Right around Valentine’s Day, the Monarch butterflies wintering in Mexico get frisky again.

”They start mating and getting really sexy,” says Chip Taylor, the 76-year-old director of the Monarch Watch research and conservation program at the University of Kansas.

Toward the end of February, the butterflies begin their annual migration north from their mass gathering site in a Mexican forest, laying eggs, then dying as a new-born generation continues on. The cycle of life and death and life repeats itself for as many as four generations over as many as 3,000 miles, until the final wave wings out across widespread destinations in the Midwest and the Northeast and Canada.

None of the butterflies that begin the journey are still alive at its end, but the knowledge of their winter home somehow survives. The proof comes in the early fall, as they begin to head south, these butterflies not reproducing and able to survive the whole way, retracing the route of their predecessors.

The Monarchs were beginning this annual journey a dozen years ago, when thousands of them fluttered through the smoking ruins of the World Trade Center in the immediate aftermath of the 9/11 attacks.

“Souls,” one firefighter softly exclaimed.

And souls are exactly what the migrating Monarchs are traditionally believed to be in Mexico, a notion that has seemed confirmed every November for as long as anybody could remember, when the Monarchs’ annual arrival coincides with the Day of the Dead.

Only this past November, the Monarchs arrived late. And there were far fewer of them than ever before. They occupied only 1.65 acres of the forest that is their final winter destination, whereas in other years they had occupied as many as 45 acres. Their numbers are estimated to have dropped this year to 35 million from a high of 1 billion in 1996.

Part of the reason for this precipitous decline is likely climate change.

But the biggest factor is thought to be an accompanying decrease in milkweed, where the Monarchs lay their eggs along the way.

And that can be traced largely to the greatly expanded planting of genetically altered soybeans and corn that are able to tolerate herbicides. The accompanying use of these poisons has proven deadly to milkweed.

One twist to it all is that the milkweed is the only plant that can provide Monarch larvae with a chemical that makes the adults toxic to predators, which are warned away by the bright colors of the wings.

For some time, Monarch Watch has recruited amateur trackers to help it monitor butterfly populations and document the migration. The organization is now also asking volunteers to plant milkweed, most particularly in Texas, where the first wave lays its eggs, but also wherever else the Monarchs might venture.

“We need gardeners all over the country to be involved,” Taylor says. “It’s really easy. Take an existing garden and just plant some milkweed plants and a few nectar plants.”

Seeds and instructions for milkweed can be obtained via the organization’s website, www.monarchwatch.org. People already growing milkweed include Justine Drennan, the youngest daughter of John Drennan, a New York Fire Department captain who was critically burned in a Manhattan fire in 1994 that killed two other firefighters. He fought for life for a biblical 40 days before he finally succumbed.

At the burial, Fire Chaplain Mychal Judge was just finishing the Our Father prayer when a Monarch that must have been nearing the end of its northward migration alighted on the flowers set beside the coffin. The butterfly closed its wings as if in prayer at the Amen, then opened them again and fluttered off into the sunlight.

Seven years later, Judge and a dozen of the firefighters who had gathered at Drennan’s grave that day were themselves killed at the World Trade Center. And, in the aftermath, those thousands of Monarchs passed through, their colors even brighter against the smoke and ash.

Drennan’s widow, Vina, got a call from an older daughter saying that a butterfly was banging against her window.

“Well, let it in and give it a beer,” Vina Drennan said.

This week, Vina reported that Justine is planting milkweed. Other supporters of the milkweed effort include Mexican billionaire Carlos Slim.

Whoever you are, there can be no better way than planting milkweed to honor the souls of those you miss on Valentine’s Day, when the surviving Monarchs down in Mexico are getting frisky.