One of the best-known butterfly species, the beautiful monarch butterfly is renowned for its spectacular, long-distance annual migrations. The adult monarch butterfly is brightly coloured, with orange upper wings, interlaced with black veins and surrounded by a wide, black border marked with numerous white spots. While the striking colouration of the upper wings serves as a visual warning to predators that this species is poisonous, the under surface of the wings is duller orange, and helps to camouflage this species against tree bark and other substrates when at rest. Male and female monarch butterflies can be readily distinguished by the fact that the adult male is slightly larger than the female and has a black spot on each hind wing.
Like the adult, the fully-grown monarch butterfly caterpillar is also highly distinctive, possessing bold, yellow, black and white bands over the entire five centimetre-long bodies, with a pair of long black filaments near the head and a pair of shorter filaments towards the rear. During metamorphosis the caterpillar forms a lime-green chrysalis, marked with gold spots and a black, horizontal band edged with gold.
Range: The monarch butterfly has an expansive range extending throughout much of the New World, from southern Canada, south through the entire United States to Central and South America. The two best-known populations are the North American western and eastern migratory populations, which travel vast distances between British Colombia and California, and from Southern Canada, through the eastern United States to central Mexico.
Status: The exact number of individuals insects is not known and yet to be assessed by the IUCN. But it is noticed that about one third of the population is reduced at their usual hibernation ground.
Habitat: The monarch butterfly can be found in a variety of temperate and tropical open habitats. As both the adult and larval stages rely on milkweed for food, the monarch butterfly is typically found at sites, such as fields, meadows, weedy areas, marshes, and roadsides, where these plants are common. During the winter, migratory populations hibernate in fir, pine, oak and cedar forests.
Biology: Although all monarch butterfly populations share the same basic biology, it is the migratory populations, in particular the eastern North American population, which display the most spectacular behaviour. The eastern population migration commences at the summer breeding grounds, which range as far north as southern Canada. During the summer several successive, short-lived generations of monarch butterfly are produced, which complete the entire lifecycle from hatching through metamorphosis to reproduction and death within a period of two to five weeks. The final summer generation, however, has a much longer lifespan, and commences a mass, southward migration in the autumn from the breeding grounds to the wintering grounds, covering distances as far as 4,800 kilometres at speeds of up to 130 kilometres per day. These butterflies, which originate from a breeding range spanning over 100 million hectares, concentrate in forests, in areas which cover less than 20 hectares.
Here they form some of the largest single species aggregations known, numbering millions of individuals, which blanket the trees on which they roost. The butterflies remain in a state of relative inactivity for most of the winter, occasionally taking moisture and flower nectar on warm days, but as spring approaches, many commence mating, before returning to the northern breeding grounds. The females lay eggs during the journey, and while most of the winter generation die before reaching the original breeding grounds, once subsequent new generations have become adults, they continue to head north, thereby re-colonising the entire North American breeding range. This two-way, north-south yearly migration is unique amongst butterflies and moths.
Female monarch butterflies lay eggs, usually singly, on a variety of milkweed species, sticking them to the underside of the leaves. After four days, the caterpillar hatches, and eats almost constantly increase in mass by almost 2,000 times over a 9 to 14 day period, before undergoing metamorphosis. This rapid growth is accompanied by five moults, known as “instars”, in which the caterpillar sheds its smaller skin. The caterpillar then forms a chrysalis in which metamorphosis take place over a period of 9 to 15 days. Once emerged, the adult monarch butterfly remains reliant on milkweeds, feeding on nectar from the flowers, although it may also take nectar from a variety of other flowering species.
Both the caterpillar and the adult are poisonous to most vertebrates due to the accumulation of toxic chemicals produced by the milkweeds. These are accumulated by the caterpillar during feeding, and remain present in the adult’s tissues throughout its life. When attacked, by naïve birds for example, the toxin causes severe vomiting, and ensures that the predator avoids the monarch butterfly in the future. Despite this powerful defence mechanism, monarch butterfly caterpillars are preyed upon by some invertebrates, such as wasps and ants, which are less affected by the toxins.
Threat: Although the monarch butterfly is not considered to be globally threatened, the North American migration is recognised by the IUCN to be an endangered biological phenomenon. This is mainly due to the variety of threats faced by the butterflies at the winter sites, including logging and clearance for agriculture in Mexico, and coastal land development in California. In addition, outside the wintering sites monarch butterflies are further affected by the use of pesticides, habitat loss, the loss of milkweed populations and parasites.
Conservation: In order to protect migratory populations of the monarch butterfly at the wintering grounds in Mexico, in 1986, the Monarch Butterfly Biosphere Reserve was established. This World Heritage property includes more than half of the overwintering colonies of the monarch butterfly’s eastern population.
In 2008, the Commission for Environmental Cooperation created The North American Monarch Conservation Plan, which details a strategy for conserving and maintaining the Monarch butterfly population through international cooperation between Canada, The United States and Mexico.
The plan’s aims include the decrease or elimination of deforestation at the wintering grounds; addressing habitat loss and fragmentation along the migratory route and at the breeding grounds; public education about the threats faced by this species; and increased monitoring during migrations. Their work will help to ensure that this spectacular butterfly is preserved and that its extraordinary migratory journey will be witnessed by future generations.