Complex sentence: In the midwestern USA the black cutworm is considered to be a serious pest of corn which is very susceptible at the one-leaf stage but by the 4 or 5-leaf stage it is relatively unaffected.
Split into simpler sentences: In the midwestern USA, the black cutworm is considered to be a serious pest of corn. Corn is very susceptible at the one-leaf stage, but by the 4 or 5-leaf stage, it is relatively unaffected.