Your apples probably had been stung by apple maggots.  The best way to protect them is to bag them.  That involves putting a sandwich bag over the apple when it is about the size of a green olive.  The bottom two corners are clipped to allow moisture to escape and the top is zipped and then stapled on their side of the stem.  The bag remains on until the ripe apple is picked.  This is time consuming initially but is one and done.  You can try spraying but it must be done several times on a precise schedule.  Here is information: https://extension.umn.edu/yard-and-garden-insects/apple-maggot