Complex sentence: While in their naturalized state, scale insects live in the canopy and cause little if any damage, in large numbers they promote the growth of sooty moulds, and cause canopy dieback and tree death.
Split into simpler sentences: While in their naturalized state, scale insects live in the canopy and cause little if any damage. However, in large numbers they promote the growth of sooty moulds, and cause canopy dieback and tree death.