SciTech

Making sure monarchs return

It’s special to gently place a brightly colored monarch butterfly on a child’s nose and watch her face light up with joy and excitement as she feels the tickle of fragile legs or the brush of a scaly wing. It is perhaps even more remarkable when I place a butterfly on a teacher’s nose and watch that same sense of wonder come alive in someone who is perhaps not quite so close to youthful exuberance.

No matter who you are, having a butterfly on your nose is a magical experience.

Monarchs are the black-and-orange beauties commonly seen in North Carolina in spring and fall. Their numbers are greatest in the mountains and at the coast as they follow these geographical landmarks on their southward fall migration. These small, fluttering creatures have one of the most remarkable insect migrations in the world, annually traveling from breeding grounds in the eastern and central United States and Canada to a few small areas of forest in the Sierra Madre near Mexico City to overwinter.

Each year, the individuals that make this 3,000-mile trek do so for the first time: They are the great-great-great-grandchildren of the butterflies that migrated north the previous spring.

Generally, monarchs are relatively short-lived. During the spring and summer, each butterfly goes through a life cycle lasting about five to nine weeks – from tiny, striated egg (three or four days) to black, white and yellow-striped caterpillar (10 to 14 days) to gold-flecked green chrysalis (10 to 14 days) to orange and black butterfly (two to five weeks).

But late summer through the winter, the butterflies cease breeding, and the adults live much longer – up to nine months – making their long-distance migration to Mexico possible. After that long-lived generation overwinters and spring arrives, the butterflies mate as they prepare to leave the Mexican mountaintops. After only a short distance, the northward migrants lay eggs and pass their genes on to the next generation, which continues the journey north. It is truly an astounding feat that each year new butterflies return to the same specific locations in Mexico.

Populations declining

There is concern that this phenomenon may not survive the strain that has been placed on it, as the species is challenged on a number of fronts. Since 2006, monarch butterfly populations have declined significantly. Observers of the species have seen far fewer all across North Carolina and elsewhere in its range.

These anecdotal observations are supported by scientific data. Each December, surveys are completed of the overwintering population in Mexico. Because the butterflies are faithful to the same sites year after year, scientists know where to go to assess the population. During the past 20 years, the average amount of land occupied has been about 6 to 7 hectares (a hectare is about the size of half a football field), with a peak in 1996–97 at 20.97 hectares.

Last winter, monarch butterflies occupied only 0.67 hectares – about 10 percent of the space they typically average. That’s a tiny amount by comparison, and it is sad and a little frightening to consider. We may be running out of time to see this majestic natural spectacle.

What is causing this population decline? A key to any species’ survival is the preservation of critical habitat. For the monarchs, this means sites for breeding, feeding and resting. For many years, emphasis was placed on the protection of monarch overwintering grounds in Mexico. The oyamel fir trees, in which the monarchs roost during winter months, play a critical role in the species’ well-being. With the creation (1986) and expansion (2000) of the Monarch Butterfly Special Biosphere Reserve, steps have been taken toward maintaining a safe haven for monarchs during winter, though illegal harvesting of trees in the region remains a problem.

But new evidence suggests a far greater impact from loss of habitat in the United States and Canada.

Herbicides killing milkweed

One of the most important types of habitat for breeding monarchs is farm fields. Monarch caterpillars feed exclusively on plants in the milkweed family, and these are often found within and on the edges of agricultural land. Two major factors seem to be contributing to the decline of milkweed habitat for monarchs. Since the introduction of herbicide-resistant corn and soybeans in 1997, the use of herbicides on farm fields has increased dramatically.

Farmers are now spraying herbicides on crops during the growing season, which has essentially eliminated milkweed that formerly would have grown in and around fields. Furthermore, with high corn prices, due in part to the use of corn in the production of ethanol, marginal farmland at the edges of existing fields is being converted to agricultural use.

These margins traditionally would have been home to numerous native plants, including milkweed for monarch caterpillars. Scientists estimate that more than 150 million acres of habitat have been lost since 1996.

Some variation in population is expected as a result of annual weather fluctuations: Unseasonably cold or hot weather is not ideal for monarch migration. But if the overall decline in population is due to human factors, action on our part is needed now.

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