How is a tree described when it is not affected by harmful insects?

The term that best describes a tree that is not affected by harmful insects is "immune." When a tree is referred to as immune, it implies that it has developed natural defenses or resistance to particular pests or diseases, preventing them from causing harm. This immunity can be a result of genetic factors, inherent characteristics of the species, or certain adaptations that make it less appealing or susceptible to the insects.

While the other terms have related meanings, they do not specifically convey the concept of protection from harmful insects. "Resilient" relates to the ability to recover from stress or damage, which does not necessarily indicate that a tree is unaffected in the first place. "Thriving" describes growth and health but does not specifically relate to insect resistance. "Vigorous" indicates strong growth and vitality but lacks the specific connotation of immunity to pests. Hence, referring to a tree as immune most accurately captures the idea of being unharmed by these damaging insects.