stones throw:

Perhaps the reason your little green worm infestation has not generated much comment is that the answer seems so obvious: Spray them with something. It does not help that no one knows whether you are in Alaska or Arizona; there are lots of places in zone 6a. Few of us here are entomologists and qualified to identify little worms on the basis of their color, but if they are eating holes in leaves, you want to be rid of them whatever they are. Whether they will need to be controlled each year may depend in part on whether you control them now. Allowing them to have their way is practically guaranteed to increase their numbers.

Whether you decide to use a common insecticide or a biological control is up to you. There are several insecticides that would be effective and easy to apply to small trees with a 1-quart trigger sprayer, and they are readily available in the nursery sections of bigbox stores, and most specialized nurseries. Ask the staff what they recommend. Bt is itself a living organism, which takes longer than chemicals to have an effect on caterpillar type creatures, but can also be effective if it is important to you to avoid chemicals of any kind. But Bt has a short shelf life, so make sure the stuff you buy is fresh. Bt cannot be stored and used from year-to-year as its potency quickly degrades. Bt is also most effective when used early in the outbreak of caterpillar infestations.

I don't suppose it makes much difference, but it isn't clear from your message whether the trees in question were purchased in 7-gallon pots then planted in the ground, or planted in 7-gallon pots after purchase. If you purchased potted trees from a local nursery, that same nursery ought to be able to advise you on controlling pests of fruit trees.

Don Yellman, Great Falls, VA