My two most delicate plants- Zanzibar and Madam Butterfly are dropping leaves. Not the older leaves, but many of the younger leaves instead. They are both rather small plants with small diameter stems. I've looked at both with a fine tooth comb and see no pests. No spider mites, or any more visible creatures. Anyone else have this happen? What could the culprit be? Otherwise the plants look healthy and after spraying them with soapy water the leaves start coming back almost immediately and they look healthy. However, it recurs once I stop the soap spray or it rains frequently. Could it be light colored thrips that I can't see? Uncharacteristic of snail damage- and I do have plenty of snails. The plants don't look like they have any root problems. The Madam Butterfly does have some leaves that are a bit deformed. What gives?