I don’t know about Curculio. Well, I do, but I blame cosmetic damage on them, not summer drop. I have, in the past, bagged with Maggot Barriers (Footies). Applying the bags is a step in thinning the crop because you have to decide how many bags you have left and how much gumption remains actually to put them all on. Then you whack the remaining fruit off. It’s a way of harnessing your frustration to make sure you don’t get too greedy and leave too great a crop load on the tree. Inevitably, however, the tree has other ideas, and some of the carefully bagged fruit always drops.
PC in most cases doesn’t cause fruit drop. Well it does on peaches and plums, near-100%, but on apples it is more like 5% or less. I am a bit surprised you are losing nickel-sized apples, if clusters have been thinned to only one apple that one apple usually is hanging on very well. So, I really don’t know what is going on for you. Maybe the bags themselves are for some reason inducing the fruit to drop; keep a few non-bagged controls this year to see.
Another possibility is the trees have not yet reached maturity. Apples tend to hang on to fruits when immature better than some other types of fruit tree, but it is still a factor for apples.
Can you please share what you use to spray (and when) to prevent losing my fruitlets on stone fruit? I have many trees and got zero fruit last summer, everything dropped. I think it was curculio as each drop had a tiny bite in the fruit. Really want to buy the right spray this year!
I had a lot of drops the first couple of year of bagging. Now there are only a few that fall. My opinion is that surround spray allows the fruit to get dime-nickle size with reduced insect damage. I also think that learning to quickly decide which fruit is more dominate in the cluster helps. My trees are older now and I also thin as I bag. The first couple of years I had heavy June drops but not so much anymore. Now I need to work on alternate year fruiting on my apples (goldrush).