Monarch butterflies can usually be seen flying amongst the flowers in backyard gardens and parks between September and March in New Zealand. They have an important role to play in helping to pollinate many flowering plants. As monarchs are becoming increasingly scarce, each year the Monarch Butterfly New Zealand Trust encourages the public to tag and release monarchs, and to monitor their flight movements. This information is to help record the monarchs' migration patterns and to identify any environmental changes.
The large and brilliantly-coloured monarch butterfly is among the most easily recognizable of the butterfly species. The wings are covered with tiny scales of differing colours to give us the patterns we see. Monarch butterflies communicate with colours and scents. They signal to other animals that they are poisonous by having bright orange wings. The males attract females to mate by releasing chemicals from scent glands on the hindwings.
Monarch butterflies go through four stages: the egg, the larva (caterpillar), the pupa (chrysalis), and the imago (adult butterfly). During mid-summer, the process goes from egg to adult in about 23 days. A butterfly on 28 February may come from eggs laid before 5 February – but very unlikely to be from an egg laid after that date. Once adults, monarchs have an average lifespan of six to eight weeks in the summer, and 6–8 months if the butterfly pupates in winter. There can be up to three generations of monarch butterflies born each year. (See Monarch Butterfly New Zealand Trust: FAQs)
The monarch butterfly lays eggs on milkweed plants. It takes about 4-10 days for the eggs to hatch into a caterpillar. Times are longer for cooler conditions, than in the peak of the summer.
The baby caterpillar spends its time eating plants in order to grow. In 2–3 weeks, the caterpillar grows to about 2,700 times its birth weight! (See Science Learning Hub)
The main food source for caterpillars (larvae) comes from the Gomphocarpus genus, which includes the swan milkweed and balloon plant. The Swan plant is so called in NZ, because the green seed pods shaped like a swan, the stem being the long neck, and the sap is milky-white. Adult monarchs also feed on the nectar of milkweed flowers, as well as other plants.
As the caterpillar grows & becomes too big for its skin (exo-skeleton), it moults or sheds its skin five times. For photographs and a description of how the caterpillar looks after each of the five stages of moulting (known as instar stages), see University of Minnesota's Monarch Life Cycle.
After the caterpillar is fully-grown, it will find a sheltered place to attach itself in an upside down 'J' shape. It will then start the process of transforming into a chrysalis over the next 10-28 days. The monarch’s scientific name, Danaus plexippus, means ‘sleepy transformation’.
The monarch butterfly will emerge hanging upside down from the pupa with an enlarged abdomen that is full of fluid. By hanging downwards, gravity will help them to pump the fluid from their abdomen into their wings. This allows the wings to expand and dry so that the monarch can use them to fly.
The adult butterfly has an average lifespan of six to eight weeks if it pupates during the summer, but this extends to 6–8 months if the butterfly pupates in winter. The primary role of the adult stage is for the monarchs to reproduce—to mate and lay the eggs that will become the next generation, with three generations occurring each year.
Garden plants rich in nectar will attract monarch butterflies. Marigold flowers will also help repel aphids from attacking swan plants. Spring flowers, such as Verbena, Cineraria, are ideal for butterflies coming out of overwintering who are very hungry for nectar. (For examples of nectar plants, see: Monty's World and Monarch Butterfly NZ Trust.)
Monarchs are more sensitive to cold than other butterflies and are less common in the far south of NZ. When the air temperature drops to 12.8°C around April or May, monarchs start flocking to sites that tend to be in milder coastal locations where the temperature remains at least 10°C. When the temperatures warm up in Spring (around September/October), butterflies move inland to reproduce. (See Science Learning Hub and The Butterfly Musketeers.)
The monarch butterfly's main habitat is urban and suburban gardens. Swan plants are the main source of food. However, as larvae grow they can quickly strip the plants, leaving hungry caterpillars looking for food. To help monarchs to locate available swan plants, the Monarch and Milkweed Matchmaking New Zealand Facebook page was set up in 2019. Did you know that a monarch can smell a milkweed from as far away as 2 kilometres! (See Monarch Butterfly New Zealand Trust: FAQs.)
Butterfly experts in NZ and the United States are concerned that the number of monarchs have been declining. The National Geographic reported that numbers had plunged more than 80 per cent over the past 20 years in America. The Monarch Butterfly New Zealand Trust encourages member of the public to tag butterflies with a light sticker and identification number provided by the Trust (see Youtube video), and to report sightings. The aim is to gather information about the migration and overwintering habits of monarchs, and the indicators of environmental change. See the Trust's Monarch Sightings Map and online form for reporting sightings.