Fruit flies. Fruit flies just looove guavas. I think it’s the smell. If you want to eat a guava that doesn’t have worms, you need to take really good care of them. When the guavas are growing, you need to cover them in a kind of linen, so the flies can’t put their eggs on them. read more
Couldn't find specific info for S. Africa but the culprit in much of our area that fits is the guava fruit worm, Argyresthia eugeniella. Rock was spot on; this is a moth. My recommendation would be treatment just prior to the appearance of the moth with BT var. kurstaki, as this is the strain that is deadly to moths. read more