Status, Distribution & Conservation

The Monarch Butterfly, Danaus plexippus L., in Canada

General Biology

Monarchs, like all butterflies, have four distinct stages in their life cycle: egg, larvae or caterpillar, pupae or chrysalis and imago or adult butterflies. Mated adult females lay their eggs on a variety of Milkweeds. The egg stage in Monarchs lasts about 4 days, larvae grow and develop through 5 instars (growth stages) in approx. 15 days and the pupal stage lasts from 9 to 15 days (Urquhart 1987). Adult butterflies live 2-6 weeks in the summer broods and up to 8 months in the migratory stage. Monarchs are most vulnerable to a wide variety of mortality sources in the larval and pupal stages.

Monarchs obtain cardenolides (cardiac glycosides) - bitter, emetic compounds akin to digitalis - from their Asclepiadaceous hostplants which serve to protect them to some degree from their own predators (Brower 1984). The bold coloration of both larvae and adults advertises their unpalatability in such a way as to "train" their predators to avoid them (Brower 1984; Dempster 1984). Each different kind of Milkweed has a characteristic profile of cardenolides (a chemical "fingerprint") as do the butterflies that feed on them (Roeske et al. 1976). The concentration of cardenolides declines with age so that unpalatable butterflies become progressively more palatable as they age (Malcolm et al. 1993; Alonso-Meija & Brower 1994).

Classification and Description

The Monarch, Danaus plexippus, is the only Canadian representative of the family Danaidae or Milkweed butterflies (Monroe 1978). The Milkweed butterflies are closely allied to the Nymphalidae or Brushfoot butterflies and show similar reduction of the adult forelegs. The adult is probably the most familiar butterfly in North America, with its large size (wingspan approx. 10 cm) and distinctive orange and black colour pattern. The Viceroy (Limenitis archippus, Nymphalidae), a mimic of the Monarch, is somewhat smaller and has a very similar appearance in both colour and pattern (they can be differentiated from Monarchs by the curved vein which parallels the hind wing margin), so similar in fact that accurate identification of Monarchs can be confusing to the novice. Male Monarchs may be differentiated from females by the presence of patches of specialized pheromone scales ("alar" spots) on the centre of the hindwing of males. Fully grown larvae are large and conspicuous, transversely striped in black, white and yellow, and are considered to be aposematic or warningly coloured.

Reproductive Biology

Southern Canada offers perfect conditions, from June through August, for Monarch butterfly reproduction. Ovarian development (that is the time it takes eggs to develop) is optimal at 28° C and egg maturation takes longer both above and below this temperature (Barker & Herman 1976). There are two broods through most of Canada except for southwestern Ontario where 3 broods can occur (Holmes et al. 1991; Cockrell et al. 1993). Larvae are most successful on large plants which can support the full growth of the caterpillar (Cohen & Brower 1982), however, adult females often prefer to oviposit (lay eggs) on tender young plants (Borkin 1982; Urquhart 1987). Isolated plants are preferred over large patches, and plants on the edge of patches are preferred over central plants (Suzuki & Zalucki 1986; Zalucki & Suzuki 1987; Zalucki 1993). Herman (1981) suggests that reproductive diapause in the migrating generation is induced, likely by photoperiod (day length), in the larval or pupal stage. Females may mate up to 10 times, receiving a nuptial gift in the form of a spermatophore (sperm plus vital nutrients and salts), which makes males an important resource for females (Suzuki & Zalucki, 1986).