NATURAL HISTORY

MONARCH BUTTERFLY } Danaus plexippus plexippus

FAMILY: Nymphalidae

DESCRIPTION: The monarch is a large orange butterfly that flies with its wings held in a “v” shape. The wings have prominent dark veins and two rows of white spots at the edges, and the body is dark. The wingspread ranges from 89 to 105 millimeters (about 3.5 to 4 inches). Males are bigger than females and have a visible dark spot over a vein on their hind wings.  

HABITAT: Monarch caterpillars need milkweeds as host plants, which provide toxins to protect them from predation. Adults feed on nectar from a variety of flowers. Monarchs roost in trees, often near bodies of water. Monarchs in northern states roost primarily in conifers and maple trees, while monarchs in the south commonly roost in pecan and oak trees. Monarchs overwinter in forests, forming clusters of butterflies in a state of diapause, which is like hibernation. The forests provide protection from wind and storms and exposure to dappled sunlight to keep the butterflies warm enough not to freeze but cool enough not to break diapause and deplete the fat reserves they need to survive the winter. In Mexico, monarchs overwinter primarily in oyamel fir trees, and in California they cluster in gum, pine, cypress and sycamore trees.

RANGE: The vast majority of the world's monarchs migrate from Mexico to Canada, breeding in the United States and southern Canada during summer, and overwintering in Mexico. A smaller population breeds west of the Rockies and overwinters on the coast of California, though this population intermingles with the larger eastern population. Small, nonmigratory peripheral populations have been introduced into many areas outside of the continental United States since the 1800s.

MIGRATION: Monarchs are known for their spectacular multi-generational migration from Mexico to Canada. Butterflies fly north from inland Mexico beginning in late February and following the progression of blooming milkweed as spring and summer progress. They start laying eggs on fresh milkweeds in northern Mexico, Texas and southern Oklahoma, and to a lesser extent, Louisiana, Florida and other Gulf states, generally between mid-March and the beginning of May. In late April the first-generation butterflies — offspring of the migrants from Mexico — continue to move north, laying eggs throughout the mid-South into the Midwest and North.  Then in June, the main colonization of the Midwest and North takes place with two to three more generations being produced there before migration south begins in August. Monarchs west of the Rockies mostly migrate to sites on the California coast to overwinter, though some fly to Mexico.

BREEDING: Breeding adults first mate a few days after metamorphosis. Females lay eggs on milkweed after mating. During an average summer in North America, several generations of breeding butterflies will be produced. Monarchs in the fall migratory generation go into reproductive diapause for the winter. Spring and summer generation monarchs live from two to five weeks, and winter generation monarchs live from five to nine months.

LIFE CYCLE: Eggs take 3 to 8 days to develop and hatch into caterpillars. Caterpillars take 9 to 14 days to go through five instar stages before pupating. The first instar larva, just out of the egg, is solid pale green and translucent. The second instar larva develops a characteristic pattern of white, yellow and black transverse bands, and the tentacles start to grow. The third instar larva has more distinct bands, particularly on the abdomen. The fourth instar is characterized by a new banding pattern on the thorax, and white spots on the prolegs near the back of the caterpillar. The fifth and last instar larva has a more complex banding pattern and white dots on the prolegs. The body mass of fifth-stage caterpillars has increased about 2,000-fold from first stage instars. Larvae must eat constantly to ingest enough milkweed to increase in mass so dramatically within a few weeks. Larvae in the final stages of development stop feeding to search for a location to form a pupa, or chrysalis — the last stage of development before the emergence of the adult butterfly. The fifth stage larva attaches itself securely to a chosen leaf or branch with a silk pad, latching on with its hind legs and hanging down. The larva then molts to reveal an opaque, blue-green chrysalis adorned with gold dots. At normal summer temperatures, adult morphology develops within a few weeks. The cuticle of the chrysalis becomes transparent and the monarch's characteristic orange and black wings become visible. At the end of metamorphosis, the adult emerges from the chrysalis, expands its wings and flies. Monarch metamorphosis from egg to adult takes as little as 25 days during warm summer temperatures, and up to 7 weeks during cool spring conditions.

FEEDING: Adult monarchs gather nectar from many different kinds of flowers, but monarch caterpillars only eat milkweed leaves. Milkweed provides toxic compounds to protect the monarch from predators.

THREATS: Monarchs are threatened by the loss of milkweed in the Midwest due to spraying of Roundup (glyphosate) on genetically modified crops that has nearly wiped out milkweed from the core of the butterfly's range. They are also threatened by sprawl, global climate change, pesticides, development of their overwintering groves in California, and logging of their overwintering grounds in Mexico.

POPULATION TREND: Monarchs across North America have declined by more than 80 percent over the past 20 years. In the mid-1990s there were approximately 1 billion monarchs, but as of winter 2013-2014, the population had declined to 35 million. The annual "overwintering" count of monarchs released in February 2017 confirmed that butterfly numbers had fallen by 27 percent from the 2016 count, indicating an ongoing risk of extinction for this butterfly.

Monarch butterfly photo courtesy Flickr/Debbie Long.