got a twisted willow which is being eaten by millions of catterpillars

(an insecticide might be the answer) Would love to see a pic of that.

These may be sawflies, I had some that crawled off a gooseberry plant in a pot ( waiting to be planted) onto a dwarf willow. It took a couple of years to get rid off it. I laboriously picked off all the caterpillars. Or try a systemic insecticide, this goes into the sap of the plant and kills anything that tries to eat the leaves. Not very organic, but sometimes you have to resort to a spray. Read the labels on the containers before you choose one to buy so that you get the most 'organic' one.