The Monarch Butterfly, Danaus plexippus is one of the world's most famous butterfly species and is another example of a migratory insect. The monarch butterfly is best known for its annual mass flights between Southern Canada and overwintering sites in California and Mexico. Each monarch migration event can involve one to three billion individuals and occurs over multiple generations. Each fall, adult butterflies make the southward trip to overwintering sites, but the return journey is completed by their descendants. Monarch butterflies are unable to survive the cold winters of the northern and central regions of North America. This is why unlike locusts, migration in monarch butterflies is a regular event triggered not by high population densities, but by changing photo periods that precede winter temperatures. This migration allows monarchs to avoid lethal winter conditions and track their host plants, milkweeds. Milkweeds are the only plants that monarch caterpillars feed on. Adult monarch butterflies on the other hand, feed on nectar from many types of flowers in addition to milkweed. Shortening day length induces the Oogenesis-flight syndrome in migratory females who don't produce eggs before migration. Migrating monarch butterflies are therefore in a state of reproductive diapause which is achieved by suppression of juvenile hormone production in the migrating adults. One amazing thing about migrating monarchs is that not only do they locate the same overwintering habitats as their ancestors, but they often return to the exact same tree. Upon arrival at overwintering sites, the adult migratory butterflies remain largely inactive for approximately 4.5 months until the following spring. These overwintering adults generally roost together to maintain favorable microclimatic conditions within the cluster just as penguins in the antarctic cuddle together for warm. The delay of reproduction and winter inactivity that accompanies the southward migration allows the butterflies the ability to persist through the winter season at overwintering sites. Stored fat reserves provide essential energy for migration. However, they are not enough to last the entire trip. So monarch butterflies must feed heavily during the migration. In fact, they often arrive at the overwintering sites with more stored fat than where they started, which also contributes to survival of the dormant adults as they overwinter in the south. During the winter, some individuals may arouse briefly to drink and feed on nectar, but usually return to the cluster within a couple of hours. Before the return migration, the monarch butterflies gradually become more active, reproductive diapause ends, and oogenesis, or egg production, begins. The individuals will mate before initiating the trip north. On the way, these monarch butterflies lay eggs on the new leaves of milkweed plants found in regions north of the overwintering sites and die shortly after. The first generation of caterpillars will feed on milkweeds, complete their development, and continue northwards as winged adults before stopping to reproduce. The entire northward migration can take between two and five generations to complete. What makes monarch migration so fascinating is that we still don't know exactly how they navigate so precisely between breeding and overwintering sites. Remember, unlike migratory birds, individual migrating monarch butterflies do not return to overwintering sites. Instead, it is their descendants that make the next trip south. These individuals are able to locate places visited by their ancestors that they themselves have never been to in their lifetimes. How do they do this? Well the research suggests that several potential navigational mechanisms are used in monarch migration. One proposed explanation is genetic memory. This suggests that new generations of monarch butterflies are born with memories incorporated into their genomes allowing individuals to recognize and follow landmarks despite having never been exposed to them before. While recent research shows aversions to certain stimuli may be heritable, there's little evidence to support the idea that entire maps are genetically transferred into the brains of offspring. Another proposed method of navigation with greater scientific support is a sun-based compass, whereby the butterflies orient themselves relative to the position of the sun in the sky. This mode of navigation requires a form of circadian rhythm or at least some form of internal timing that allows the butterflies to correct their flight orientation and maintain a correct bearing as the sun's location changes in the sky throughout the day and over the course of the seasons. Air currents may also play a role in monarch navigation. Monarch butterflies take advantage of southward moving cold fronts to aid southern migrations in the fall. With the help of tailwinds from these jet streams, individual monarchs can travel distances exceeding 1,000 kilometers without needing to refuel and build up fat reserves. In fact, monarch butterflies can fly at altitudes over 3,350 meters above ground for up to 12 hours covering distances of up to 640 kilometers a day. Other suggested navigational mechanisms include orientation by magnetic fields and chemical markers left behind on plants by individuals from previous generations. Geographical features such as mountains may also influence flight direction by funneling monarch butterflies in specific directions. The monarch butterfly is a well known conservation symbol in North America. The overall population numbers and the geographical distribution of the species worldwide however, provide no evidence that the species is endangered. So why does this lepidopteran garner so much attention from conservation groups? The major threat to monarchs is not a decline in overall population size, but a threat to their migratory success due to the destruction of their migratory and overwintering habitats. Populations of migratory monarchs have experienced declines and there have been significant losses to their habitats with no signs of decline in non-migratory populations. As monarch butterfly migration is such a stunning example of insect migration unrivaled by anything seen in other insect species, it is a biological phenomenon that warrants conservation attention. The threat to monarch butterfly migration results primarily from the destruction or alteration of overwintering sites in both Mexico and California and the reduction of host plants along migratory corridors. This is due to a combination of factors such as urban development, large scale logging operations, and farming practices. As forests are thinned, colonies of overwintering monarchs are exposed to unfavorable environmental conditions which increase the chances of mortality. The reduction in available habitat also limits the ability of the monarch colony to respond to environmental changes by shifting their location as they have done historically. In addition to the effects of habitat loss at over-wintering grounds, recent research suggests that the use of herbicides to eliminate their host plants, which are weeds in agriculture and other managed lands, has impacted monarch butterfly survival and migration success. Extreme weather events have also contributed to declines in monarch populations as have unseasonably low or high temperatures. One extreme storm event was estimated to have resulted in 50 percent mortality of overwintering monarchs for that year. The frequency of such extreme storms is predicted to increase with climate change. The monarch butterfly is not just charismatic and aesthetically pleasing, it is also an ecologically important organism. Like most butterflies, adult monarchs are important pollinators of flowering plants. Habitat loss, herbicide use, and changing climate in recent years have stacked the odds against the migratory populations of these insects and have reduced the migratory success and survival of these populations. However, monarch butterflies are not doomed to face extinction. Multinational cooperation has resulted in a conservation plan to protect their overwintering habitats and ensure the necessary abundance of larval food plants is available to support migratory monarchs. These efforts, in combination with large scale monitoring efforts based on citizen contributions, has brought monarchs to the forefront of North American conservation. As an insect with great aesthetic appeal, the monarch butterfly has become one of the flagship symbols of conservation in North America. Efforts to conserve the spectacular overwintering monarch colonies and overwintering habitats have raised awareness for conservation across the continent. Sometimes, we conserve animals because we appreciate their beauty and their unique behaviors. The enjoyment we obtain from having these animals around is reason enough for conservation. Additionally, butterflies are important pollinators for many plant species that are critical to ecosystem function. Monarch caterpillars are not typically used in pest or weed control, although other insects can be used for this as we will see in a later module. You can contribute to monarch conservation as well, or maybe even that of other native pollinators in the vicinity of your homes. Consider planting native nectar plants which both the monarchs and other pollinators feed on in your garden. Garden organically to reduce the impact of synthetic chemicals on butterflies and other pollinators. Consider becoming a citizen scientist and help monitor monarch populations in your area. Finally, you can also pass on the message, educate others about the importance of conserving ecologically important insects like the monarch butterfly by telling them about the roles these incredible organisms have in our daily lives.
