Termites have certain requirements like darkness, a food supply and moisture which must be what your mulch pile provides. You could start shifting the pile, moving it and breaking up the pile to let in light, let the moisture dissipate. Without knowing how far the mulch pile is from anything else in the yard the simplest, most environmentally safe thing is to pour boiling water over the pile and keep doing it until you have reached every termite. Then shift the pile and pour more boiling water. Another alternative is to go ballistic and burn the entire mulch pile but...
