  We show that the insulating states of magic-angle twisted bilayer graphene
support a series of collective modes corresponding to local particle-hole
excitations on triangular lattice sites. Our theory is based on a continuum
model of the magic angle flat bands. When the system is insulating at moir\'e
band filling $\nu=-3$, our calculations show that the ground state supports
seven low-energy modes that lie well below the charge gap throughout the
moir\'e Brillouin zone, one of which couples strongly to THz photons. The
low-energy collective modes are faithfully described by a model with a local
$SU(8)$ degree of freedom in each moir\'e unit cell that we identify as the
direct product of spin, valley, and an orbital pseudospin. Apart from spin and
valley-wave modes, the collective mode spectrum includes a low-energy
intra-flavor exciton mode associated with transitions between flat valence and
conduction band orbitals.
