thanks for your question. So many times we hear people, especially keen gardeners, say how much they hate caterpillars because they eat their cabbages or their flowers or their trees. The fact is that they are not being entirely fair on the caterpillars who are rather choosy as to which plants they eat - or to be more exact, their parents are. How many times have you had something like spinach or turnip greens put on your dinner plate and been told to eat it because it's good for you even though you may think it's revolting? This is what caterpillars have to put up with.
Butterflies lay their eggs selectively on specific plants so that when they hatch, the young caterpillars have an immediate source of food. For example, the great white butterfly lays its eggs on cabbages which is why it is sometimes called the 'cabbage white'. The Orange Tip butterfly favours cuckoo flowers and garlic mustard. My personal favourite, the Cinnabar Moth, lays its eggs on ragwort; I have a huge bush in front of my house and so I am never short of specimens. Look for little red, orange, or white eggs on the underside of leaves that are about the size of sugar or rice grains. Here are two great pictures from Jay Cossey's Butterfly Gallery; one of a Cabbage White caterpillar, and one of a SwallowTail egg (normally found on fruit trees).
If you have some caterpillars and you are wondering what to feed them, just go back to the bush where you found them and take some leaves; failing that, try a selection from your garden and note which ones they prefer. As a tip, do not pick just the leaf, but take a short piece of stem with it and wrap the cut end in wet tissue. This will keep the leaf succulent and tasty for much longer and save you from having to denude your garden to feed your caterpillars.
Fussiness over food is not uncommon in the insect world. I have some indian stick insects that will only eat bramble leaves, whereas other species will only take privet or rose leaves. In the same vein, some carnivorous insects specifically prey upon certain other species, and it is that trait that can make insects so very useful. Ask any gardener who is fond of his roses what he thinks of ladybirds (or ladybugs if you're Stateside), and he will tell you that they are the best little greenfly killer in town.
Feed your caterpillars carefully, and if you find that they only eat dandelions, export them to England and make yourself rich as you save thousands of us from having to weed the lawn.
I hope you enjoy the release of some splendid butterflies or moths. Justin Roux