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Monarch Migrants

Filed under: Bugs — Lowell Christie -- December 15, 2006 @ 5:42 am

Monarch ButterfliesJust how far can a butterfly fly – assuming it has the urge? Down the block? Across town? All the way to the county line? Much, much farther if you happen to be a monarch – up to 3,000 miles.

Like Snowbirds, they fly south, hundreds of millions of wings filling the air. We’ve watched them flutter along the Massachusetts shoreline, and we’ve seen pine trees hung with living lace where they have settled to roost overnight. We’ve accompanied them across the Nevada desert, asking why they couldn’t find a less arid route. It’s little wonder that the monarch migration is one of the world’s great natural events.

Actually, the monarch is the only butterfly to make this massive, species-wide migration. Others may do a certain amount of moving about in search of food, but no one else in the insect world follows as predictable a path to as certain a location. Why do it then?

What follows is still speculation, but it’s informed speculation by lepidopterist Lincoln Brower, zoology professor at the University of Florida. He points out that most butterflies in the subfamily Danaiinae (to which the monarch belongs) are tropical, and cannot survive a cold winter – not as eggs, larvae, pupae, or adults.

On the other hand, monarchs eat only milkweed, and it’s a particularly widespread plant farther north. Brower suggests that back in the glacial epoch monarchs began migrating farther and farther north to take advantage of that food supply, without cutting ties to their southern home.

The story isn’t quite that simple, however, for the insects don’t head south to find food for Christmas dinner. In fact, they’re about to go into semihibernation. Monarchs have very special temperature requirements at this time – they need it warm enough to keep them from freezing, yet cold enough to prevent their bodies from burning the stored fat they’ll need to fly back north.

Before going further, we need to point out that there are two populations of monarch butterflies. Their ranges overlap a bit, but basically, monarchs west of the Rockies (U.S. and Canadian) overwinter along the cool, foggy California coast. Their relatives east of the Rockies travel much farther, all the way to a few mountain valleys in central Mexico.

When you consider that the monarch’s California wintering grounds are in the middle of some of the world’s most expensive real estate, it’s not surprising that the monarchs had to adapt to new species of trees. Originally, monarchs draped themselves over the branches of Monterey cypress and Monterey pines. As development forced these trees out of existence, the butterflies moved on to eucalyptus. Had they not been able to make a successful adaptation, millions of California monarchs would no longer be around.

The earliest monarchs arrive in late September, followed by the greater mass in October and November. Many come from Canada, crossing mountains, deserts, and cities before reaching the ocean’s edge.

As they settle on the trees, monarchs overlap their bodies and wings shingle-fashion, completely covering branches. This resting posture offers them protection and warmth, and their combined weight keeps the tree branch from tossing about in the wind. Monarchs can’t fly at temperatures much lower than 55 degrees, so if they become dislodged, they may die.

Since monarchs taste bitter, predators aren’t much of a problem – but tourists sometimes are. In fact, Pacific Grove, California, “Butterfly Town USA,” fines anyone caught molesting a monarch within its city limits.

California’s monarch wintering grounds have been known for more than a century, but the Mexican ones weren’t found until 1975. University of Toronto Professor Fred Urquhart spent a lifetime working on the problem. His first task was to figure out how you tag a butterfly. He tried various markers and labels, but he found they were either too heavy or they washed off in the rain. Finally, someone suggested the gummed labels used on products you buy in the stores – the ones that have to be soaked off, and still leave a sticky residue. It worked.

That problem solved, Urquhart advertised for helpers to tag monarchs. By 1972 more than a thousand amateur lepidopterists in the United States and Canada were netting and tagging butterflies by the thousands. As recovery records filtered in, the data pointed to Mexico, to somewhere not far from Mexico City.

Kenneth Brugger gets the credit for actually finding the first Mexican wintering site. He’d volunteered to travel throughout the area in order to ask questions and to look for unusual concentrations of monarchs. No one could have predicted what he would find on January 9, 1975.

He saw hundreds of millions of monarchs packed into just a few acres of a remote mountain valley. There were incredible concentrations, far greater than anything seen in California. Subsequent searches turned up only a handful of other sites, which causes great concern among monarch admirers north of the border. To have the entire population of eastern monarchs packed into a half-dozen mountain valleys makes them dangerously vulnerable. And, sadly, we may have found the sites only to see them destroyed.

For time beyond memory, the monarchs wintered safely hidden in the dense mountain forests, but now Mexico is logging great tracks of timber to support her exploding population. Those very mountainsides used by the monarchs were actually scheduled for cutting.

Mexico cares about the monarchs, but it’s a desperately poor country. There’s hope that enough money can be made from tourists to compensate the local economy for the loss of income from timber. If not, the outlook is grim. If the choice must be made between jobs and butterflies, the monarchs may well get the short end of the deal.


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