common name: monarch butterfly
scientific name: Danaus plexippus Linnaeus (Lepidoptera: Nymphalidae: Danainae)

Introduction - Distribution - Description - Biology and Life Cycle - Hosts and Monarch Toxicity - Conservation Status - Natural Enemies - Selected References

Introduction

The monarchs, Danaus plexippus Linnaeus, are among the best known of the world's butterflies, due to their remarkable ability to migrate, wide distribution, and charismatic appearance. The last Pleistocene glaciations in North America instigated migration to Mexico in the east and to Californian coast and deserts in the west. In the western U.S., the overwintering colonies are smaller and more numerous, while in Mexico, they are few, but more spectacular, with billions of butterflies concentrating in one spot.

adults

monarch migrating

Distribution

Danaus plexippus is found throughout the Americas and Australia, with individuals reported in New Guinea and Western Europe. Sedentary populations that are found in Mexico, Central and South America (including the Caribbean islands) are somewhat different from migratory populations of D. p. plexippus found in North America. Several subspecies, such as M. p. megalippe (Mexico, southern U.S.) and M. p. menippe (South America) have been described. Monarchs fly from sea level up to 2,500 meters.

Some of the D. p. plexippus reach Cuba instead of Mexico, where they mix with the resident population of D. p. megalippe, from which they noticeably differ in behavior and wing length and shape (Dockx 2007).

adult emerging in Florida

migrating adult in Mexico

Description

Orange-and-black warning coloration of monarchs is noticeable, and its memorable pattern is directed at repelling insectivorous birds. Experiments conducted with captive blue jays showed that monarchs indeed are toxic (Brower et al. 1968). Being distasteful due to ingestion by larvae and sequestration by adults of cardenolides that are toxic to birds, monarch serves as a model for several mimetic species and is frequently confused with the viceroy (Limenitis archippus (Cramer, 1776)) and queen (Danaus gilippus (Cramer, 1775)) butterflies. That mimicry is considered to be Müllerian, with all species involved being distasteful to some degree and contributing to each others' defense (Ritland and Brower 1991).

viceroy - monarch mimic

viceroy - monarch mimic

Other than its mimics, the monarch can be confused with very few other butterflies. However, on the islands of Hispaniola and Jamaica, a smaller Jamaican monarch, Danaus cleophile (Godart, 1819), can be found flying together with the D. plexippus.

Jamaican monarch

Biology and Life Cycle

Monarchs lay their eggs singly on underside of the leaf and sometimes on the flowers of different milkweeds (Asclepidaceae: Asclepias).

egg

Link to video of egg hatching

The first instar larva is white with a black head. The larva develops a more conspicuous striated yellow-and-black coloration in the second instar. This coloration varies depending on subspecies in mature larvae. Larvae also develop two thoracic and two abdominal dorsal filaments.

first instar larva

fourth instar larva

fifth instar larva feeding

close-up of fifth instar larva

Link to video of caterpillar from egg to chrysalis

The pupa (chrysalis) is formed by the larva hanging on a substrate, such as underside of leaves and twigs, usually away from the host plant. When formed, it is green with gold markings.

pupa

Link to video of caterpillar forming chrysalis

adult emerging

Link to video of adult emerging

Development from egg to adult takes less than a month. In North America, the monarchs go through at least four generations a year, before they start migrating south in the Fall.

Adults are strong fliers and can fly for 11 hours straight. In the Fall, enough fat is stored in the adults to allow a continuous 1000 km flight without feeding. Some make a journey of a total of 4000 km to reach overwintering sites in the Sierra Madre de Oriente, where they settle inside the coniferous forest of the state of Michoacán. Monarchs also fly across Gulf of Mexico with overwater flights of 600 km.

monarch covered trees

close-up of trees

adults migrating

close-up of migrating adult

A tagging program was initiated by F. A. Urquhart of the Royal Ontario Museum in the 1950s and is continued to this day. It allowed scientists to determine the migration path of monarchs.

tagged adult

Monarchs feed extensively on the way, accumulating body fat sufficient to last them through the winter. At the overwintering sites in Mexico, the monarchs spend over four months in a reproductive diapause. They feed and drink as the weather warms up, but return to their resting sites.

adults drinking

adults sunning before feeding

adult feeding

Courtship behavior of monarchs has been described in detail and consists of a mating ritual, where male grasps the female in the air and brings her down to the ground, where mating occurs. Mating occurs several times, both during the summer and at the overwintering sites. Mating attempts frequently fail due to resisting by females (e.g., Frey, 1997).

