The familiar Monarch Butterfly is one of the most common butterflies in Florida.
The Monarch Butterfly (Danaus plexippus) is found worldwide, in six subspecies. With its bold markings of black stripes on an orange background, it is probably the most familiar butterfly in the United States. On the Hawaiian island of Oahu there is a white color morph that makes up about ten percent of the population; it is also found in Australia, New Zealand and Indonesia. In Florida there are two subspecies: the D. p. plexippus is, like most of the population in the US, migratory and travels to Mexico each winter, while the D. p. megalippe, which ranges from Georgia through Florida to the Caribbean, is a permanent year-round resident. The Southern Monarch, ranging across South America, is considered to be a separate species D. erippus, and the Jamaican Monarch is classed as the species D. cleophile.
The Monarch is best known for its annual winter migrations—the longest known for any insect species. There are two distinct migratory paths. Populations east of the Rocky Mountains head south to Mexico, gathering by the millions in the Mariposa Monarca Biosphere Reserve. A small proportion of this population also overwinters in Florida. West of the Rockies, the butterflies overwinter in sanctuaries in Arizona and California. In spring, the butterflies disperse back to their home ranges. This migratory process is at least partly genetic. Captive-bred butterflies are capable of migrating to their winter quarters, and while the individuals who overwinter in Mexico do not live long enough to reach their home grounds in the eastern US, their succeeding generations are able to find their way back even though they have never been there before. Recent DNA studies have identified genes in the butterflies which seem to be important in navigation and migration: these genes are present in both migratory and nonmigratory populations but are “turned off” in the nonmigratory butterflies.
Like many butterflies, the Monarchs are very specific in their choice of host plants. The eggs are laid, one at a time, on Asclepias species of Milkweed plants during the spring and summer. These hatch into a tiny pale green caterpillar. As the caterpillar feeds on the Milkweed, it is able to extract cardenolide toxins from the plant sap and store these in its own body. The stored toxins make the caterpillar taste unpleasant to birds, and the predators quickly learn to avoid both the caterpillars and the adult butterflies. The young caterpillars, once they have begun to store the toxins in their body, no longer need their newborn camouflage, and turn from an inconspicuous pale green to a brightly-colored pattern of black, white and yellow bands, which warn the birds of their unappetizing taste.
After about two weeks, the caterpillar, now about two inches long, finds a suitably protected place, hangs from a silken anchor, and molts to form a chrysalis. These jewel-like cases are pale green with a gold band and gold dots. Inside, the caterpillar pupates and transforms from a larva into an adult butterfly, emerging after a week or two. Unlike the caterpillars who only feed on Milkweed, the adults can sip nectar from a wide variety of plant species. Once they emerge from the chrysalis, however, Monarchs only live for a few weeks—long enough to find a mate and lay eggs.
The adult Monarchs contain the same cardenolide toxins as the caterpillars, and their bright orange and black markings are a warning to birds, who learn not to eat them. The Monarch’s chemical protection is also taken advantage of by the unrelated Viceroy Butterfly, which lacks any toxins but has evolved to mimic the Monarch’s warning colors, thereby gaining protection too.
Although the Monarch Butterfly is one of the most widespread species in the US, its population has been measurably declining. Some of this is due to the loss of Milkweed habitat because of the destruction of wild fields and the use of herbicides on agricultural fields. The migratory populations are dependent upon a limited number of overwintering grounds in Mexico, and these have been reduced and damaged by illegal logging activities. The Mexican Government is making efforts to protect and expand the wintering areas, and the US Government has begun the process of listing the Monarch under the Endangered Species Act. The Canadian Government is also taking steps to protect its Monarch populations. In many areas, homeowners are being encouraged to plant patches of wild Milkweed in their lawns and gardens to serve as food sources for Monarch, and many state highway departments are now planting butterfly food plants along roadways and median strips.
Because Monarch Butterflies are attractively-colored and breed readily, they are often captive-bred in “butterfly gardens” by science museums, school groups, and individual fanciers. Some institutions mass-breed and release thousands of Monarchs at once. This is a controversial practice that is opposed by many entomologists since it can lead to the spread of parasites from crowded captive populations to the wild. Most of the educational butterfly gardens do not release their butterflies to the wild, but keep them permanently inside enclosures where visitors can see them.