Why Were There So Many Orange Butterflies in Boulder Creek?

You notice the movement before you understand it. Some warm afternoon this month, the air over Boulder Creek fills with small orange butterflies, dozens and then hundreds of them, all headed the same direction.

They didn't hatch here. They're painted ladies, and this spring they're moving through California by the thousands. What you're watching isn't a local hatch so much as a migration passing overhead, and it only turns up big in certain years. This is one of them.

What you're actually seeing

The butterfly is the painted lady, Vanessa cardui, one of the most widespread butterflies in the world. Up close it's orange and brown with white spots and black-tipped forewings, a bit ragged and mottled rather than boldly patterned.

It flies fast, low, and erratically, rarely stopping for long. That restless flight in a steady direction is the tell. You're watching a migration, not butterflies drifting around a garden.

They're easy to mistake for monarchs, so here's the quick way to sort them:

• Painted ladies are smaller, roughly a two-inch wingspan, mottled orange-brown with white flecks.
• Monarchs are larger, closer to four inches, with bold black veins across deep orange and a border of white dots.

Monarchs are the ones that overwinter on the coast, at spots like Natural Bridges. The fast orange river pouring through the valley in March is painted ladies.

Why so many this year

Painted lady numbers swing wildly from year to year, and the switch is rain, hundreds of miles away. These butterflies spend the winter in the deserts of the Southwest and northern Mexico. When those deserts get a wet winter, the spring wildflower bloom explodes, and all that fresh growth feeds an enormous crop of painted lady caterpillars. A few weeks later, that boom takes to the air and heads north.

UC's naturalists tie the huge years directly to big wildflower blooms, and the butterfly biologist Art Shapiro has long noted their numbers are enormous in years with large blooms.

So this isn't an every-year show. Big migrations are irregular, with standout years like 2005 and 2019, and long stretches in between when you'd barely notice them. This spring is a strong year, though the old-timers among butterfly watchers say it's not quite the wall-to-wall spectacle of 2019.

Where they're all going

They're passing through, not moving in. The main California migration runs north from the desert through the Central Valley toward Oregon and the Pacific Northwest, with a branch that swings through the Bay Area and over the Santa Cruz Mountains.

The round trip takes several generations. The butterflies you're seeing now will breed to the north, and their descendants make the return trip south in the fall. The ones crossing Boulder Creek today are one leg of a relay that spans the West and the seasons.

If you want to give them a hand, the same thing that helps them helps everything else: native, nectar-rich plants and no pesticides. Thistles and mallows are classic painted lady host plants, and a patch of blooming natives gives migrants a place to refuel.

Will it happen again?

Maybe, and it comes down to the deserts, not to us. Another big migration needs another wet desert winter to trigger the wildflower bloom that feeds the caterpillars, and that varies year to year in ways no one can promise in advance.

One thing is worth watching for next year. Forecasters are calling for a strong El Niño this coming winter, and wet winters can prime the Southwest deserts for exactly the kind of bloom that produces a painted lady boom. That's a genuine possibility, not a prediction, since it depends on where and when the rain actually falls.

Their numbers are a kind of weather report from a place you can't see. Watch the deserts this winter. If they green up, next spring could be another good one.

Frequently asked questions

What are the orange butterflies all over Boulder Creek?

They're painted ladies (Vanessa cardui) on their spring migration, small orange-and-brown butterflies with white spots and black forewing tips. In big years, thousands stream north through the Santa Cruz Mountains.

Are they monarchs?

No. Painted ladies are smaller and mottled orange-brown; monarchs are larger with bold black veins and a white-dotted border. Monarchs overwinter on the coast, while the fast-moving spring swarms are painted ladies.

Why are there so many this year?

A wet winter in the Southwest deserts produced a big wildflower bloom, which fed a huge crop of caterpillars. That population boom becomes the spring migration. Numbers vary a lot year to year.

Will there be another big migration next year?

Possibly. It depends on whether the deserts get another wet winter and a strong bloom. A strong El Niño is forecast, which could set that up, but it's not guaranteed.

How can I help the butterflies?

Plant native, nectar-rich flowers, including painted lady favorites like thistles and mallows, and skip the pesticides. Migrants need places to refuel along the way.

Last reviewed July 20, 2026. The painted lady biology here follows longstanding observations from University of California naturalists and butterfly biologist Art Shapiro.

Seen something worth noting in the valley's wildlife? Tell us, and browse more Boulder Creek nature and seasonal notes while the migration is still passing through.