When the cloud of dust clears, the door of the four-wheeldrive swings open. “Señor, for two hundred pesos I will take you to the butterflies,” it sounds from inside the car. The driver beckons invitingly and takes a sip from the can of beer in his hand. Hearing that I prefer to keep walking, the wagon continues its trip, growling dangerously, to the high reservation where millions of Monarch butterflies gather every winter.
Monarch butterflies are found throughout much of the Americas, from Peru to southern Canada. In North America you find them only in summer; in autumn they migrate south where they overwinter in large groups. Monarch butterflies from western North America overwinter in California, where they gather in sheltered places along the coast.
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The Monarch butterfly
The monarch butterfly (Danaus plexippus) gets its name from its majestic size. Adults can grow to about five centimeters and have a wingspan of up to ten centimeters. The butterflies look beautiful with their rust-brown wings topped with dark-colored veins. The black wing edges are covered with white spots – a typical characteristic of butterflies of the genus to which the monarch butterfly belongs.
For monarch butterflies from the central and eastern United States and Canada, however, the Rocky Mountains impede their passage to warm California. A small proportion of them winter on the Gulf of Mexico, but the majority migrate to the wintering grounds in central Mexico. It has long remained a mystery where the millions of monarch butterflies from central and eastern North America spend the winter.
By tagging monarch butterflies, it was hoped to determine the migration route. One of those concerned with this was Canadian biologist Fred Urquhart. In 1975, two of his collaborators discovered by chance the wintering habitat on the steep southwestern slopes of the Sierra Madre, nearly three thousand meters above sea level. Until then, it had been known only to local people.
An hour and a half after leaving Angangueo, a small mining town in the Sierra Madre about a hundred kilometers west of Mexico City, I reach a windswept plateau: the entrance to the reserve. Everywhere on the ground are dead monarcas, as the monarch butterflies are called here. A man with a tawny face approaches me and offers himself as a guide. A moment later we are standing in front of a group of trees completely covered with monarch butterflies.
There must be tens of thousands, maybe hundreds of thousands. Like fish scales, the animals cling to the tree trunks. Branches bend deeply under the weight of the monarch butterfly clusters hanging from them. Sometimes even one breaks off, says the guide. The forest offers the butterflies protection from wind, snow and frost. Because of the forest’s cool climate, the butterflies’ metabolism is at a low ebb, so they hardly draw on their fat reserves.
The fog and clouds that often hang around the mountain slopes ensure that it is moist enough. The butterflies stay densely packed on oyam pine (Abies religiosa). Why the insects do so is unknown. Nor is it known why they return to the same trees each winter. If they accidentally fall to the ground, they crawl into bushes and onto young shoots to escape attackers.
A little further on, an orange-colored carpet covers the ground. A thin layer of water glistens between the stones. The butterflies feast en masse on the moisture. While hibernating, they eat little, but do drink water occasionally. Because they are cold-blooded animals, they must warm up sufficiently before they can become active. In sunny spots in the forest, you can see thousands of butterflies flying up at once. The guide picks up one and carefully spreads its wings open.
“Macho, a male,” he says, pointing to the black spot on the hind wing. The spot – in reality a small sac – contains a pheromone: a sexual lure with which the male tries to attract females. The basis for the pheromone is pyrrolizidine, a toxic substance found in the honey of certain flowers (especially crucifers). With two hair tufts on their abdomen, the males occasionally stroke the pouch to moisten it with the pheromone.
As temperatures rise in late winter, monarch butterflies become increasingly active. They fly further down the mountainside and mate with each other. In mid-March they leave the wintering grounds and fly toward the northeast. The time of departure listens closely. If the butterflies leave too early, there are few silk plants left (on which the monarch caterpillars feed) and the females cannot deposit their eggs.
The females deposit the eggs on the underside of the leaves of the silk plants to protect them from the sun. After four days, the caterpillars crawl out of the eggs. They have distinctive yellow, black and white bands on their bodies and carry two fleshy protrusions on their head and abdomen, which serve as a sense of touch. The caterpillar stage lasts between 13 and 22 days, depending on temperature. The final transformation, pupation from caterpillar to butterfly, takes an average of two weeks.
The new generation of monarch butterflies lives for only about six weeks, much shorter than their overwintering counterparts, which sometimes last as long as eight months. About two to three more generation changes follow in fairly quick succession. The newly pupated butterflies continue their journey north, spreading throughout the United States and southern Canada. As the end of summer approaches, the monarch butterflies gather in large groups at night. Preparations begin for the long journey south.
Before that happens, the butterflies must have plenty of fuel. They stuff themselves with nectar, which they store in their bodies in the form of fats. They must rely on this fat supply until spring. During migration, the butterflies therefore stop occasionally to refuel with nectar. Like migratory birds, monarch butterflies fly low above the ground when the wind is against them and high in the air when the wind is with them.
They orient themselves to the position of the sun. The landscape, a river valley or mountain ridge for example, can also influence the direction of flight. In addition, there is evidence that butterflies are sensitive to the Earth’s magnetic field, as has been demonstrated in other animals. Some other migratory butterflies, such as the atalanta and the thistle butterfly, are known to be able to fly at night as well. They reach the wintering grounds in central Mexico in late October.
