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Nature’s View: Monarch Butterfly Migration

Ray Harden rayharden2@aol.com
Ray Harden rayharden2@aol.com

This is the time of the year when monarch butterflies migrate through Iowa. The phonological calendars state that the peak time is from September 6th through September 20th. These colorful orange and black insects are heading to their winter roosting area in the mountains of southwest Mexico.

The discovery of the monarch winter nesting ground was made on January 2, 1975 by the scientists Ken and Cathy Brugger has been called one of the greatest biological discoveries of the Twentieth Century.

The insects spend the winter in an area about half the size of Dallas County that is covered with dense stands of fir trees. Estimates have been made that the butterfly population

is more than 100 million insects in some years. One site that was a hundred yards in diameter was reported to contain thirty million of these brightly colored insects.

Chris Adkins, a naturalist for the Dallas County Conservation Department, has led monarch tagging programs with local school children for more than fifteen years at the Kuehn Conservation Area near Redfield. In 2009 forty students from Earlham Elementary School set a record when they caught and tagged 146 monarchs in two hours. Many of the tagged specimens have been recovered from various areas of the United States. In 2001 a monarch that was tagged in Dallas County by Adkins was recovered in Mexico. It had flown 1,556 miles to Cerro Pelon Mountain west of Mexico City.

Scientists believe the Monarchs know when to start their 2,000 mile migration because of the angle of the sun. Also, the small particle of iron in their brain cells serve as an internal compass that allows them to find their way. As they fly south through Iowa they feed on the blooming prairie flowers of late summer, such as goldenrod, asters, sunflowers, and thistle and at the Kuehn Conservation Area there is an abundance of their favorite flower- blue prairie sage. The nectar from the flowers provides them with the needed energy for the migration south.

The monarchs leaving Iowa in September will not return but their offspring will, usually about the second week in May when they are migrating north as far as the southern part of Canada. They follow the growth of the common milkweed- the only plant on which the monarch caterpillars feed.

Weather is always a factor that affects the butterflies. In 2005 they were blown into Iowa a month early because of a severe spring storm and many died because the flowers they needed were not in bloom. Millions of them died when an unusual snow storm and freeze occurred in their wintering grounds in Mexico. This summer Iowa’s hot dry weather dried the insects’ eggs and lowered the nectar content of prairie flowers.

The butterflies’ population fluctuates with the weather but the evidence is showing a long term decline in the numbers of monarchs and it is happening at an alarming rate. Their wintering habitat in Mexico is being destroyed; in the past two decades 59% of their roosting trees have been logged. In the Midwest the herbicide glyphosate is being used to kill the weeds in corn and bean fields but it is also killing the plants that the insects need to survive. Also, they are losing more habitat as pasture and CRP lands are being put into row crops.

Butterflies, along with bees, play a crucial role in disseminating pollen that allows for plant diversity. The same environmental factors that are detrimental to monarchs also affect other beneficial insects and potentially the entire food chain. The government of Mexico is taking steps to protect the monarch’s habitat and in the Midwest more prairies and butterfly gardens need to be planted to make sure these beautiful butterflies continue to migrate through Iowa.

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