Some plants are more susceptible to insect attack than others. Plants have many mechanisms of defense against insects. Plant leaves may contain natural chemicals that adversely affect insects feeding on them.
Some plants have extremely hairy leaves or thick, waxy cuticles that deter insect feeding.
Other plants have developed a tolerance to insect feeding that allows them to sustain substantial defoliation without suffering serious injury. These are all considered host plant resistance. In some cases, certain varieties of a plant are more susceptible than other varieties. For example, many varieties of tatarian honeysuckle are extremely susceptible to attack by the honeysuckle witches'-broom aphid, but at least two varieties, 'Clavey's Dwarf' and 'Arnold Red', appear to be highly resistant. Using resistant cultivars in the landscape can significantly decrease the amount of maintenance necessary for those plants.