An enormous, mottled-orange cloud of painted lady butterflies that drifted over Salt Lake City Thursday morning was just a fragment of what may be one of the largest butterfly migrations of the century.

"It is equal to or larger than any we know of," said Clyde F. Gillette, who has tracked the migration since it crossed into Arizona March 5. "It is so extensive that it covers an area from the Pacific Coast to Utah's eastern border and perhaps beyond."Gillette, founder of the Utah Lepidopterist Society, adds that the migration may also be one of the longest lived. "It has been going on for eight weeks," he said, attributing its longevity to repopulation along the way.

"The ones we are seeing now are undoubtedly the second generation," he said.

He explained that the painted lady doesn't have a migratory route in the normal sense but rather migrates in response to overpopulation. The current migration apparently began in the breeding fields of northern Mexico and will end when the swarm thins out sufficiently or encounters a climate too cold to endure.

"They don't make a return flight," Gillette said.

Large numbers of the butterfly were sighted near Cedar City recently, but only a few had crossed the heavily populated Wasatch Front until Thursday. Gillette, who has been writing scientific papers about butterflies for almost 50 years, says he can't recall ever seeing so many in the Salt Lake area.

"The whole sky was full of them for as far as you could see," said Steven Ivie, Kearns, who watched as many of the painted ladies fluttered into his yard in apparent exhaustion.

"I guess they were stressed out," Ivie said. "When they hit the ground, they couldn't get back up again."

The USU Extension Service reported getting dozens of calls about the swarm from throughout the valley and as far away as Wendover. Several airline pilots reported seeing the cloud of butterflies Thursday morning over West Jordan.

"There were about a million of them," said a resident of the Oquirrh Shadows subdivision in West Jordan. "They were everywhere you looked."

The painted lady is not a pest and poses no threat to Utah agriculture or urban landscaping, Gillette said. For energy along its migratory path, it generally feeds on dandelion and some fruit blossom nectar. Occasionally, it does harm commercial bean crops, he said.

The migration is likely to continue for some time, Gillette said. "You can expect more of them when it warms up again."