Snout Swarm
Will last year’s explosive snout butterfly migration make a repeat performance?
By Elaine Robbins
On the Friday before Labor Day weekend last year, the snout butterfly migration through South Texas started out slowly enough. But within a few hours, some areas looked like a scene out of an Alfred Hitchcock film. “Since 1500 hrs (and they are still streaming through) tens, if not hundreds and thousands, of American snouts are streaming through our neighborhood,” reported butterfly watcher Jan Dauphin from her home in Mission. “We have witnessed several of these movements through the Valley in the past, but nothing like this. You literally have to brush them off your clothes before going inside. Neighbors are standing outside watching them, cars are stopping to watch. All seem to be heading northeast.”
It wasn’t the first time that South Texas had experienced a migration of such mind-blowing proportions. In fact, it’s fairly common to see population explosions of this species, according to Mike Quinn, invertebrate biologist at TPWD. In September 1921, an estimated 25 million per minute migrated through South Texas. The phenomenon continued for 18 days and involved about 6 billion of these butterflies, which resemble dead leaves.
Last fall, after hearing the reports, Quinn was curious to find out what triggered the population explosion. He decided to test the prevailing theory, which is that the mass migrations are set in motion by a period of drought followed by abundant rains. He gathered weather data from the past year and found that the theory held water: Rainfall in the July preceding the migration had peaked at six inches above average, following nine months of below-average rainfall. “The drought is thought to suppress the parasites that keep the butterfly’s population in check,” he says. The following rains encourage the spiny hackberry — the snout’s favorite caterpillar food plant — to leaf. Females then seek out the shrubs to lay their eggs, and under these ideal conditions of fresh leaves and few parasites, more of their caterpillars mature.
The theory would have ended there if an enterprising lepidopterist named Larry Gilbert — a biology professor at the University of Texas — hadn’t decided to test the sex of all those south-roaming snouts. In 1976 he studied the gender ratio of the snouts passing through Chaparral Wildlife Management Area and came up with some surprising results. In July of that year, the ratio soared to an astounding 167 males to 2 females — up from a nearly equal ratio just a month before.
That ratio may explain the behavior that naturalists John and Gloria Tveten observed during a snout migration in the Rio Grande Valley in 2004. The butterflies were emerging from their chrysalises in a “mass emergence,” they reported. “Many were coupled mating pairs, and in quite a few of those pairs, a male with dried and fully expanded wings was mating with a newly emerged female.” Given the shortage of females, explains Quinn, “the migrating swarms are primarily males searching for females that haven’t yet mated.”
Will South Texas witness another population explosion of snout manhood this fall? “If this year’s drought continues, and if we get summer rains,” says Quinn, “we should see a swarm of South Texas snouts.”
