WEATHER »

Butterfly Royalty’s Humble Beginnings

Monarchs Depend on Milkweed


If you live in Santa Barbara or Goleta, hopefully you’ve had a chance to go to the Coronado Butterfly Reserve and Ellwood Main Monarch Grove in Goleta to witness the amazing butterflies en masse.

The butterflies, of course, are Monarch butterflies, scientifically known as the species Danaus plexippus. Monarchs are part of a very old lineage of butterflies; their family (Nymphalidae) first evolved around 90 million years ago, meaning they flew around with the dinosaurs.

Monarchs belong to a subfamily of butterflies referred to as milkweed butterflies (Danainae), which includes about 300 species worldwide, four of which, including the Monarchs, are native only to North America. As the name implies, Monarch butterflies have an intimate relationship with milkweed. Not only do they depend upon it as their sole source of food, but Monarchs also use it as their defense against predators. Consequently, destruction of milkweed can threaten the grand migration, and very existence, of Monarch butterflies.

Once laid on a milkweed plant, the pinhead-sized Monarch egg hatches after three to eight days. The caterpillar eats continuously for the next two weeks, until it is about two inches long. In captivity, some Monarch breeders have been able to make these caterpillars eat others plants, but it is not recommended as they often will not develop to maturity. In the wild, Monarch caterpillars will only eat species of milkweed.

Milkweed, which encompasses all plants of the genus Asclepias, is so named for the sticky sap it secretes when cut. This fluid contains potentially harmful toxins called cardenolides; these compounds are heart-arresting. When Monarch caterpillars eat milkweed they store the toxins in certain areas of their body. This process seals the cardenolides off from harming the Monarchs, but gives both the caterpillars and adult butterflies a chemical defense against most predators; the stored cardenolides are distasteful and toxic to most animals.

To advertise their toxicity to predators, Monarch larvae display “warning coloration.” The larvae are brightly colored, with contrasting vertical bands of black, white, and yellow. However, despite being fairly poisonous, the caterpillars still have a very high mortality rate due to predation; less than 12 percent become mature caterpillars, as they are eaten by ants, wasps, spiders, cockroaches, parasitic flies and wasps, and others.

Ten days after the caterpillar has matured, and become a chrysalis, the adult Monarch butterfly emerges. Like the caterpillars, the butterflies are also quite colorful: The upper sides of their wings are tawny-orange, with black veins and small white spots. Their wingspan reaches four inches. It’s no surprise the British named these brilliant insects in reference to royalty: the “Monarch” of the butterflies.

But Monarch butterflies are best known for the epic winter migrations, which cause them to journey thousands of miles each year. While most insects living in temperate areas can withstand a long, cold winter, Monarchs cannot, so they fly to warmer locations. Throughout the year, Monarch butterflies reproduce several times, so only one of about five generations makes the migration, which is up to 3,000 miles one way. The Monarch butterfly is the only known butterfly to make such a long migration.

Every fall, the Monarchs migrate southward to winter homes which are either in Mexico or California. Millions of Monarchs in eastern North America and Canada fly to certain protected mountain locations in central Mexico. At the same time, Monarchs from the western United States go to locations along the California coast. Amazingly, year after year, they arrive at the same location their ancestors came to six generations ago! The butterflies accomplish this feat using a sophisticated internal circadian clock combined with special photoreceptors for ultraviolet (UV) light from the sun.

During the winter, Monarch butterflies huddle in spectacular, yet quite vulnerable, swarms. Monarchs over-winter from November to December, forming layers of bundled butterflies on tree branches. In California, over 300 such over-wintering areas that have been found. They are mostly along the coast in sheltered groves or forests of Monterey pines, Monterey cypresses, or, most often, eucalyptus (Eucalyptus globulus). The Coronado Butterfly Reserve and Ellwood Main Monarch Grove in Goleta is one such site. The wooded locations not only provide a moderated microclimate that protects the butterflies from strong winds, allowing them to conserve heat and energy, but also act a food source. On warm days, the mostly inactive butterflies still fly around and seek nectar from flowers, often from the flowers on eucalyptus trees.

