Like all butterflies, moths, and other insects with a lifecycle that involves complete metamorphosis, individual monarch butterflies (Danaus plexippus ) go through four stages from eggs to larvae (caterpillars) to pupae to adults. Unlike other insects, as a species monarchs also undergo an annual cycle that involves several generations and a migration that covers thousands of miles.
Monarch butterflies are native to North and South America, but have were spread by humans throughout much of the world in the 1800s. They first appeared in Hawaii in the 1840s, then spread throughout the rest of the South Pacific in the 1850s and 1860s. In the early 1870s, the first monarchs were reported in Australia and New Zealand. These different populations have adapted to their new habitats with an amazing range of behaviors, but the annual migration undergone by the North American monarchs makes them unique among insects.
It is thought that there are three distinct populations of monarchs in the United States. One breeds east of the Rocky Mountains and overwinters in the mountains of central Mexico. Another breeds west of the Rocky Mountains and overwinters on the California coast. These populations constitute separate breeding pools with little genetic exchange. A third population is found in southern Florida. This population is genetically less isolated and probably receives significant influx from the eastern population in the fall and possibly the spring.
The monarch life cycle
An individual monarch's life cycle begins when a female lays an egg on a plant in the milkweed family (Asclepiadacae). Dozens of milkweed species are found throughout North America, and most of these are suitable hosts for monarchs. Common milkweed (Asclepias syriaca ) in the northern United States and Canada, is probably host to more monarchs than any other species. It is difficult to tell just how many eggs each butterfly lays during her life, but the average female in the wild is thought to lay 300–500 eggs. In captive females average about 700 eggs over two to five weeks of egg laying. Eggs usually hatch three to seven days after they are laid.
The newly hatched larva's first food is its egg-casing, but it soon begins eating the plant on which it was hatched. Only after consuming milkweed does the larva develop the characteristic yellow, white, and black striped pattern recognized by children and adults throughout the world. Monarch larvae undergo five stages called instars, shedding their old skin between stages to allow for growth. The main activity of the larva is eating, and during the larval period of about eight to 14 days, they increase their mass about 2,000-fold.
The third stage in the monarch life cycle is the pupa, or chrysalis. When the monarch is ready to pupate, it seeks a protected location and spins a white silk pad using a spinneret located just under its mouth. It attaches itself to this pad and dangles head down. About a day later it sheds its skin for the last time and the gold-spotted green chrysalis, or pupa, is complete. This stage lasts eight to 15 days. The day before the adult butterfly emerges, its folded wings are visible through the pupa case, and it is possible to see the black, orange and white color pattern that is the monarch's trademark. This pattern is due to tiny scales that cover the monarch's wings and body. On the day that the butterfly emerges, the colors are very distinct and the pupa no longer has any green coloring.
Adults live for two to six weeks, spending their time gathering nectar from flowers, mating, and laying eggs. Four to five generations repeat this cycle throughout the spring and summer.
The monarch's annual cycle
Butterflies that emerge in late August and September put reproduction on hold. Instead of eating, mating, and laying eggs, the adults fly south. Individuals in the eastern North American population, live up to nine times as long as their spring and summer counterparts. They travel to old growth oyamel fir (Abies religiosa ) forests high in the transvolcanic mountains of Michoacan, Mexico. West coast monarchs also begin a migration that will end in the eucalyptus and Monterey pine (Pinus radiata )groves of Pacific Grove, Santa Cruz, and Fremont on the Central California coast.
Starting in early November, the monarchs form compact roosts thickly covering the trunks and branches of hundreds of trees. They remain in a nearly dormant state until the temperatures warm in February, when they become more active, mating, and seeking water and nectar. In mid-March, they fly north to look for the milkweed plants in order to produce new generations of monarchs that will continue the migration cycle. Recolonization of the summer breeding grounds is a two-step process. Monarchs that have overwintered make part of the return trip. Then they lay eggs and die. Their offspring continue the journey north until the breeding range is reoccupied by late May or early June.