Despite the huge number of monarch butterflies that gather annually for migration – some estimates speak of one hundred million – the species is getting worse. One of the main reasons for this is that the number of silk plants has declined sharply. Silk plants serve as host plants for the monarch butterfly; the females deposit their eggs on them and the caterpillars eat the leaves.
These perennial plants used to occur in large numbers on the prairies of North America, but due to current agricultural practices, they are less abundant today. Many silk plants are being killed by herbicides. In addition, many butterflies and caterpillars die as a result of pesticides. California wintering grounds are also under severe pressure due to urbanization.
Yet measures are also being taken to protect monarch butterflies. In 1984, conservationists established the Monarch Project to protect California’s overwintering sites. Efforts include getting landowners to set aside part of their land for the butterflies. They are also trying to influence legislation and new infrastructure plans so that the butterflies are better protected.
Most dire is the situation in the wintering grounds in Mexico. The wintering area here is very small, consisting of only eleven to fourteen sites, often covering an area of only a few hectares. However, millions of butterflies do gather there. This combination, a high concentration of butterflies in a small area, makes the wintering area in the Sierra Madre especially vulnerable.
Monarch butterflies gather in huge numbers every year on the same oyam pine trees in the wintering area. How subsequent generations manage to find these trees so accurately is a mystery.
One of the main problems facing conservationists in Mexico is logging. Oyam pine is a valuable timber species, and large tracts of forest in the wintering area are falling prey to logging. The open patches created by logging have the same effect as a tear in a winter coat: wind and cold can enter undisturbed. Mass mortality of monarch butterflies will therefore become increasingly common, researchers believe.
In 1992 and 1995, for example, it is estimated that between five and seven million butterflies froze to death after severe snowstorms in the wintering grounds. Still, there are initiatives to protect butterflies even in Mexico, often in cooperation with foreign conservation agencies. In 1985, five butterfly reserves near Angangueo were granted protected status. Whether this will be enough to sustain the annual arrival of monarch butterflies over time remains to be seen.
The economic importance of logging is great – even ín the reserves, logging has not yet been completely eliminated. Perhaps tourism can offer an alternative. Some Mexican landowners now transport tourists to the reserves instead of cutting down trees. Although they sometimes have to face the fact that not everyone wants to take advantage of this.
Toxic corn pollen
Three American entomologists recently discovered that pollen from genetically engineered corn plants is toxic to caterpillars of the monarch butterfly. The caterpillars can ingest the corn pollen by eating silk plants, which often grow near corn fields. Corn pollen can be dispersed by the wind at least sixty meters away.
The engineered corn plants are equipped with a gene from the bacterium Bacillus thuringiensis (Bt). This makes them produce a deadly toxin against the corn borer, a caterpillar that can be a major crop pest. In the Netherlands and Belgium, the corn borer does not occur and Bt corn is not applied. However, in the United States, a quarter of the corn fields (eight million hectares) are already planted with Bt-maize, and this percentage is expected to increase further in the coming years.
Nearly half of North American monarch butterflies spend summers in the corn belt, the midwestern part of the United States where much corn is grown. The toxicity of the engineered corn pollen came to light in a laboratory experiment. In it, researchers put monarch caterpillars on three different diets: silk plant leaves coated with Bt-maize pollen, leaves with regular corn pollen and leaves with no pollen. The caterpillars fed Bt-maize pollen ate less, grew slower and, in addition, after four days nearly half had died.
The other caterpillars were all still alive at that time. Until now, it was assumed that the engineered Bt corn, which was approved in the United States in 1996, is not harmful to organisms that do not threaten corn production. The debate over the dangers of genetic engineering of agricultural crops has now revived in full force. Incidentally, the use of insecticides as a pesticide against the corn borer is also harmful to the monarch butterfly.
Caterpillars of the monarch butterfly feed on plants of the silk plant family (Asclepiadaceae). By eating the leaves, monarch caterpillars ingest toxins (cardiac glycosides). The toxins remain present in the exoskeleton of the monarch butterfly. They themselves are not affected by this, but in birds or mice that eat the caterpillars or butterflies, the toxins cause cardiac disorders.
The monarch butterfly is an aposematic butterfly species: it betrays its toxicity through its orange-black colored wings. The caterpillar does the same with the distinctive black, white and yellow bands on its body. For other animals, this is the signal to avoid monarch butterflies. The color pattern has universal validity in nature.
The Canary Islands, for example, are home to monarch butterflies that deposit their eggs on host plants that are not poisonous. Even though this makes the butterflies and caterpillars harmless to birds, their appearance still deters predators. Some other butterfly species take advantage of this.
The North American viceroy butterfly (Limenitis archippus archippus) is very similar to the monarch butterfly. Although the viceroy butterfly is not poisonous, most animals ignore it. They confuse the butterfly with its venomous doppelganger. This imitation of a poisonous species by a harmless one is called mimicry.