Tenant has roaches - how to get rid of them

[Brendon Woirhaye]

We are landlords in California.  The subject property is a 6 unit (duplex plus fourplex).

About 10 months ago, one of our tenants had problems with roaches.  We distributed a letter to all tenants in the building about roach behavior and control, and had their place sprayed.  Things seemed to be solved.

A couple months ago, we did an inspection and found that they once again had roaches, and that they were feeding the little buggers (dirty dishes on the countertops, overflowing garbage, living in their refrigerator, eating dinner on the living room couch, not mopping up spills).

We have sent them a letter telling them that they need to clean things up, and to work with the on-site manager to set up a time to admit the exterminator.  They are refusing the exterminator, claiming that their child became ill (months later) from the last spraying.

We are concerned that:

a: the roaches will spread to other units

b: the tenants may claim retaliation if we give them a 60 day notice

Any advice from the community?