FYI - I gave up on the ammonia idea. A 10% ammonia solution does kill them but only after continued exposure to it. I did a test by spraying them with ammonia and rinsing it off after about five minutes. They lived. I don't think orchids would do well if their roots were subjected to extended periods of exposure to the ammonia. I ordered some of the Orcal Slugfest (25% metaldehyde). We'll see how that works.