Pest resistance is partly the plant and substantially the condition it is in.
Most serious insect damage here follows stress. A healthy pine pushes attacking bark beetles out with resin; a pine that has been short of water for a season or two cannot make enough pitch and the beetles get in. Spider mites explode on plants that are hot and dry. Borers go for trees already weakened by sunscald or a trunk wound. In each case the insect is the thing that finishes it and the stress is the thing that opened the door.
Which makes water and correct siting the most effective pest control available, and far cheaper than treatment. Two or three deep waterings in a drought year protects a mature tree that would cost thousands to remove and replace.
Genuine resistance exists on top of that — varieties selected against specific pests, and plants whose foliage or chemistry simply does not appeal. It is worth having, particularly on anything that will be maintained, because a susceptible plant becomes a recurring treatment line rather than a one-time problem.
What it is not is a substitute for keeping a plant healthy. A stressed resistant plant gets eaten too.