I don't like the idea that it can be made somewhere but not at home. It just needs a little thought.
Horn worms are larva for Sphinx moths. There are probably 30 different species in North America and they all feed on different plants. The big green one that I have seen for sell at mulberry farms is either a tomato horn worm or a tobacco horn worm. Both of them will eat willow leaves. They have to be started on that to begin with, you can't swithch food plants once they start eating.
I think the foods you buy are probably ground up leaves, maybe freeze dried and powdered, mixed with agar agar or gelitin some how. Might need something like fruit fresh to prevent mold.
Might be fun to experiment with.
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Wade's Weptiles & Wodents