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What's involved in raising caterpillars in mesh bags on host plants?

Caterpillars can be grown on exposed host plants and still protected from predators by enclosing them and some of their host plant in mesh bags. Large mesh bags, some with clear plastic “windows”, are commercially available. An alternative is to go to your local craft/fabric supply store (around here we have JoAnn’s and Michael’s) and buy organza wedding bags. These are small bags with drawstrings, made from organza mesh material. They’re intended to be used as gift bags for bridesmaids and other members of wedding parties, and they come in various sizes. I always keep a supply of the 3×4” size and the 4×6-1/2” size on hand.

If you use organza bags, enclose the leaves and small branch of the host plant in a bag, pull the drawstrings tight, and then knot the drawstrings. While knotting makes it more difficult to get the bag off when you want to, it also prevents tiny caterpillars from crawling out of the bag, which can happen if the opening isn’t secure enough. So knotting the drawstrings is important.

Make sure the caterpillars do not run out of food

There are a couple of things to be aware of when raising caterpillars in mesh bags: The first is that periodically they will eat everything enclosed in the bag, and run out of food. How frequently this happens depends on the size of the bag, how much plant material it contained, and the size and number of the caterpillars enclosed, but eventually it happens. I check all my bagged caterpillars at least once each day, to evaluate if there’s enough food left for them to feed for another day. When they run out, you’ll need to move the bag and caterpillars to a fresh branch. More on moving the bags in a minute, but first I want to mention the other thing to be aware of:

Keep the bags clean of frass

The caterpillar droppings (technically called frass) that accumulate in the bags are toxic to the caterpillars. Not to mention that they can also host molds which are also toxic. For this reason, the frass needs to be removed from the bags. The most convenient time to dump the frass is when you have the bags open because you’re moving them to a fresh food source.

Frass can reportedly leach into soils and compost and subsequently be absorbed by plant roots, posing a danger to caterpillars that eat the plants, if they are host plants. So it’s best to dump the frass in a tray or bucket and dispose of it someplace where it can’t be reintroduced into host plants.

Sanitize bags between uses, or if they get too dirty

As mentioned above, both frass and the molds they can host are toxic to caterpillars. When a bag is done playing host to a round of caterpillar(s), or even sooner if it gets too dirty, wash it and sanitize it. I put them in zippered lingerie bags and wash them in hot water in the washing machine.

Moving bags to a fresh food source

When the caterpillars need to be moved to a fresh food source, these are the steps that I follow:

  1. Open the bag and remove it from the old host material.
  2. Dump the frass.
  3. Some caterpillars may be on the bag, and some may still be on the plant. Don’t worry about the ones on the bag — they’ll just move with the bag to the new plant. For the caterpillars that are on the old plant, use pruners to snip off the bits of plants that the caterpillars are hanging on to, and simply drop the bits of plants and caterpillars into the bottom of the bag. Don’t worry about moving the caterpillars onto the new plant material — they will move themselves when they get hungry.
  4. Find fresh host plant material and carefully slide the bag over it, making sure that no caterpillars fall out.
  5. When you close the bag over the new material, if it’s one of the organza wedding bags, be sure to knot the drawstrings.

Chrysalises formed in mesh bags

If caterpillars forms chrysalises on the inside of the mesh bag itself, carefully turn the bag inside out, so the chrysalises are on the outside, and tack it up in a safe place for the butterflies to emerge.

If the caterpillars form chrysalises on plant material inside the mesh bag, use pruners to snip off the bit of plant material that the chrysalis is attached to, and hang it in a safe place for the butterfly to emerge.

Leaving chrysalises inside a mesh bag until the butterfly emerges can be dangerous for the butterfly, unless you’re sure that it has enough free space to hang and drop its wings without obstruction.

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