Butterflies versus beetles

The butterflies flitting in the sky above seem ablaze, as sunlight filters down into the Sierra Chincua forest in Mexico’s Monarch Butterfly Biosphere Reserve. Millions of the orange-and-black insects are just arriving, as part of their annual 2,800-mile journey from Canada and the United States.

But this year, because of the worst drought in nearly 70 years, an infestation of bark beetles has hit this 138,000-acre reserve, where tourists from around the world come to view part of a migration pattern considered one of the globe’s most extraordinary.

Authorities have already identified more than 7,500 beetle-infested fir trees – three times the normal amount in any given year – and have raced to cut them down. “We can expect to find more infected trees,” says Rosendo Caro, director of the reserve.

The bark beetle has always been present in the forest, but because of drought, the volume of resin that acts as a natural defense is much lower than usual. Beetles that usually are repelled by a flood of resin when attempting to enter trees have instead been able to burrow into them, sapping the trees of crucial nutrients.

The needles of affected trees turn yellow and red. In some cases, authorities mark the tree and wait to see if the infestation ceases on its own, but most must be cut down so the beetles do not reproduce inside and spread to other trees.

About 173 acres of the 32,000 acres in the reserve’s nucleus have been infected so far this year. Without trees to house them, the butterflies are threatened by cold winter air and rains.

Bark beetle damage in forests isn’t confined to Mexico. Forests across North America have been devastated, mostly because of drought, says Jaime Villa Castillo, a forestry expert at Mexico’s National Forest Commission.

On a recent day, deep in the forest, groups of men practiced “sanitation logging” to help prevent the spread of the bark beetles. They cut down trees, buried the infected bark underground, and used cables to haul heavy logs onto trucks. The work is slow, with trees inspected one by one, and it will have to stop when the butterflies arrive to spend the winter huddling in large groups on branches that droop under their weight.

Some environmentalists have criticized sanitation logging. They say that insecticides are a better option, because they kill the beetles while allowing the trees to live.
But government officials say that the law does not permit the use of insecticides within the reserve. “The only way to do this is to cut the trees down,” says Mr. Villa Castillo, who visited the forest on a recent day to monitor the project.

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