The Regal Monarch

It’s that time of year. Garden centres will make sure they’ve stocked up on swan plants. Harassed parents will be visiting to placate weeping offspring, whose monarch caterpillars have stripped their host plants bare. Nurseries will happily sell a few tiny seedlings, enough to feed voracious caterpillars for a few more days. The cycle will continue until the butterflies finally disappear for winter. Where do they come from, and where do they go?

Monarch butterflies (Danaus plexippus) are a recent immigrant to New Zealand, and unlike most of our introduced butterflies they don’t come from Australia, but from North America. Because they can travel long distances, they’ve dispersed around the Pacific, South-East Asia, and Australia, and in the late 19th century found their way to New Zealand. Because monarchs introduced themselves, they’re technically classed as a native animal, like the Welcome Swallow and Silvereye, and are our largest native butterfly.

The swan-shaped seed pods: There are many species of milkweeds in the monarch’s home, North America, but the swan plant is the only one that’s widely cultivated here. The seed pods can be saved and used to grow a new crop of butterfly hosts next year. photo: Creative Commons BY, Sid Mosdell / Flickr

The swan-shaped seed pods: There are many species of milkweeds in the monarch’s home, North America, but the swan plant is the only one that’s widely cultivated here. The seed pods can be saved and used to grow a new crop of butterfly hosts next year.
photo: Creative Commons BY, Sid Mosdell / Flickr

The biggest impediment to monarchs becoming established here was a lack of host plants. In North America, their caterpillars feed on many different plants in the milkweed family, but none of our New Zealand native plants will do. The introduced milkweed they eat here is swan plant (Gomphocarpus fruticosus), although they can also stomach the noxious weed moth vine (Araujia sericifera), and in emergencies can be fed squash or pumpkin. But the number of monarchs is determined by the number of swan plants we cultivate for them.

Stripey and inedible: The swan plants that monarch caterpillars eat are poisonous, full of noxious chemicals called cardiac glycosides that can actually cause heart attacks if eaten in large quantities. Monarchs are able to store the noxious tasting chemicals and make themselves inedible to birds; the bright colours make caterpillars memorable and ensure a bird will only try eating one once. photo: Creative Commons BY-NC-SA, Vicki DeLoach / Flickr

Stripey and inedible: The swan plants that monarch caterpillars eat are poisonous, full of noxious chemicals called cardiac glycosides that can actually cause heart attacks if eaten in large quantities. Monarchs are able to store the noxious tasting chemicals and make themselves inedible to birds; the bright colours make caterpillars memorable and ensure a bird will only try eating one once.
photo: Creative Commons BY-NC-SA, Vicki DeLoach / Flickr

A female monarch butterfly lays hundreds of eggs, then dies. The eggs take about a week to hatch. The caterpillar can take as little as two weeks to reach full size, eating voraciously, and then it forms a chrysalis in which it stays for about a fortnight. The whole life cycle from egg to adult butterfly can take only a month, although it’s much slower in cooler weather. In the right conditions, one female could produce hundreds of offspring, although almost all of them die from disease, starvation, or being killed by wasps.

Hanging around for a bit: When the caterpillar reaches full size, it attaches itself to a leaf, forms a J shape, and sheds its skin one last time, revealing the pupa. The pupa has to do a little dance to rid itself of the shed skin, and then reshapes itself and hardens to form a chrysalis. photo: Creative Commons BY, Sid Mosdell / Flickr

Hanging around for a bit: When the caterpillar reaches full size, it attaches itself to a leaf, forms a J shape, and sheds its skin one last time, revealing the pupa. The pupa has to do a little dance to rid itself of the shed skin, and then reshapes itself and hardens to form a chrysalis.
photo: Creative Commons BY, Sid Mosdell / Flickr

In their home, North America, monarchs make a extraordinary journey each winter. From as far north as Canada they migrate thousands of kilometres to Mexico, where they find their way to a few sheltered valleys and overwinter in their millions, covering the trees. The weight of massed butterflies bows the branches to the ground, and the fluttering of their wings sounds like a constant roar. In spring they head north again, laying eggs as they go. Because each butterfly lives a few months at most, several generations pass before next winter; the butterflies heading south have never been to Mexico before, and yet find their way to the same trees in the same valleys. Nobody knows how.

Into the chrysalis: When the caterpillar gets big enough, it makes a hard waxy case about itself called a chrysalis, from the Greek chrysos or gold. The jade green chrysalis has golden spots, and is attached to the leaf by a hook called a cremaster. photo: Creative Commons BY, Sid Mosdell / Flickr

Into the chrysalis: When the caterpillar gets big enough, it makes a hard waxy case about itself called a chrysalis, from the Greek chrysos or gold. The jade green chrysalis has golden spots, and is attached to the leaf by a hook called a cremaster.
photo: Creative Commons BY, Sid Mosdell / Flickr

In New Zealand, monarch butterflies don’t migrate in this way. They can’t survive the winter in most of the country, so congregate on a few trees in overwintering spots in Northland, Hawke’s Bay, Nelson, and Christchurch. (There may be an overwintering spot near Whanganui, so let the Museum know if you come across a tree full of monarchs this winter.) When the weather warms up they become active and start laying eggs again.

Nearly ready to hatch: After two weeks in the chrysalis, the caterpillar has rebuilt its body into a butterfly, and is about to emerge. photo: Creative Commons BY, Sid Mosdell / Flickr

Nearly ready to hatch: After two weeks in the chrysalis, the caterpillar has rebuilt its body into a butterfly, and is about to emerge.
photo: Creative Commons BY, Sid Mosdell / Flickr

To ensure monarchs have enough to eat, we can plant swan plants in weedy corners or empty sections, using seed saved from the previous year’s plants. If the seedlings can be kept frost-free over winter, they’ll have a chance to get large enough to host a good crop of caterpillars. But regardless of how many swan plants you grow, or how big they get, there will always be hordes of caterpillars waiting to strip them bare. The best strategy is to pluck off eggs or caterpillars and dispose of them while they’re still small, so that just two or three survive to adulthood per plant. If you’re too soft-hearted to do this, well, there’s a garden centre owner rubbing his hands together with glee, looking forward to a profitable caterpillar famine.

Attractive perfume: Male monarch butterflies have scent pockets on their hindwings, into which they deposit tiny drops of the nectar-scented perfume that attracts females. When they’re about to mate, they brush the scent from these pockets with their hind legs, to ensure the female butterfly stays attracted to the honey-like odour. photo: Creative Commons BY-NC, TexasFlower / Flickr

Attractive perfume: Male monarch butterflies have scent pockets on their hindwings, into which they deposit tiny drops of the nectar-scented perfume that attracts females. When they’re about to mate, they brush the scent from these pockets with their hind legs, to ensure the female butterfly stays attracted to the honey-like odour.
photo: Creative Commons BY-NC, TexasFlower / Flickr

Dr Mike Dickison is Curator of Natural History at the Whanganui Regional Museum.

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2 comments

    1. Hi Terry, the butterflies find the correct host plant through a combination of sight, smell, and taste. They prefer to land on green things, and may even seek out just the right shade of green; their eyes are probably more sensitive to shades of green than ours. Once on a plant they can sense the chemicals each one gives off by tasting/smelling with sense organs in their feet. Sometimes you’ll see a monarch land on a leaf and drum with its big front feet; this is a female, which has bigger feet than a male, and she’s “tasting” to see whether the plant she’s sitting on is one of the milkweed family and suitable food for her caterpillars. Thanks, Mike (Natural History Curator)

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