Hi JulietP
I don’t have any actual experience of this, but as no-one else has answered your question, will do my best.
Bearing in mind that ant powder is killing insects, I imagine that it could harm hedgehogs. If nothing else, potentially killing their food. If they ate insects killed by it, they would ingest some of the substance. I certainly wouldn’t want to use it, or any man made chemicals in the garden, but it is difficult to control what your neighbours do. Perhaps a little gentle persuasion to try a more ‘organic’ method if he feels strongly about the ants being there?
