Complex sentence: The immature stages of these spittlebugs congregate on the young shoots and derive their nourishment by sucking the tree's sap, and while doing so they secrete pure water, which is the cause of the "weeping" effect.
Split into simpler sentences: The immature stages of these spittlebugs congregate on the young shoots and derive their nourishment by sucking the tree's sap. While doing so they secrete pure water, which is the cause of the "weeping" effect.