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 (4). 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 3,000 miles at speeds of up to 80 miles per day (4) (5). These butterflies, which originate from a breeding range spanning over 100 million hectares, concentrate in forests, in areas which cover less than 20 hectares (4). Here they form some of the largest single species aggregations known, numbering millions of individuals, which blanket the trees on which they roost (2) (6). 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 (2). 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 (4) (6) (8). This two-way, north-south yearly migration is unique amongst butterflies and moths (5) (8).
Female monarch butterflies lay eggs, usually singly, on a variety of milkweed species, sticking them to the underside of the leaves (2). After four days, the caterpillar hatches, and eats almost constantly, increasing 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 (6). 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 (2). 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 (2) (9). 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 (6).