Monarch butterflies

New Zealand’s most identifiable butterfly is the monarch (Danaus plexippus). Although found in many places around the world, the monarch is considered a New Zealand native because it became established here on its own. Scientists believe that monarchs were blown from New Caledonia and/or Vanuatu to Australia via cyclones and then blown over to New Zealand a few years later.

New Zealand’s most recognised butterfly

The monarch is recognisable for two reasons – its main habitat is urban and suburban gardens and it’s our largest common butterfly. We welcome monarchs into our gardens by planting their larval food – milkweed species such as swan plants – and enjoy watching their amazing journey through metamorphosis. Monarchs are more sensitive to cold than our endemic butterflies. They do not survive the harsh winters, so are less common in the far south.

The monarch’s life cycle

The monarch butterfly has one of the best-known life cycles in the insect world.

During mid-summer, the process goes from egg to adult in a month or so. The butterfly has a lifespan of 60–70 days during the summer, but this extends to 6–7 months if the butterfly pupates in autumn.

Monarchs (and all butterflies) are sensitive to air temperature. Their growth is the strongest in warm weather, and cooler temperatures signal a change in behaviour. Monarch butterflies in North America migrate from the colder northern regions to overwinter in California and Mexico, with some of the butterflies travelling thousands of kilometres! As the temperature warms, they return north to reproduce.

Adaptations to life in New Zealand

Unlike their northern relatives, monarchs in New Zealand do not appear to migrate. They have adapted their behaviour to suit local conditions. When the air temperature drops to 12.8°C, monarchs look for overwintering sites. Tagging data shows that most butterflies stay within the area where they eclosed (became adults). Overwintering monarchs prefer sites that are sheltered from the wind, have trees with a rough bark surface on which to cling and have a nearby source of nectar. The butterflies are mostly inactive, but on warm days, they fly, bask in the sunlight and feed. When the temperatures warm up, butterflies become active again.

Swarms form regularly to overwinter at places such as Tauranga Bay in Northland. The Monarch Butterfly New Zealand Trust (MBNZT) was initially formed by Jacqui Knight to protect this site. Other overwintering sites include areas of Hawke’s Bay, Nelson and Christchurch. The MBNZT encouraged citizen scientists to tag butterflies to learn more about the migration and overwintering habits of monarchs. The tagging project gathered data from 2006–2021.

Threats to the monarch

Monarchs use warning colouration and toxicity from their larval swan plant food source as defence mechanisms. This is enough to keep most vertebrate predators away, with the exception of the shining cuckoo.

Invertebrates are a different story. The brown soldier bug (Cermatulus nasalis) uses its hollow beak as a straw to suck the insides out of larvae. Praying mantids (Miomantis caffra Saussure and Orthodera novaezealandiae Colenso) eat them. The Tasmanian paper wasp (Polistes tasmaniensis) picks up larvae and carries them away.

An often-overlooked threat to monarch larvae is their dependence on swan plants. Without these plants, monarchs could not breed here. Four species of milkweed have been brought into New Zealand. The plants can reproduce naturally, but more likely, gardeners plant them to attract butterflies. The voracious larvae can quickly strip the plants, leaving hungry caterpillars to starve.

There are some other food sources available. Tweedia (Oxypetalum caeruleum) is a common garden plant with small blue flowers. Larvae will only eat the new shoots. Mature larvae (those that are in their last instar) do not need the toxins in the milkweed plants, so they can successfully pupate if they are fed alternative foods. Two suggestions include draping slivers of pumpkin or thin slices of cucumber on the milkweed stems. This will not work with small caterpillars – they won’t survive. It’s worth remembering that monarchs lay a huge number of eggs, so losing a few larvae to predators or starvation is all part of the process.