Why fewer Monarch butterflies are surviving their winter migration to Mexico
GWEN IFILL: The Monarch butterflies that spend their winters in the forests of Mexico before returning north each year are appearing in fewer numbers.
Independent video journalist Ross Velton filed this report for us on why their numbers are dwindling.
ROSS VELTON: Tradition says they're the souls of the dead returning to earth. That's why Monarch butterflies arrive in Mexico around the start of November, when lost loved ones are being remembered on a holiday called the Day of the Dead.
The Monarchs sometimes fly over 2,000 miles from the U.S. and Canada to spend winter in these mountain forests in Central Mexico.
EDUARDO RENDON-SALINAS, World Wildlife Fund (through interpreter): More than a decade ago, practically two decades, the butterfly colony here in El Rosario occupied up to three hectares.
ROSS VELTON: That's nearly 7.5 acres in the part of the Monarch butterfly biosphere reserve where the most butterflies are often seen. But the forest has been missing some souls.
EDUARDO RENDON-SALINAS (through interpreter): During 2012 to 2013, which was the period we monitored last year, we determined that the butterflies only occupied 1.9 hectares.
ROSS VELTON: Or just over 4.5 acres. That's the lowest number of Monarchs in the past 20 years, and a nearly 60 percent drop on the year before.
And this winter could be worse. The Monarchs as a species aren't in danger, but their migration might be. The World Wildlife Fund tracks the number of butterflies arriving in Mexico on one of nature's great journeys.
EDUARDO RENDON-SALINAS (through interpreter): The Monarch butterflies which hibernate in Mexico are those butterflies which live, which as adults in Southern Canada and the north and center of the U.S. During September and part of October, they fly south. And the greater proportion of butterflies in this big region come to hibernate in Mexico.
ROSS VELTON: But something's happening at the start of this journey, so that fewer butterflies are coming down to Mexico. Herbicides to protect crops like corn and soybeans from are also killing the butterflies' food source, a plant called milkweed.
CRAIG WILSON, U.S. Department of Agriculture: The female butterfly will deposit one egg on the underside of a leaf. And when the caterpillar emerges, it starts to eat the leaf.
ROSS VELTON: That's something Craig Wilson tells children when he visits schools as part of the U.S. Department of Agriculture's Future Scientists project. And he's got more to say about the benefit of milkweed to monarchs
CRAIG WILSON: It has a poisonous sap. I don't know if you can see it bleeding there when I pull a leaf off. And that latex sap protects the caterpillars.
ROSS VELTON: Because it gives them a nasty taste, which puts off predators.
Molly Keck, an insect expert at Texas A&M University, says milkweed isn't just under threat from herbicides.
MOLLY KECK, Texas A&M University: We're developing, and so these natural areas, there are buildings in neighborhoods that are popping up, so there's less places where milkweed has the potential to grow.
ROSS VELTON: Few animals like changes to their surroundings. But the loss of milkweed isn't the Monarchs' only problem. The adult butterflies get energy from nectar as they fly down to Mexico.
MOLLY KECK: When we have extreme droughts like we have been experiencing in the United States, there are less nectar-producing plants and we see less butterflies, we see less bees, we see less of all the pollinating insects.
ROSS VELTON: The falling numbers add a sense of urgency to research into Monarch behavior. Here in College Station, Texas, for example, researchers are trying to find out how genetics and the butterflies' body clock affect the migration. The Monarchs' problems have also fired up citizen scientists.
The children at this school in Texas have planted a milkweed garden to try and attract Monarch butterflies. It's an example of one of the many projects all over the country to try and protect the migration.
ROSS VELTON: Mike Quinn took part in another one. It involves looking for milkweed and signs of Monarchs.
MIKE QUINN, citizen scientist: Monarch Larva Monitoring Project is a way for citizen scientists across U.S. and Canada to collect quantifiable data that can be compared scientifically with other sites and at the same site over time. And this gives us a bigger, more accurate picture of the Monarch population levels.
ROSS VELTON: The Monarch population flying south to Mexico hasn't always had a great welcome. The type of fir trees in the reserve where the butterflies like to spend winter have suffered from several decades of illegal logging. Now thousands of new trees are being planted back into the reserve.
This is a tree nursery supported by the World Wildlife Fund. The idea is to give those working there an income, and therefore a reason to reforest. And this nearby forest has been grown to provide wood for sale, meaning less pressure on the reserve.
MAN (through interpreter): The Monarch butterfly is a marvel. But to get a marvel, like a flower in a garden, you must take care of the garden. And this is the garden of butterflies.
ROSS VELTON: EDUARDO RENDON-SALINAS says there's been a growing bond between the people and the butterflies.
Old men from El Rosario have formed groups to protect them. Their job includes making sure tourists don't get too close. Rendon-Salinas says illegal logging on a big scale has stopped in the heart of the reserve. But he knows things take time to heal.
EDUARDO RENDON-SALINAS (through interpreter): This butterfly is a female. It's in perfect physical condition. There are no signs of it having been attacked. It looks complete. However, it's dead. Very likely, it dropped to the ground during the night. And, in places such as this, where there are gaps in the tree canopy, the temperature falls during the night, and the butterflies die.
ROSS VELTON: The butterflies that survive Mexico might one day be seen back at the school in Texas, because the same Monarchs that migrate do a round-trip. So the kids tag them.
Alina Garcia has just done her first one.
STUDENT: They just have to travel so far. I mean, I couldn't really handle that pressure. And I don't know how they can handle it.
ROSS VELTON: They get help handling it from friends on both sides of the border. When he comes to the reserve, Rendon-Salinas breathes warm air on butterflies that can't fly because of the cold, so they will fly again.