Pest Infestations Who is Responsible?

At the start of any Tenancy the property must be given to a Tenant free of vermin and infestation, if your Tenant has only recently moved in and is experiencing a problem of the pest kind, then it would be the responsibility of the Landlord to remedy the problem.
If however the Tenant is doing something to attract or cause the infestation then the Tenant is liable to remedy the problem. If they refuse then a breach notice may be issued.