Final answer:
Pharaoh ants are difficult to control because they have developed resistance to common chemical treatments and can spread when aggressively targeted with sprays.
Step-by-step explanation:
Pharaoh ants are particularly hard to control because they are resistant to most sprays and baits, and the use of residual sprays can cause the colony to fragment, which leads to the spread of the infestation rather than its control.
This phenomenon is an example of how pests can evolve resistance to chemical controls, making eradication increasingly difficult over time.
Such resistance, along with the potential costly use of chemicals and the risk of non-target impacts, has spurred interest in alternative pest control strategies, such as biological control tools, which aim to be both effective and environmentally responsible.