Chronic bugs: Whose fault - tenant, neighbor, unit, exterminator?

[Harman N.]

Hi all,

I have a duplex with recurring pest (roaches, mice) issues. When I bought the building both units were rented, but one of the tenants complained of mice and bugs and asked to be released from the lease, which I allowed. 

Since then the new tenant in that unit has complained several times about pests, and the property manager has ordered 3 treatments in 5 months. Incidentally, the tenant who's been in the other unit the whole time has never complained of bugs, but this tenant does live in filth. 

So my question is, whose fault is it? Is it that:

• a) Both of the tenants in the one unit with complaints have each fostered bugs in their place
• b) The holdover tenant in the other unit (who hasn't complained, but lives in filth) is impacting the whole building
• c) It's something specific in the unit, maybe requiring a deep, deep clean
• d) The exterminator company's treatments are ineffective

I'm leaning towards (b) but would love to hear the community's thoughts.

- Harman