Because of their incredible yearly migration cycle, monarchs depend on resources in widely dispersed locations, making them especially vulnerable to disruption of their habitat . In their breeding range throughout the United States and southern Canada, they need fields, roadsides, gardens, and prairies filled with milkweed for larvae and nectar sources for adults. During their fall migration monarchs need safe flyways and nectar sources to fuel their long journey. Finally, the migratory generation requires an intact habitat in their wintering one that will protect them from temperature and humidity extremes during their winter rest. Just as with migratory song birds, preserving the monarch's migratory life cycle requires international interest, knowledge, and cooperation.
Monarch population size
There are no reliable estimates of monarch population size during the breeding stage of the annual cycle. However, estimates of absolute numbers during the overwintering period exist. During the winters of 1985 to 2002, researchers estimated that overwintering monarch densities are approximately 10 million butterflies per hectare, with 6–12 hectares of land used for roosting. However, estimates of monarch mortality during a major winter storm in 2002 suggested that this estimate is too low, and that densities may be more like 50 or 60 million monarchs per hectare of occupied forest.
Sources of monarch mortality
About 90–95% of monarch eggs never reach adulthood, and predators, milkweed defenses, and environmental conditions can all cause mortality during the immature stages. Predators and environmental conditions can also kill adults. Habitat destruction limits the number of monarchs reaching maturity during the summer breeding period, and also affects the number that survive the overwintering period.
Invertebrate predators such as insects and spiders, and diseases caused by bacteria, viruses, fungi , and other organisms kill many monarchs in natural populations. Invertebrate predators of monarchs include both native and introduced flies and wasps that lay eggs in living larvae or on the leaves that larvae eat. The wasp or fly larvae that emerge from these eggs ultimately kill the monarch larvae.
Monarchs accumulate chemicals called cardenolides (also called cardiac glycosides) that are present in the milkweed. These cardenolides are poisonous to most vertebrates, and monarchs face little predation from frogs , lizards, mice, or birds during the adult breeding stage. However, vertebrate predation is a major source of mortality for overwintering monarchs. Both birds and mice consume significant numbers of overwintering adults.
The milky latex present in milkweed leaves is contained under pressure in a system of vesicles. When the plant is punctured, the latex flows out of these vesicles and coagulates on contact with air. The latex serves as a defense to the plant, since it is similar to glue after coagulation. It can kill small larvae by sealing their mandibles or gluing their entire body to the leaf. While monarch larvae can cut off latex flow by trenching the leaves, about 25% of first instar larvae die after becoming mired in the latex of some milkweed species.
Monarch eggs do not hatch in very dry conditions. Dry weather can also reduce the population of monarch by killing milkweed and reducing the amount of nectar in flowers. Very hot weather also causes mortality. Studies have shown that prolonged temperatures above approximately 95°F (35°C) can be lethal to all stages. Likewise, temperatures below freezing can kill monarchs. All stages can survive if temperatures are only a few degrees below freezing for short periods, but extended periods of severe cold and wet kill them. In the overwintering colonies, the highest mortality occurs when adult monarchs become wet from rain or snowfall, then are subjected to freezing temperatures. Since temperatures extremes are likely to be greater in forests that have been thinned, logging can exacerbate the effects of winter storms.
The most important source of human-caused mortality in the breeding range of monarch is habitat loss, especially the destruction of milkweed and nectar sources. Milkweed is considered a noxious weed by some people, and is often destroyed. Herbicides used in agriculture and on roadsides and lawns kill both nectar plants needed by adults and milkweed needed by larvae. Monarchs are also exposed to insecticides used to control insects such as mosquitoes. Research conducted in 2001 found that most monarchs in the upper Midwest probably originate in agricultural fields, especially corn and soybean fields. This means that any agricultural practices that affect milkweed abundance or monarch survival in these fields could have large impacts on monarch populations.
There is also concern that genetically corn containing Bt toxin is a hazard to monarchs. Pollen produced by this corn contains the toxin. Pollen may be blown onto milkweed plants growing in and near cornfields. Since many monarchs that migrate to Mexico breed in the corn belt of the central United States, and milkweed growing within cornfields constitutes an important food source for these monarchs, exposure to this risk is likely to be high. The most commonly used Bt strains express levels of toxin in their pollen that are unlikely to kill monarch larvae outright. Sublethal impacts have not been closely studied.
[Karen S. Oberhauser Ph.D. ]
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