Who covers the cost of professional treatment
In California, the landlord generally pays for professional extermination of a cockroach infestation, because keeping the rental free of vermin is part of the landlord's habitability duty.
When roaches infest a unit or a building, hiring and paying a licensed pest-control company is typically the landlord's responsibility. Tenants are expected to keep their space reasonably clean and to cooperate with treatment — for example, by preparing the unit the way the exterminator asks — but they usually should not have to pay for the infestation itself.
The picture can change if a tenant's severe neglect clearly created the problem, in which case some cost might fall on the tenant. But for the ordinary building infestation, the bill belongs with the landlord. If yours is trying to charge you for extermination that should be their responsibility, a free review can help you understand what is fair.
This is general information about California tenant rights, not legal advice. Every situation is different.