Everyone who raises monarchs has a story about the butterfly that appears in a room unexpectedly from nowhere.
I took some big caterpillars to an event to show children, and on my way home several escaped into my car. I couldn't find them. It was summer and the car became an oven during the day, so I was sure they wouldn't survive. But a few weeks later, as I was driving down the street, a butterfly brushed across my face. At first, I thought it was outside the car, but there it was sitting on the dashboard, looking back at me as if to say, "Let's go, faster!" I found another chrysalid on the ceiling of my car, about to eclose. How they survived the heat is a mystery. But you should protect eggs from direct sunlight.
At each stage of raising monarchs, there's a little loss. This is normal, so don't feel bad if all your eggs don't become butterflies. In the wild, typically a female will lay roughly 700 eggs (max. known 1300), but only two of 700 (0.29%) need to survive in the wild for the population to sustain itself.
People experienced with monarchs are doing well if they can get 6-10 butterflies out of 10 eggs. So, let's consider the various ways caterpillars can die or go missing.
Eggs: An egg is laid with "glue" on the bottom that attaches it to a leaf. Sometimes the attachment goes awry, or the egg later falls off when the milkweed stem is bumped or brushed. You'll probably lose a few eggs on the way home with them. Check carefully the bottom of the container they were in.
Tiny caterpillars: Upon hatching, the first thing a caterpillar does it eat its eggshell. The shell is made of chitin, the same as the caterpillar's skin; they need more chitin each time they molt, so it's a good idea for them to lay away a chitin supply. But when crowded together they might be tempted to also eat their neighbor's shell (along with the neighbor itself). Eating your brother has the added benefit of reducing competition for milkweed.
I found this out the hard way. I had a milkweed stem with 10 eggs hatching, but a day later there were only about 5 tiny caterpillars, and one larger caterpillar, with a big smile on its face. I had counted carefully and excluded spiders, so I knew who the guilty party was.
Tiny caterpillars can fall off the food, or you might drop one when you transfer it, because it clings to your brush with silk. You think it is transferred, but it comes away attached to your brush dangling on silk, drops down, and gets lost. So, after transferring, as you withdraw the brush always wrap the thread of silk around the back of a leaf (even if you can't see the silk).
Large caterpillars: As caterpillars get larger, I don't believe they eat one another. But other predatory insects and spiders do. Either cover your habitat bin with mesh or put your jar with milkweed in a saucer of water, to make a moat. It's important to check fresh milkweed for spiders and other insects. Ladybug beetles, their larvae, assassin bugs, spiders, and many more will eat monarchs. If large caterpillars are exposed to the outdoors, wasps will carve them up and carry them away, piece by piece.
Caterpillars will also wander off and get lost if their food is unsatisfactory. The worst thing is if the milkweed food dries out and the leaves curl up. The caterpillars will stay inside the rolled-up leaves; you'll have to unroll each leaf by hand looking for them.
But if the food is good, caterpillars never wander... with two exceptions. Each time they molt their skins, they may leave the food and rest on the side of the container a short distance from the milkweed. And, when large and ready to form the J, they always roam. When you first see roaming of large caterpillars, it's time to put a breathable thin cardboard cover on your bin (a manilla file folder).
Caterpillars can also die from disease, accidents (such as children dropping them), or parasites. Tachinid flies (like a house fly but with red eyes) may lay eggs on a caterpillar. The eggs hatch to a larva that eats the caterpillar from the inside. I have more problems with accidents than disease.
If you get milkweed food from agricultural areas, the food may be poisoned. The caterpillars will stop eating, and possibly die. Solution: get food from a different place and see if they resume eating. Even applying a lawn fertilizer with built-in pesticide (for grubs) can poison caterpillars growing on milkweed in a garden next to the treated lawn. Because of pesticides, it's best to stick with the same milkweed source once you establish that it's safe. Washing milkweed won't eliminate pesticides, but it might reduce disease (OE). I don't wash milkweed, but I do check it for predators.
When caterpillars are small, you transfer them to fresh food by hand. Once about half grown, they can transfer themselves if you lean a fresh stalk of food against the old stalk. (This is a major advantage of raising groups of caterpillars on one stalk.) Now here comes the tricky part: Did you transfer all the caterpillars, or leave one behind on the old food? For this reason, I don't throw out the old food right away. I put the old food in a bin and check the rim once in a while. Tomorrow, you may see a caterpillar crawling on the rim, saying "what about me?" I also label each stalk with the number of caterpillars on it, so I know how many I expect to transfer.
J's and chrysalid: Finally, it's time for the J to form--you know because a large caterpillar starts to wander--and you cover the bin. About half the time, chrysalids form on the milkweed plant. Once, one of the large caterpillars ate a chrysalid! But this is rare.
Chrysalids are fragile, especially when freshly formed. If you drop one on the floor, it will shatter like a chicken's egg. A chrysalid can survive small injuries (with the loss of a little green blood) but not major accidents. They need to develop in a suspended position. You can use a straight pin, dental floss, or glue to attach one to a twig anchored in a small jar, or to the roof of your mesh cube. Chrysalids sometimes die of disease--parasitism by tachinid flies is a possibility.
Eclosure (emergence from the chrysalid) is a critical time. If the butterfly dies late in the chrysalid, fails to emerge, or is weak on eclosure, it may have the OE parasite. The butterfly needs space to emerge, because it twists this way and that as its wings dry. If the chrysalid is too crowded by foliage, or if it's hanging from the side of the bin, its wings may become deformed.
The biggest danger is if the butterfly falls down as it emerges. This could happen if it is disturbed by wind or children. Normally, a butterfly will survive falling because it can climb up and hang again. But it can't climb the slick walls of a plastic bin. It can climb up the walls of the mesh cube. If you have an emerging butterfly that has fallen, offer it a bit of paper towel to cling to, and raise it up to a place where it can hang. If many are emerging in a plastic bin, tape paper towels to the wall of the bin.
I strongly urge teachers to let the butterfly eclose in the open in plain sight, so children can see it clearly. The butterfly is fragile and shouldn't be handled for about 4 hours after eclosure, because the wings are drying and hardening. They won't fly or feed for the first 3-4 hours. It's convenient to have the butterfly eclose in the mesh cube or place it in the cube an hour or so after it ecloses. But after 3-4 hours if in the open, it may take its first flight toward light (overhead lights or a window). If it goes to the lights, turn them off. If it goes to a high window you can't reach, wait. It may slide down after a while.
The final way a butterfly can go "missing" is for it to eclose before your class starts. What a shame! All that work, and the children missed it! Butterflies eclose in the morning--if located by an eastern window, as soon as the sun hits them.
Here's how to delay eclosure until class starts. If in the afternoon you see black and orange wings through the transparent chrysalid shell, the butterfly will emerge the following morning. So, when you leave for the day, place a dark cloth over the chrysalid's container. The butterfly will emerge about half an hour after you remove the cloth the next day. If you see the orange and black on Friday afternoon, take the chrysalid home, cover it, and place it in the fridge for the weekend. (Mist the chrysalid several times a day because a fridge is dry.) On Monday, take it to school, but leave it covered until time for the show.
In summary, you can see that falling off the food, accidents and other mistakes are often a larger cause of mortality than disease or predation.
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