Answer: If you have seen the actual caterpillar, it might be caterpillars or larvae of a moth such as the cecropia moth, Hyalophora cecropia.
These can be picked off by hand or, if there are many or you have many plants, you could spray using a product that contains Bt (Bacillus thuringiensis). Or, if the problem is fairly minor with some occasional damage and not threatening the life of the plant at all, you could ignore it -- the birds and also spiders will help provide natural control. This would be true for whatever type of caterpillar it could be.
Good luck with your elderberries.
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