I would wait until high 60s or 70s. That is when their metabolism gets active and spawning will follow soon thereafter.

http://srac.tamu.edu/tmppdfs/144249-280f...dddd2f05577e352

Geographic range for culturing

tilapia in ponds is dependent upon

temperature. The preferred temperature

range for optimum tilapia

growth is 82° to 86° F. Growth

diminishes significantly at temperatures

below 68° F and death will

occur below 50° F. At temperatures

below 54° F, tilapia lose their resistance

to disease and are subject to infections

by bacteria, fungi and

parasites.

Tilapia generally stop feeding when

water temperature falls below 63o F.

Disease-induced mortality after handling

seriously constrains sampling,

harvest and transport below 65o F.

Reproduction is best at water temperatures

higher than 80o F and does

not occur below 68o F. In subtropical

regions with a cool season, the number

of fry produced will decrease

when daily water temperature averages

less than 75o F.