Monarch Butterfly (Danaus Plexippus - Mystic Aquarium

Monarch Butterfly
(Danaus Plexippus)

Photo: Lincoln Brower ©

Size: Adult monarchs have a wingspan of approximately 9 to 11 cm and their weight
ranges from 0.25 to 0.75 grams, with an average of about half a gram, which is about
weight of a paperclip! Males are slightly larger than females.

Appearance: Monarchs have bright orange wings with black veins, and black edges that
contain white spots along the margin. The underside of the wings is duller orange, so that
when the wings are folded in rest, the butterflies appear camouflaged as they cluster or
rest singly in trees or on other substrates. Males have a black spot on a vein on each hind
wing. Females have slightly more brown scales in the orange patches of their wings, and
more black scales over the wing veins, making the veins appear wider.

Range/Distribution: There are monarch butterflies in Australia, New Zealand, Hawaii,
the Caribbean, North America, Mexico and South America. However, only the Australian
and North American monarchs exhibit seasonal movements. North American monarchs
form two fairly distinct populations, although recent research has shown that there is
some movement between the eastern and western populations. The western migratory
population breeds in the western United States and Canada, and winters near the
California coast. The eastern migratory population breeds in the central and eastern
United States and in southern Canada, and winters in central Mexico (in the eastern part
of the state of Michoacán and western part of the state of Mexico). The monarchs that
spend the winter in the mountains of central Mexico or eucalyptus groves of coastal
California are the final generation of a cycle that begins anew each year. Most of the
butterflies in this final generation begin their lives as larvae in the northern United States
or southern Canada, and then migrate up to thousands of kilometers to specific
overwintering sites. After spending several months at these sites, they fly north and east,
starting the cycle again.

Habitat: During the summer butterflies can be found around milkweed plants where they
lay their eggs or in areas where they can obtain nectar from flowering plants. In winter,
the western population clusters on the Eucalyptus groves of California and the oak-pine-
oyamel fir forests located in the mountains of central Mexico.

Food: Larvae eat milkweed leaves and adult monarchs feed on nectar from flowering
plants.

Predators: During the winter black-headed grosbeaks and black-backed orioles prey on
adult monarchs. After capturing a butterfly, the bird takes it to another branch and eats it.
Because of their thick beak, grosbeaks eat butterflies by biting them roughly, usually
removing their abdomen from the rest of the body. Orioles are able to cut through the
exoskeleton (skin of the butterflies) with their beak and then empty the contents of the
abdomen and throats with their tongues.

There are also five species of mice that feed on butterflies that have fallen to the ground.
The most conspicuous of these species is the scansorial black-eared mouse Peromyscus
melanotis. The mouse lives on the ground in the colonies and takes advantage of the
abundant food source the butterflies provide during winter.

Mortality is especially high for immature monarchs, with approximately 5% to 10%
survival rates from egg to the end of the larva stage. For example, fire ants (Solenopsis
invicta) kill up to 100% of monarch larvae in some locations in Texas. Tachinid fly
parasitoids cause high mortality rates that vary both with location and year. Other
important predators of monarch eggs and larvae include spiders, true bugs, ladybug
larvae, lacewing larvae, and paper wasps.

Life Span: During summer, the generations of adult monarchs live around two to six
weeks, while those that migrate may live up to eight or nine months and are not
reproductively active until the end of the winter in Mexico. The cool conditions at the
roost sites in Mexico, the inactivity of the butterflies, and the lower metabolism during
diapause contribute to the longer lifespan of the migratory generation.

Mating Behaviors: Monarchs are unusual among butterflies in that males are able to
force females to mate. The male pursues the female in flight and takes her down to the
ground where they copulate, remaining attached for many hours. The male transfers a
spermatophore to the female, which contains both sperm and a protein-rich material that
she is able to digest and use in both her own tissue and that of her offspring.

Conservation status: Although the monarch butterflies are not considered as a species in
danger of extinction, the migration of eastern North American population has been
classified as an endangered biological phenomenon due to the threats to the monarch’s
habitat during its annual cycle of breeding, migrating and overwintering.

Threats: North American Monarch butterflies need milkweed plants to lay their eggs in
summer and a dense, healthy forest to overwinter in Mexico. Milkweed is diminishing

due to the use of herbicides, whereas the overwintering forest habitat is being impacted
by commercial logging, domestic wood harvesting, conversion to agriculture, and other
human pressures.

The extraordinarily dense concentrations of monarchs in the Mexican sites make forest
conservation and rehabilitation essential priorities for the long-term survival of the
monarch butterfly’s migration.

What’s being done: A number of continent-wide monarch conservation initiatives have
been endorsed by cooperative activities of the governments or government agencies of
Canada, Mexico and the United States. Non-governmental organizations are also playing
a huge role in monarch conservation.

Among these organizations, the Monarch Butterfly Fund (MBF), stands out for its
commitment to involve individuals in the three countries to conserve the monarch’s
extraordinary migratory phenomenon.

In North America, MBF board members are actively monitoring monarch and milkweed
populations with the help of ordinary citizens, including children, students and teachers
who are involved in tagging programs and the creation of butterfly gardens.

In Mexico, MBF supports activities to restore and conserve the forest ecosystem and
improve the quality of life of the local communities who are poor and marginalized. MBF
focuses on reforestation programs and educational programs, offering communities
options to use their resources sustainably.

Fun Facts:
• The monarch is the only butterfly to make such a long, two-way migration, over
2,500 km to reach their winter destination.

• Butterfly wings are densely covered with scales. Scales help butterflies insulate
their bodies and improve the aerodynamic efficiency of the wings.

• Monarchs only lay their eggs on milkweed plants. Wild females probably lay
from 300 to 400 eggs over the course of their lifetime, although captive females
can lay, on average, approximately 700 eggs in two to five weeks.

• Milkweed contains cardenolides, molecules that induce nausea, vomiting,
diarrhea and cardiac arrhythmias in animals with backbones such as frogs, lizards,
mice and birds. As larvae feed on milkweed they sequester these molecules giving
monarchs a chemical defense against these predators.

• The two bird species and mice that prey on the monarchs in the wintering sites
have evolved to be able to either tolerate or avoid these toxins.

• Larva molt (shed their skin) up to five times before pupating and when the old
skin peels off, the larva eats its old skin before it starts eating more milkweed!

• Favorite nectaring flowers for monarchs include: Purple Coneflower, sedum such
as “Autumn Joy”, Joe-Pyed Weed, Rough Blazing Star, Azaleas, Monarda and
Phlox

• The overwintering sites in Mexico were “discovered” in the 1970s thanks to an
ingenious butterfly-tagging program started by Canadians Fed and Norah
Urquhart in the 1930s. They enlisted volunteer “research associates” in 1952 to
tag butterflies. Tracking the individual butterflies suggested that northeastern
monarchs overwintered somewhere in Mexico and finally in 1975 an American
salesman called Kenneth Brugger found the sites with the help of a local peasant.

• In the past, Purepecha Indians that live in the region where monarchs overwinter
considered monarchs the souls of the dead and interpreted their arrival as the
announcement of the visit by the dead, since their arrival coincides with the
second day of November when Mexicans celebrate “El Día de los Muertos” (the
Day of the Dead).