Orley “Chip” Taylor didn’t think twice about driving through a rainstorm last month on his way back from Mexico.

But that storm, it turned out, resulted in the largest mass death of monarch butterflies ever known.

“Nothing you can say gives the visual description of this kind of carnage,” said Taylor, director of Monarch Watch, the Kansas University-based organization that tracks the butterflies.

The rain  rare for Mexico in January  was combined with unusually cold temperatures in the 20s. It killed between 220 million and 270 million monarchs in the mountains of central Mexico, where the butterflies spend their winters.

About 75 percent of the butterflies in the Sierra Chincua and Rosario areas were wiped out. The two sites, along with a few others, form the entire breeding stock of monarchs that migrate annually through the eastern United States, including Kansas.

Though monarch numbers are now lower than they ever have been, Taylor said the deaths shouldn’t have a long-term effect.

“It will take more than a year for them to recover,” he said. “Monarchs aren’t going to become extinct because of this incident, but it will take them awhile to get back to their normal numbers.”

Taylor said the next few months would be critical for the butterflies. They’ll face increasing fire ant populations in Texas and drought in the Midwest, he said.

Taylor plans to travel Feb. 24 to Mexico to evaluate the situation. This year’s monarch numbers will depend on how the weather has affected the remaining butterflies, he said.

The butterfly deaths will help Monarch Watch’s tracking program. The tags from dead monarchs will be easier to retrieve than tags from live monarchs, he said.

In typical years, between 600 and 700 tags are recovered. This year, Taylor expects more than 1,000.

But that will strain Monarch Watch’s budget, which pays Mexican workers 50 pesos  or about $5  for every tag they submit to the program. Taylor said he’s trying to raise $3,000 more to cover the increased expense.

Taylor said monarch numbers dropped to a previous all-time low in 2000 before rebounding last year. He said this year’s drop is another round of the cycle.

“A lot of things in nature are kind of on a roller coaster,” he said.