Monarch butterflies (Danaus plexippus) are famous for the epic annual fall migration they undertake to Mexico and Southern California. What is little known is that their range extends beyond North America to the Pacific, as far south as New Zealand. North American and New Zealand monarchs are the same species, so biologists believe that these butterflies island-hopped their way from North America across the Pacific, eventually reaching New Zealand. There, they could thrive thanks to the abundance of milkweed plants—their favorite food source—introduced by European settlers.
The monarch caterpillars shown in this blog were spotted in a private garden in Nelson (Tasman, South Island) last month.
Monarch caterpillars undergo five instars (i.e. development stages) before becoming a butterfly: they molt five times, each time into a larger, differently shaped and colored larva. Once they are 2,000 times larger than their first-instar self (the stage reached on the photos), the caterpillars are ready to go into pupation, their final stage of development, which lasts 10 to 14 days depending on weather conditions.
The larvae were feeding off a balloon cotton-bush (Gomphocarpus physocarpus), a species of milkweed native to southeast Africa introduced in New Zealand and various other countries.
Milkweeds are paramount to monarchs who are specialist herbivores of this type of plants. The milky latex sap of milkweeds contains cardenolide, a steroid that protects monarch caterpillars against vertebrate predators by making them bitter-tasting and toxic. The larvae possess the ability to ingest, sequester and store cardenolide throughout their lifetime.
However, feeding from milkweed is not harmless to monarch caterpillars: they are also sensitive to cardenoline and deadly intoxication can therefore occur. Moreover, the sticky milkweed latex often glues the larvae’s mandible together, causing them to die from starvation, especially in the early instars.
Cardenolide does not deter wasps, the monarch’s fiercest enemy. New Zealand has one of the highest wasp density in the world: German wasps, common wasps and Asian paper wasps were accidentally introduced in New Zealand and are now severely impacting the local monarch population by predating on eggs and larvae.
Unlike their North American counterparts, New Zealand monarchs do not undertake a fall migration to Southern California and Mexico. Instead, they migrate locally to areas with mild winter temperatures, like Christchurch (South Island) and Tauranga Bay (North Island).
Location: Nelson, Tasman, New Zealand