On Oct. 3, a 70-mile wide mass was seen on the weather radar over the Denver area. It was thought to be a cloud-like pattern but ended up being a swarm of painted lady butterflies.

The painted ladies continually migrate from Baja, California to Canada. In the early spring, they ride storm fronts. “Colorado gets one called the Albuquerque low, it centers down in New Mexico because storm fronts that are low move counterclockwise,” said Matt Camper, an entomology professor at Colorado State University. The Albuquerque low pushes the storm front towards the Front Range and they ride the storm front as if they are surfing. This is what pushes the butterflies further north.

Most of the time, this number of butterflies are seen during the early springtime. “We are seeing the fall return migration with this year because of the good moisture we saw in the spring and the fair moisture we had throughout the season,” Camper said. Because this fall is not extremely cold, and there were good survival rates of the caterpillars, students are now able to see the painted ladies are in their adult stage. The painted lady butterflies do this same migration yearly from the north to the south.

The reason this migration was abnormal is that it stayed visible. The butterflies are all migrating at the same time, and the last generation of butterflies had a really good survivorship, so they have joined the new generation for this year’s migration. “It was just one of those weird things where everything just lined up for the migration numbers to be spectacular this year,” Camper said. Because the number of butterflies is so high, you might assume that it is dangerous, but it is not. Some larvae are considered beneficial because they feed on thistle, a garden plant we see as invasive.

The major question is how the butterflies showed up on a weather radar. The weather radar picks up dense things in the atmosphere, so the density of the butterfly storm was the reason it was able to be seen.

“So many butterflies in a clump moving over the generally same movement just like a cloud front moving across,” Camper said.

Butterflies will be seen here and there, but up 500 hundred feet, there are streams of butterflies heading down. They do this as a way to pop down to find some flowers for nectar. This year’s migration is definitely a spectacular sight and one for the books.