Thank you for your pictures.  Sorry for the delay in responding, I've been out of the office since Friday.

This appears to be an insect called Plum Curculio. 

There are few effective non-chemical methods for controlling plum curculio in tree fruits. However, home orchardists can help reduce future populations by promptly picking up and destroying any fallen fruit from the ground (particularly from May
through June).

Adult populations can be suppressed in the spring with well-timed applications of effective insecticides immediately after petal fall. Insecticides labeled for use against plum curculio, and available for homeowners include carbaryl (Sevin), malathion and kaolin clay (Surround). Care should be taken when using carbaryl
and malathion because these products can be highly toxic to pollinators and beneficial insect predators, and may promote outbreaks of secondary pests (e.g., mites).

If the tree is small, look for other infected from to remove.  This will also help the tree put it's energy towards the remaining fruit. 

Our spray schedule for Nectarines, can be found in our Home and Grounds Pest Management Guide starting on page 3-14. https://resources.ext.vt.edu/contentdetail?contentid=2704&contentname=2021%20Home%20Grounds%20and%20Animals%20PMG%20-%20Home%20Fruit