I live in the Albaicin and I have a balcony with geraniums.  They keep dying. They lose condition and get holes in the stem.
I opened one of the stems and there was a small grub inside. It looked like it was eating it from the inside.

What can I do?

You have got the "taladro de los geraneos"
You can buy a spray at a garden center or in Plaza Larga for this.

There is a page with more information for aficionados of geraniums on this page.

https://granadafoto.com/photos/geraniums_albayzin