I imagine it boils down to a question of expense. Pests can probably arrive through: - People's boots/clothes. - The water. - The soil/fertiliser. - T

I imagine it boils down to a question of expense. Pests can probably arrive through: - People's boots/clothes. - The water. - The soil/fertiliser. - The air. Each of those inputs would need to be filtered. And the cost of filtration (or the entry/exit protocols for the workers) would need to be balanced against the cost of losing a crop.