Nerd alert: Friends in the garden

Now that I do an extra 2 hours of travelling to work and back every day, I'm getting home after dark, which leaves me no time to do my usual garden scouting (watering, checking for pests and diseases, looking for new fruit/flowers, and seeing what's ready to eat). The upside of this is that I get up excited on the weekend to see the changes in the garden over the week. One Saturday this month I was picking the caterpillars off my Tuscan kale (as you do...) when I noticed that one looked a bit suspicious! He wasn't moving, had some yucky black spots on him, and was laying on a pile of yellow cushions. There was also a little black insect sitting nearby.

Apanteles glomerata adult and pupae

I'm quite familiar with these caterpillars, they are the larvae of the white cabbage butterfly (Pieris rapae) and they eat big holes in the leaves of brassicas. I've seen the butterflies flitting about my broccoli and kale, laying tiny little yellow eggs, and I've washed them off when cleaning the leaves to eat. So I know they are as small as a pinprick, not big and fluffy like these things.
I've also studied some entomology as part of my horticulture degree, enough to recognise the little black critter as a tiny wasp. They are closely related to ants, and have a similar body shape with 3 distinct segments.
After a bit of research, I've discovered the wasp is a parasite called Apanteles glomerata, who lays her eggs inside caterpillars. After hatching, the baby wasps eat their way out of the offending caterpillar, and spin little yellow cocoons. When they come out of the cocoons, they are fully developed wasps (albeit very small) who will find a mate, and then an unlucky caterpillar. Although it's a grisly way to feed your children, it's quite handy for me, as wasps like these control the numbers of caterpillars in my veggies so that there are more for me to eat.
For me, the moral of the story is this- not using pesticides in my garden means safety for the insects that are actually helping me out. I hope the wasps that come out of these cocoons hang around my garden and find every one of those pesky caterpillars! It's a beautiful system (in a disgusting way) and I'd hate to interfere.