The fact that so many Monarchs over-winter on eucalyptus trees seems a bit strange given the fact that eucalyptus was first introduced to California by humans in the late 1800s; it is a nonnative plant. Additionally, it’s thought that humans may have helped create the established Monarch migration paths by bringing milkweed plants westward across the U.S. This occasionally makes for conflicting conservation efforts, as the eucalyptus must be protected despite the fact that it is nonnative, to conserve the Monarchs’ migration routes and overwinter sites.

Despite conservation efforts, overwintering of Monarchs remains a high mortality point in their lifecycle. Although the butterflies are poisonous, they are not completely safe from predators: Some birds and rodents are known to kill and eat overwintering butterflies in Mexico, although it is mostly unknown whether the birds seen eating the butterflies are naive-eating one for the first time-or eat them regularly. Spiders, wasps, praying mantids, and dragonflies will also eat the butterflies. Additionally, many butterflies can be lost to freezing; 2002 was a particularly devastating year, as atypical freezing in overwintering sites in Mexico resulted in 250 million dead butterflies, amounting to 80 percent of the Monarchs that were resting there for the winter.

In the spring, around February and March, the butterflies that survive the winter mate and start the journey north, reproducing along the way. Monarchs in Mexico lay each successive generation of eggs a bit further north, taking three to four generations before they reach the northern United States. The butterflies in California migrate back to the Western states.

Along the way back, the butterflies must find milkweed plants to lay their eggs on. A female butterfly can lay around 40 eggs a day, for a total of 700 eggs in her lifetime. Many groups have made efforts to distribute milkweed plants across migration routes to maintain Monarch populations in an area or help them return to previously inhabited locations.

Because milkweed is so central to the life of the Monarch, these butterflies have become established in areas people introduced milkweed to, areas often far from the butterfly’s native North American territory. In the mid-1800s, the soft, fluffy white seeds that milkweed plants produce were used to stuff pillows, mattresses, and life-jackets. These articles were used by people around the Pacific islands, and consequently caused milkweed to quickly spread to eastern Australia and surrounding areas, such as New Zealand; Monarchs quickly followed, arriving in the mid- to late-1800s. Interestingly, in these non-native areas, the butterflies have adapted their migration behaviors. In Australia, the butterflies have a much more limited seasonal migration, sometimes traveling from inland to the coastal areas; in New Zealand they still overwinter in masses, but undertake no long-distance migrations.

For directions to the Coronado Butterfly Reserve and Ellwood Main Monarch Grove in Goleta, where you can see the amazing Monarch butterflies in action, check out Ray Ford’s article “Ellwood Monarch Groves.” The butterflies generally start arriving in late September, although they are most spectacular in December and January. Be sure to abide by all the precautionary signs; the butterflies must conserve strength for the mating season. They begin mating in February and March, right before dispersing to start the annual cycle over again. To watch the stunning caterpillars mature first-hand, plant some native milkweed, which is sold by many local nurseries.

For further reading on monarch butterflies, see Wikipedia’s “Monarch (butterfly),” TheButterflySite.com’s “Monarch Butterfly Website,” David Cavagnard’s article “Relief for Weary Monarch Butterflies“, the Live Monarch Foundation’s Free Milkweed Seeds website, Ray Ford’s article “Ellwood Monarch Groves,” Margaret Connell’s article “The Monarchs of Ellwood, ScienceDaily’s “How Butterflies Fly Thousands of Miles Without Getting Lost Revealed by Researchers,” ScienceDaily’s Molecular Basis of Monarch Butterfly Migration Discovered,” ScienceDaily’s Habitat Destruction May Wipe Out Monarch Butterfly Migration,” Milkweed Farm’s “Native Milkweed Catalogs” (listed by state), ScienceNOW’s “Uncovering Butterflies’ Past,” Karen S. Oberhauser and Michelle J. Solensky’s The Monarch Butterfly, and George Gibb’s The Monarch Butterfly.

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells.



event calendar sponsored by: