Legal Question in Landlord & Tenant Law in California

You are mistaken. You don't have the power to reject a 24-hr. notice, and a 24-hr. notice is valid for any business purpose related to the unit. Receiving notices, including service of process, at the premises is an agreed and necessary service under the tenancy. No trespass occurred. Furthermore, bugs do not automatically make a premises uninhabitable, and they are not always the landlord's responsibility. Depending on how they entered and why they stay, they may be the tenant's responsibility. So your landlord may have a perfectly valid UD case against you. Unless the health department has come out and issued a citation, it is extremely unlikely that you had a right to withhold rent. Tenants must really dot their "i's" and cross their "t's" when withholding rent, or the habitability defense will fail and the landlord will win an eviction and money damages. I strongly suggest you either try to patch things up with your landlord and try to work out a settlement of your issues with each other, or start looking for a new place to live.