I've noticed a only a few subtle difference, such as B. giganteus in the last instar would be much larger than B. fusca. Also, fusca nymphs appear to have much more colorful markings.
Early instars may be difficult to tell apart but B.giganteus nymphs are mostly maroon and B.fusca are mostly black. Also, of course, B. giganteus subadult nymphs are twice as big.
