It is an entrance reducer Bruce, I've had lots of trouble with robbers due to my hives location (on a rooftop in an industrial park). I opened the reducer to the larger size opening about a week ago and it was a "free for all".
Manowar, when you say it was a 'free for all', that would imply your hive was suddenly 'attacked' by bees from another hive? Unless there's another hive in VERY close proximity - you wouldn't notice robbing as soon as you took the entrance reducer off. If you were noticing ALOT of bees suddenly at the entrance, they were probably your own, happily the larger entrance.
Your hive is very strong, I wouldn't be concerned about robbing at this point. I would be more concerned, based on the fact that they're on a black tar roof in Texas that they are getting adequate ventilation. It could get hot in there really quickly once your temps start rising.
sherry