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