The ball fell to Threlfall 20 yards out and he thumped the it back into the top corner over outstretched Torquay goalkeeper Michael Poke and in off the underside of the crossbar.
Lathrope was one of five players recalled by Torquay caretaker-boss Geoff Harrop, looking after first team affairs following the midweek dismissal of Alan Knill.
Torquay could easily have gone in further ahead at half-time as recalled striker Elliot Benyon headed Anthony O'Connor's cross against the crossbar in the 20th minute.
Another O'Connor cross, in the 34th minute, resulted in Morecambe goalkeeper Paddy Roche having to scramble the ball away from the foot of his post after striker Karl Hawley's header had come back off the woodwork.
It could have been a different story if Morecambe skipper Mark Hughes' 16th-minute header from Threlfall's corner had not flown just over the crossbar.
- Sports & Recreation
- Damon Lathrope