As wiggy notes, if an engine surges but quickly recovers and doesn't continue to surge, no immediate action is required (especially since, as wiggy also notes, it's often not obvious which engine it was). If it keeps surging then a
controlled throttle retard is warranted when work load permits. But keep your wits about you and communicate with the PM to make sure you're addressing the correct engine (again, it's not always obvious).
There was a rather infamous incident at Boeing during the PW4000 development in the 1980s (I had several friends on the event flight). They were flying a prototype PW4000 engine on the Line No. 1 767 (VA001) - with a JT9D on the other wing. During takeoff from Boeing Field, the PW4000 started surging at about 50 ft. - un-recoverable surge. The pilot flying didn't say a word, but calmly reached over and retarded
both throttles to idle
. As the aircraft started settling towards the Georgetown neighborhood, the PM took control and slammed both throttles to the firewall - the PW4000 furiously barking the whole way before it was shutdown and they circled back around and landed.
That particular Boeing flight test pilot was demoted...