Please don't buy caterpillars online-- they may be sickly and you are very likely to find caterpillars in your own yard or garden, or in that of someone you may know. Many gardeners, if you ask them, would be overjoyed for you to remove caterpillars from their plants!
|A Black Swallowtail caterpillar about to devour carrot greens.|
Different types of butterflies need different hostplants for their caterpillars to eat. In my area the most common kind of butterfly is the Cabbage White (Pieris rapae). They aren't native to the U.S., but I think they're fine for your first try. It's easy to find a plant they'll eat: anything in the cabbage family -- radishes are probably the easiest and quickest to grow, and they're happy in a flowerpot if you don't have an in-ground garden. If you spot a radish whose leaves look pretty chewed up, that's a good clue you might find Cabbage White caterpillars on it.
Other caterpillars that are popular to raise include Black Swallowtails, which eat anything in the carrot family; and Monarchs, which eat any species of milkweed. My posts about raising caterpillars are collected here. Always make sure to identify caterpillars you find so you can provide the right hostplant. When I was a little girl I tried to raise caterpillars every summer by feeding them grass, because I didn't know they wouldn't eat just any plant. Of course, they all died. But you can do better!
|Releasing one of my successful Black Swallowtails a few years ago.|
You'll need a container of some sort to keep your caterpillars in, that's closed to keep them safe. A jar is fine with a paper towel or pantyhose fastened over the top. Your caterpillars will poop (a lot!), so to make cleanup easier, line the container with a piece of paper towel or even a coffee filter. Newly-hatched caterpillars are tiny, and their poops are barely bigger than dust, but soon the larvae will grow and so will the size of their frass (the actual name for caterpillar droppings). Leaving frass in the bottom of the container provides a great medium for nasty bacteria to grow, not a good idea.
|My first set-up, with floral vial and paper towel.|
When you are ready to move up in commitment (or your caterpillars get too big for their first container), you could use an old fish tank, even one that's no longer watertight. You can also buy professional caterpillar cages made of mesh. I have several of these cages-- one purchased at the thrift store, and four of different sizes that I bought online. You can see one above, where I'm releasing a butterfly. The benefit of the professional cages is that they let plenty of air through. They're collapsible too, so you can easily store them in the off-season.
Some DIY instructions are available here; a good discussion of different ways to set up a cage is here. The latter link is to Raising Butterflies' web site; their store also has good and relatively affordable cages. I can't remember where I originally bought my cages, but they look like the ones on Raising Butterflies.
|This very hungry caterpillar is about to devour my carrot greens!|
Feeding caterpillars, part 2
When raising caterpillars, you need to provide pesticide-free food for it. Ideally you'll have grown the plants yourself, but I realize not everybody has the space or time to do so. You may get permission from your neighbors, or even be lucky enough to find a vacant lot with the right hostplant.
If you don't have enough hostplants and neither do folks you know, another way to find food is to ask people you DON'T know. Join the Facebook group Raising Butterflies and Moths for Conservation; you'll find lots of helpful information there as well as like-minded folks. A simple post asking folks who live near you for help finding caterpillar food may solve all of your problems, as the friendly members tell you where you can find safe plants or offer you some from their own yards. Don't buy plants at the grocery store, even if they're organic. Organic farmers may treat their crops with Bacillus thuringiensis (or Bt), a bacterium that's deadly to caterpillars but harmless to people.
You don't need to put an entire plant in your caterpillar cage, cuttings are fine. Keep them fresh by placing stems in a cup of water, and make sure you protect the top of the cup so caterpillars can't fall in and drown. I use floral vials I get from buying roses; these are available online or at craft shops.
|I prop my floral vials in a small glass inside the cage to keep them from falling.|
Your caterpillar will go through several instars, or stages, and molt each time between instars. Some species look different in different instars, like the Black Swallowtails I raise.The final molt will be into chrysalis.
|Early instars of Black Swallowtails, like this little guy, are black and white; later they get green stripes as well.|
When your caterpillar is close to that last molt, it will start wandering around. This is when it's essential to have your cage securely closed. My Black Swallowtail caterpillars walk a really long way, it seems, although I've never measured it. They're looking for just the right spot to make their chrysalis. At this point you should add a few twigs to the cage leaning against the walls at roughly a 45 degree angle. If you can find twigs that have a few branches, that will give your caterpillars plenty of choice.
|This little guy nearly escaped when I took the cage lid off for photographs.|
One sign that your caterpillar is almost ready will be a final poop-- this will be very liquidy, not the compact frass you're used to. The caterpillar needs to get rid of any waste before spending days or even months in the closed case of its chrysalis.
|Waste voided, this caterpillar is nearly ready to pupate.|
Next the caterpillar will attach itself to a twig or the wall of the cage. It will spin a small button of silk and attach its abdominal end (cremaster) to that. Monarchs will then hang down vertically, eventually curling into a "J" shape. Swallowtails will spin themselves a sort of safety belt to support their front end and hang parallel to the wall or twig.
|You can see the white silk belt the caterpillar spun to hold itself to the twig.|
It takes a few days before the caterpillar actually molts. If you watch it closely you may see it occasionally twitching, like someone dreaming. I'm not sure what's going on here, but perhaps it's separating inside from the old caterpillar skin. When they molt, the chrysalis shell has already formed underneath the skin.
|Halfway through shedding its caterpillar skin, you can see the chrysalis already emerging.|
Depending on the species and generation, you may have to wait a couple weeks to several months for the butterfly to emerge, or eclose, from the chrysalis. I'll talk about that another time, plus what to do if a chrysalis falls.
Have fun! Let me know how your caterpillars fare.