Fixing problems like mold and pests often falls under laws related to repairs. Please see the Repairs page for more information on how a tenant may ask the landlord to fix these issues.
This article from TexasLawHelp.org provides information on what a landlord is responsible for when it comes to mold. It explains what rights tenants have when dealing with a mold problem.
This article from legal self-help publisher Nolo details what landlords and tenants need to know about mold in rental properties and what Texas law says.
TexasLawHelp.org has published this bed bug fact sheet authored by Texas RioGrande Legal Aid. It provides information on various issues, including bed bug addendums, who pays the exterminator, and landlord retaliation.