  We study dynamical fermion effects in lattice QCD at finite temperature. The
method adopted is basically the extrapolation from negative flavour numbers
already tested at zero temperature and based on the simulation of local bosonic
theories, with an essential difference. With an appropriate choice of the
boundary conditions on the bosonic fields, called ``bermions'', it is possible
to separate the $Z_3$ breaking contribution of fermion loops to the effective
action from the one conserving $Z_3$: the former is simulated exactly at a
fixed positive and even flavour number, while the extrapolation from negative
flavour numbers is made only on the $Z_3$ invariant part of the action. We test
this approach by comparing our results on a $16^3\times 2$ lattice with those
from a hopping parameter expansion and our results on a $16^3\times 4$ lattice
with those of direct Monte Carlo simulations including the fermion determinant.
