My mother lives in a housing association flat. Last summer her home was invaded by ants and I paid for two visits from a pest control firm. Shouldn’t this be the responsibility of the housing association? They say it isn’t.
There will be a clause in your mother’s tenancy agreement stating that the housing association will keep the premises in a condition “fit for use”. If you feel that the ants are making the flat uninhabitable you may be able to argue that the housing association is in breach of the agreement and should pay for the pest control visits. You would have a better chance of success if a structural fault, such as cracks in concrete floors, was contributing to the problem. The housing association will have a complaints procedure you should follow.