By Loosestrife2
United States 
For the past several weeks while walking my dogs at night (around 10:30pm)I have noticed a beech tree which I pass along the way, with bark that glistened and sparkled in the light of my headlamp. It's appearance, I thought, was quite pretty shimmering in the night like that and quite unique as well. Perhaps this tree had a bark with some kind of crystal deposit on it arising from within or without. In any case I had never seen anything like it. Then last night the mystery was solved...I saw a mass migration of black slender not caterpillars but slugs. About 40 of them to my count climbing up the tree trunk to feed on the leaves no doubt. They were all climbing up at the same time and I surmise climbed down at the same time before sunrise to where I do not know to seek shelter from predators individually or en mass. The glistening of the tree trunk was actually an accumulation of their dried slime trails which heavily covered the tree bark from repeated feeding trips. This is the first time in my life that I have ever seen such a phenomena so my question is have any of you seen or heard of such an occurrence?
13 Sep, 2017
Answers
I've never seen them on trees before, but can sympathise with you Julien as we have been there too! Mornings there have been slime trails, not enough to track them though. questions are, how do they get in, do they come in on boots, presumably they wouldn't want to live in a dry environment indoors, do they eventually die, or are they lurking?
13 Sep, 2017
That must have been quite a sight Loose! It has been a very sluggy year here. I found scores of them when I weeded the neglected veg patch today.
13 Sep, 2017
Yes I have seen this on occasions on our sycamore tree where the snails have slowly gone up and down making beatifull trails. We have a snail that has been living in our living room for some time now and on some mornings we have long swirling silvery trails across the carpet but we just carnt find it.
13 Sep, 2017