VERY IMPORTANT - I use this method only for Mourning Cloaks.
Another factor is that I select willow that will draw water up through the stems well. We generally use Salix laevigata. Some elms would probably work also; just make sure that they draw enough water to stay fresh while the larvae eat them.
Yes, a 55 gallon barrel is ideal. I developed this technique because we raise the Cloaks on cut food. Here is what to do:
First, let me mention that I only use this method with Mourning Cloaks. You need a plastic 55 gallon barrel with a removable lid. I like the barrels with the rounded sides-the kind that actually look like a barrel, instead of the straight sided drums. A few of the larvae will pupate on the side of the barrel and the rounded sides allow them to hang away from the side. So, remove the lid from the barrel (you won't use it). About one foot up from the bottom of the barrel, cut a square hole approximately ten inches by ten inches. Hot glue some fiberglass window screen over this hole (on outside of barrel). Put some window screen in a frame large enough to cover the top of the barrel. While on the subject of the barrel; it's good to make sure that the barrel originally contained some type of food item, like apple juice, and not toxic chemicals. This takes care of the barrel.
Here's how we use it: We start out raising the Cloak larvae on cut plants in a container of water indoors, not in the barrels yet. Until they start getting larger they need no containment, as they won't leave the plants. At this stage, you can just place a newspaper under the plant to catch the frass. We do not move them to the barrels until they are large enough to pick up with your fingers. The main advantage to the barrels is the speed in which you can transfer a hundred larvae to new food. What we do is when the larvae are large enough to be carefully plucked with your fingers, you will need a water container that holds about a gallon of water, with a neck opening of approx two inches. You can find this type of water bottle in large supermarkets, but anything with these dimensions will work. The reason for the two inch neck opening is that you can stuff alot of small diameter willow branches into the container, and completely fill the neck opening, and there is no worry about the larvae getting into the water. Fill your water container, tightly stuff as many willow branches into the container as you can (make sure that the branches are about the height as the barrel). Place the water container with the willow into the barrel. Anything sticking out the top of the barrel should be gently folded around into the barrel. The idea is to get as much volume of leaves inside the barrel as possible. You wil be dropping the larvae into the barrel, not really placing them. The more volume of leaves the more likely that the larvae will fall into the leaves, and not to the bottom of the barrel. Don't worry, the one's that fall to the bottom of the barrel are not injured, and will climb back up the sides to the leaves. You can place a small branch from the bottom of the barrel up to the branches to help with this, if you want.
Here is where the real time savings comes in. When the larvae have almost defoliated their branches, I remove the screen from the top of the barrel and remove any larvae on the rim of the barrel-these I put in a small holding container. I then lift the water container, with old branches and larvae, out of the barrel. The larvae will stay on the branches while you clean the barrel, so just set them aside (now's a good time for them to get a little sun). All of the frass has fallen to the bottom of the barrel, so the larvae do not come in contact with it. Cloak frass is dry, so it is easy to dump out of the barrel. Take an extra water container, fill it with new cuttings, place in the barrel, then pluck the larvae from the old branches and drop them in on the new leaves.
I keep my barrels of Cloak larvae indoors with a fluorescent light over them. I take a small fan and set it a few feet away from the cut-out in the side of the barrel and turn the fan on low. This will gently blow air from the lower side of the barrel and up through the screen on the top of the barrel. I usually put the fan on a timer and run it for around ten hours a day (more or less depending on temperatures and humidity). When the larvae are ready to pupate, a large number of them will pupate on the screen on the top of the barrel, some in the willow branches, and a few on the sides of the barrel. You can raise up to a hundred larvae in a barrel this way. I have one hundred and nineteen in a barrel that are pupating right now, but this is a little too many. I have raised Cloaks this way numerous times with no more than 1% losses.