Bullfrogs in southern US states are active all the time. Bullfrogs in northern US states hibernate during the winter time. Mating season begins in spring and ends in early summer. In northern US states, bullfrogs will begin coming out of hibernation. They will begin eating and swimming. During the summer, bullfrogs will spend most of their time in the water. They will feed on mosquito larvae. During the fall, northern US states bullfrogs will begin hibernating. Southern US states bullfrogs will become semi-aquatic. During the winter time, they will continue to be semi-aquatic.