Yuck!  Where did they come from?  There are no box elder trees anywhere close that I know of.  Why are they congregating on my front step?  Are they planning a mass invasion when the door opens?  I just don't know what is going on!  I sprayed some bug-defense spray around the door, steps, and foundation today... we'll see how the situation looks tomorrow.