Unfortunately, there is no one-size-fits-all organic solution for this issue. Coddling moths and apple maggots - the two primary culprits of wormy apples - are difficult to control even by commercial pesticidal means, let alone organically. Pheromone traps, sticky traps and bagging juvenile fruit are the standard control options.

I'd suggest or neighbor visit their local extension office for details. Or at the very least read these helpful fact sheets on the pests, their life cycles and recommended control methods. The first addresses coddling moths, the second, apple maggots.

https://ipm.ucanr.edu/PMG/PESTNOTES/pn7412.html

https://ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/applemaggot.html

Thanks, gardengal. I've heard of most of those methods. I'm a bit disappointed that neither of those writeups mention clay spray (sold as "Surround") -- do you have any experience with that method?

Here is a picture of a natural predator of small caterpillars.

That is 2-inch angle iron support for the patio cover at my old house. There are about 200 cells designed to hold an egg and a caterpillar. I had 10 of those nests around my house keeping the caterpillar population at bay. When the egg hatches, the larvae eat the caterpillars before emerging as a wasp. I took the picture with my phone from about 2 inches away to make the point that these wasps do not have to be considered a pest. If you leave them alone, they will leave you alone. The basic trick is to have some confidence they will not hurt you and do not, under any circumstances, wave your arms around or swat at them when they fly around. Also touching the nest is a bad idea. So the suggestion is, if you are spraying the wasps around your house, stop doing that and let them take care of the worms on the apple trees.

While they may be beneficial in other circumstances, I'm not sure wasps will have much effect against the worms that typically infect apples. Both coddling moths and the flies that result in apple maggots lay their eggs either at the blossom end or under the skin surface of the apple and the larvae develop and mature inside the fruits. They are not visible or accessible to any wasps so how predatory or effective any wasps may be to prevent "wormy" apples is up to considerable discussion, if they have any effect at all.

Thank you, folks! I largely agree with all of you.

While digging in the lawn, I uncovered a number of grubs and tossed them onto the driveway, where it didn't take long for the local wasps to carry them off. But (like worms inside apples) if I'd left the grubs in the ground, the wasps would never even have noticed them.

My neighbor has only the two trees, and they're still small, so bagging may be her best bet. I've never done that myself, so any tips would be much appreciated.

Yeah, wasps won't work for those worms.