These very hungry caterpillars are eating me out of house and home.

I dreamt about caterpillars the other night. Fat wriggly beasties, ringed in black, white and yellow, squirming over my fingers. I was in the garden, carefully picking them up from the soil and placing them onto a plant. Each time I looked down, there was another. Soon the plant was swarming with caterpillars, but still they came. I couldn’t leave them on the ground, but there was no more room on the plant. More caterpillars, more squirmy fingers. Where were they coming from, and why wouldn’t they stop?

This is what raising monarch butterflies has reduced me to. What was a stray fancy a month or so ago – why not buy a couple of swan plants, bring a few more butterflies into the garden? – has become a waking nightmare, like suddenly being saddled with 20 to 30 new pets. Very, very hungry new pets.

As an Eric Carle fan from way back, I was vaguely aware that caterpillars like to eat. I did not know just how much.

The problem with monarch caterpillars, of course, is there is really only one thing they like to eat: Gomphocarpus fruticosus, commonly known as swan plant. A South African native, the swan plant came to New Zealand in the mid 1800s, around the same time the first monarch butterfly was recorded here. While monarchs originated in North America, the kahuku is considered a native to New Zealand because it flew in of its own accord, having island-hopped from the California coast to our own shores. Unlike the North American migratory monarch, the kahuku is not considered endangered, though numbers are on the decline. Growing the one plant that keeps this species alive seemed like an easy way to do my bit.

It’s not entirely true that monarch caterpillars only eat one thing. An alternative food source is tropical milkweed, which looks just like its close cousin the swan plant but with stalks adorned with tiny Carmen Miranda hats of red and yellow flowers. Tropical milkweed is the monarch butterfly’s favourite egg-laying habitat, apparently, and is what I bought ($20 for three plants) when I first embarked on Operation Butterfly Garden. My milkweed plants thrived until early December, when the eggs hatched and the caterpillars emerged; all three plants were stripped bare in not much more than a week.

And so began my descent into swan plant-induced madness. Multiple times a day I head out to inspect my caterpillar colony, do a headcount and make a rough calculation of how much longer their current living (and eating) quarters will survive. After the tropical milkweed I bought four modestly high swan plants – around $8 each – and only realised my mistake when they were demolished in a couple of days. With swan plants, size matters. On my return to the garden centre I was determined to come home with the tallest, leafiest plants I could find.

It made a difference, but only a small one. I still find myself shelling out for new plants – large size, $12 a pop – once or twice a week, each time thinking that this one, surely, is the last I’ll have to buy. And every so often I do see a member of my colony striking off on its final journey, on the hunt for the ideal spot to hang like a bat and spin itself into a green-and-gold chrysalis. But the caterpillars continue to munch on, the leaves continue to disappear, and the tiny green pellets of frass – caterpillar poo – continue to pile up.

I’ll finish my inaugural monarch butterfly season having learnt some important lessons. One: a stripped-bare swan plant can be revived, either in the garden with some fertiliser and netting to keep insects off while the leaves grow back, or in some potting mix indoors (it’ll take around a month before it’s back in an edible state). Two: Mitre 10 seems to be a better bet for swan plants than my local Bunnings, which regularly sells out over the weekend. Three: The cheapest place to buy plants is often not a garden centre at all, but through local sellers on Facebook marketplace. Wherever you buy your plants, make sure you plan ahead – you don’t want to be desperately searching for new plants while your caterpillars are reduced to chewing on the old ones’ stumps like dogs on a desiccated bone.

Which brings me to lesson four: birth control and murder. If your caterpillar colony is already getting out of control, remove any new eggs you find and, if you’re left with no other choice, perform a cull as early as you can. It’s a hell of a lot easier to throw a fingernail-length baby caterpillar down the drain than a well-fed teenager.

I’m still getting a lot of joy from my little caterpillar community. Sometimes two of them meet on a stalk and they butt heads, their antenna clashing like the horns of tiny bison. When they fall I carefully scoop them up and let them inch back onto a particularly toothsome leaf. Occasionally I find a chrysalis dangling from my tomato plants and allow myself a pat on the back for a job well done.

But if I could present them with an itemised invoice for room and board, I would.