I don't know what it is, but I usually let things be unless I know for sure it's destructive and doesn't help out the food web (like Japanese Beetles). If it's not destroying your fennel, I'd leave it so that perhaps predators will take that and not your BST caterpillars. You could remove it and put it nearby--it sounds like some type of moth, so it might eat many kinds of plants.

I'm still conflicted as to what to do! While the cat STILL hasn't eaten on the curassavica plant, and hasn't moved to the droopy syriaca plant, or touched the tuberosa shoot I plopped in the dirt. Yet, it remains active, moving around the plant. It also looks better than in has the last few days; before, it was kind of dingy looking, like it was jaundiced or something, not it's typical bright pretty colors. Now, the color seems normal, yet, it's still not eating. I may give it until the morning to decide if I should euthanize or not. I guess I keep hoping that if it's going to die on it's own, I'd rather have it happen, instead of forcing it...

The ones that are hanging limply I'd euthanize immediately, remove any other larvae from their presence, and sterilize the container. The scree (liquid droppings) contain crystals that, if eaten, can contaminate any other larvae you have. Chances are these larvae have some sort of bacterial or viral infection. Sorry. Have you checked my blog? I have some info on some of this there. Here is a link that might be useful: tdogmom's Monarch blog

I am in the Houston area and I still have lots of nectar flowers and shrubs for the butterflies. The spring and summer were so disappointing, but the fall sure has made up for the lack of butterflies. I had bushy milkweed plants, about 38 of them, until about a week ago: the cats have just exploded in numbers! My plants are just stripped of all the leaves!! Yah! Finally! I have released about 35 monarchs in the last month or so, which isn't really a large number, but considering how bad this year was early on, it is amazing. My mexican sunflowers went nuts this year and reseeded themselves. I had about 500 plants come up in one of the front beds and still have new seedlings coming up and blooming while very small in size. I really think that they did this so there would be plentiful nectar for the monarchs as they travel to Mexico. I have had so much butterfly traffic that it has certainly made up for the slow start this year. Mechelle