Monarchs begin to fly north in March, reproducing along the way. Migration north continues, with the second and third generations recolonizing the continent.

The ability to navigate to the overwintering sites is genetic and is linked to time-compensated sun compass orientation. This ability requires constant recalibration of genetic program by changing surrounding. It is linked to the activity of the central complex is a midline structure consisting of protocerebral bridge and central body in Monarch's brain.

Geomagnetic forces are probably used as monarchs get closer to their overwintering sites, since the migratory monarchs' bodies contain higher quantities of magnetic material than non-migrating butterflies. The vicinity of overwintering sites in the Mexican Transvolcanic Range contains high level of magnetic anomalies, which probably helps monarchs to find them. Social behavior and pheromones probably also play large role in choosing the overwintering site.

To understand migration, the gene expression patterns that define the migrating monarchs are being investigated. Specific genes are regulated by juvenile hormone, which is responsible for interruption of reproductive behavior and initiation of migration. Micro RNAs regulate gene expression, and each miRNA can regulate several proteins. In other systems, miRNAs are involved in epigenetic developmental events. They may be involved in initiating/mediating the migratory state in monarch butterflies. The effort to assess genome of monarchs, which proves to be smaller than in other butterflies, and more similar in size to that of mosquitoes is being made (Zhu et al. 2008).

Monarchs are great model organisms for researching many general questions about animals in general. For instance, recent studies conducted on monarchs allowed for understanding the evolution and function of cryptochrome proteins in animals (Zhu et al. 2008). Cryptochromes are critical for circadian timing in butterfly's clock mechanism. In monarchs, one of the cryptochromes functions as a likely circadian photoreceptor, while another appears to function as the major transcriptional repressor of the clockwork transcriptional feedback loop.

Hosts and Monarch Toxicity

Monarch toxicity has been linked to the toxicity of the plants upon which they feed. Monarchs oviposit on milkweeds of the genus Asclepias from which the caterpillars collect the cardiac glycosides toxic to birds. These substances are passed on to the adult butterflies, which are also toxic. The idea of automimicry (that some monarchs are more toxic than others, but that the birds, unable to distinguish between the traits, avoid all of them) has been investigated (Brower et al. 1970). Apparently, there are costs and benefits to the ingestion of glycosides, since it was shown that monarch females prefer plants with intermediate cardenolide level, rejecting higher and lower level-containing plants (Oyeyele and Zalucki, 2008).

adult feeding on milkweed

Conservation Status

Monarchs are not endangered as a species due to many sedentary populations in the south of its range. However, the deforestation around their overwintering sites in Mexico puts the northeastern population of monarch and the remarkable phenomenon of migration in danger. For instance, in 2002 severe winter weather killed off an estimated 80% of Monarchs in overwintering colonies in Sierra Chincua and Sierra Campanario, with some colonies reduced in size by 90% (Brower et al. 2004).

A few years ago, the controversy over the influence of Bt corn on Monarch mortality arose. However, though mortality due to ingestion of corn pollen does occur, it has been shown that its effect on Monarch population might not be as dramatic as was initially thought (Auman-Bauer 2001).

Natural Enemies

Birds such as black-beaked orioles and black-headed grosbeaks attack monarchs at their overwintering sites. Apparently the toxins deteriorate during the migration, hence the birds can eat them. Orioles slit open the monarchs’ abdomens avoiding most of the toxins in the cuticle. Grosbeaks eat the entire abdomen and can tolerate higher levels of cardenolides. These two bird account for over 60% of the total monarch mortality. Among other vertebrates, rats have been observed feeding on monarchs at overwintering sites.

Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants. Tachinid flies and braconid wasps are known to parasitise larvae. Several entomopathogenic organisms can infect monarchs, including a nuclear polyhedrosis virus and Pseudomonas bacteria, protozoan parasites such as Ophryocystis elektroscirrha, and a microsporidian Nosema species (McLaughlin and Myers 2007).

Selected References

Author: Andrei Sourakov, Florida Museum of Natural History
Photographs: Andrei Sourakov, Florida Museum of Natural History
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-442
Publication Date: April 2009
Copyright 2009 University of Florida

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