Barbara, 

Sorry to hear about the continued return of those bugs.  I do not find it unusual as they breed in tremendous numbers and can establish in protected places.  It looks like another visit from an exterminator is in your future unfortunately.  I would talk to them about a warranty or yearly plan so that you can have some rechecks to get them before they get too large a population.  I also found this fact sheet from PSU with some helpful tips for the homeowner to assist in their elimination. 

Good Luck!

Tim