According to the TARDIS information system, the Semestran Interlude were "a chameleonic species detected in the cloud systems of Galaxy 4." It said that they would "establish a symbiotic relationship with their hosts, from whose characteristics they derive their patterns of behavior."
At some point they landed on the island Croatoan, where they fed off humans who visited the island. Sir Walter Raleigh landed the Doctor's TARDIS on Croatoan in the 1100s, and not realising what year it was, begged them to spare him in return for the colonists at Roanoke. The Doctor had Wanchese knock out Raleigh so that he never came to Croatoan, and the Interlude never had him to help them get their prey. (AUDIO: Voyage to the New World)