Complex sentence: Balsam fir is the most susceptible host to outbreaks of the spruce budworm; annual defoliation of current-year growth for 5 to 8 years will kill the host tree.
Split into simpler sentences: Balsam fir is the most susceptible host to outbreaks of the spruce budworm. Annual defoliation of current-year growth for 5 to 8 years will kill the host tree.