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...dddd2f05577e352Geographic 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.