In early August, Canadian monarch butterflies, Danaus plexippus, start southward. Butterflies in New England finish laying their eggs and breathe their last. The new caterpillars race aginst time to grow, become chrysalids, and metamorphose into brilliant orange and black adults.
In September, monarchs following similar routes aggregate like water droplets into small groups, the groups into bands, the bands into a steady stream of butterflies moving daily along the coast of New England, the Atlantic, and central states until they form a colorful, flickering river of wings traveling toward a few mountains southwest of Mexico City. Butterflies often cover entire trees at night.
They often stop for a few days to feed at large flower beds. Inclement weather will cause them to stop. But as soon as the sun warms their bodies in the morning, they are on their way again. The journey will not end until January when the butterflies reach the Oyamel fir trees that will be their home for the winter. Here, millions of butterflies overload the trees, often bending them toward the ground. Two or three months later, they begin the return journey north to the milkweed laden plains of the United States and Canada.
The Return Trip
After having their wings tattered traveling south and losing weight over the winter, the monarchs that flew to Mexico are not strong enough to return to Canada. They struggle northward to northern Mexico, Texas, and Oklahoma, mate, lay eggs, and die. Their children feed rapidly on the new crop of milkweeds and are soon moving northward to the middle states where they mate and die. The grandchildren of the Mexican migrants finally reach the northern central states, Canada, and New England. These also mate and die. Thus, it is the great or even great great grandchildren of the Mexican migrants that again fly south when autumn comes.
Butterflies Are Tagged
The University of Kansas (UoK) and other research centers place tags on the wings of the butterflies as they migrate. When butterflies that are already tagged are captured, the tag number is recorded and sent to UoK so they can more closely plot the path these sturdy insects take. Volunteers travel down to the Oyamel forests in Angangueo Mexico to pick up tags from dead butterflies and send the information back to UoK. Native Mexicans that live near the forests also pick up tags and supplement their income when they sell the tags to visiting researchers.
Why Do Butterflies Migrate?
No one knows what makes the monarch migrate into the northern lattitudes of America, but we do know why they migrate into Mexico: cold weather kills all life stages of the monarch and unfortunate stragglers freeze to death. As in birds, the trigger for migration is the shortening day length that occurs in the autumn. Reduced day length triggers hormonal changes that prepare the butterflies for migration. The butterflies produce large fat bodies for energy; most migrating monarchs enter a state of reproductive diapause (dormancy) and do not mate or lay eggs during their journey south. Those few that do mate while migrating produce eggs that are probably doomed to die during the first night with a deep frost.
Migration is Fraught With Danger
Migration is energetically exhausting and the butterflies need to recharge their fat reserves frequently. If their food sources vanish, they may starve by the thousands. Hurricanes often blow them hundreds of miles off course, requiring the butterflies to spend much more time in migrating – often over territory that may be flooded or otherwise inhospitable. The butterflies’ wings may become torn from being blown into objects by strong winds. Early frosts may overtake and kill them as they roost overnight. Severe weather can prevent them from refueling before they use up their energy reserves. Crossing busy highways takes a large toll. Snow storms in the mountainous Mexican refuges, as occurred in 2004, can kill tens of millions of Monarchs in a single night. Predators may eat them during migration or in their wintering refuges.
Who would eat a monarch?
Milkweed produces poisonous cardiac glycosides that can trigger heart attacks, block digestion, or cause animals feeding on the plants to vomit. Monarch caterpillars eat milkweed and store these bitter poisons in their fatty tissues. Thus, the monarchs taste bitter to those that eat them. But not all milkweeds are equally toxic. Milkweeds have several genetic strains, and some of these produce far fewer poisons than others. Monarchs that are reared on less toxic milkweeds taste sweet. We often find monarchs with triangular scratches on their wings made by birds as they test the butterfly’s blood for bitter compounds. Some birds (such as grossbeak species), mice, and insects (assassin bugs and praying mantises) have developed mechanisms to prevent them from being harmed by the glycosides and may consume several butterflies in a day. Mantids eat the body of monarchs, dropping the wings (that are almost void of nutrition) to the ground.
Plants Are Being Provided For Migrants
Milkweed is necessary for monarch larvae and myriads of pollinators as well. UoK has initiated a program to aid butterflies, pollinators, and hummingbirds by enlisting schools, businesses, and other interested people in planting nectar plants along the migration paths. Many plants serve this purpose. They don’t have to be just milkweeds: butterfly bush (though invasive), liatris, autumn splendor, Echinacea, and asters. The animals depend on these plants. Secondarily, dragonflies depend on the migrant pollinators to serve as food for their own migrations.
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