The big show

Local experts weigh in on the mysterious cause of a butterfly surge this summer

Western tiger swallowtails flooded Chico this summer. It’s anyone’s guess as to why.

Western tiger swallowtails flooded Chico this summer. It’s anyone’s guess as to why.

photo by brocken inaglory

This was a tremendous year for butterfly lovers.

Most years, during the spring and summer, Chicoans would consider themselves lucky to catch a few glimpses of majestic western tiger swallowtails sipping nectar at flowering shrubs in their backyards. This year, however, they appeared nearly every day. Bright yellow and black wings fluttered around Chico yards from early spring until just recently.

In 40 years of watching the flying insects, local butterfly expert Don Miller has never seen so many large western tiger swallowtail butterflies in Chico. “I was dazzled by how many there were,” he said.

Sterling Mattoon, who has been watching butterflies since he was 5 (the Chico lepidopterist is now 82), remembers previous seasons with as many tiger swallowtails, but not for a long, long time.

What’s confounding local experts is that this year’s population boom was an anomaly. Each June, Miller coordinates the local effort in a butterfly count sponsored by the North American Butterfly Association. Some butterfly populations have declined because of activities like farming and home-building, he said. Climate change has had an effect, too.

Global warming has led to a shorter, drier, warmer growing season, which has an adverse effect on plants, he said.

“Plants’ welfare has very much to do with butterflies’ welfare,” Miller said.

The California dogface butterfly, which is the state insect, used to be found in Butte County, but not anymore, he said. In fact, its numbers have declined all over Northern California.

The “large marble” butterfly, which is “very delicate and pretty,” was relatively common but now is rare, he said. A few species of copper butterflies are now hard to find.

“This year, as well as last, numbers are down,” he said of all butterflies. “The [western tiger swallowtail] is one of a few interesting exceptions.”

So, what caused this year’s surge? It’s really anybody’s guess. Maybe something cut the population of one or more of the butterfly’s predators, Miller hypothesized. There’s a tiny wasp, for example, that can burrow into the insect when it’s a caterpillar and kill it. If something caused the population of these wasps to decline, the result would be more tiger swallowtails, he suggested.

That theory is plausible, said Arthur Shapiro, a biology professor at UC Davis and an expert on butterflies, but he couldn’t provide evidence to back it up.

Mattoon pointed to an unusual characteristic of tiger swallowtails that could explain this year’s large showing: They have the ability to live in their chrysalises (which are like cocoons) for several years instead of simply emerging as butterflies after a few weeks. If there were many butterflies resting in their chrysalises, and just the right conditions occurred, in terms of temperature and rainfall, for example, lots and lots of butterflies could appear on the scene, he said.

Shapiro stressed the mystery in what happened this year. And he noted this wasn’t the first year populations were larger than usual. “No one has the slightest idea why they are coming on strong,” he wrote in an email. “Elevated populations were first observed in 2009, when the species suddenly returned to Davis after a 10-year absence. At the latitude of I-80, the phenomenon has persisted at least from Roseville to Vallejo. This year was not one of the biggest [in Davis].”

Western tiger swallowtails live as butterflies for two to six weeks. During their short lives, the females lay eggs multiple times on the leaves of host trees, such as ashes and sycamores.

After a couple of weeks, the eggs hatch, and the new generation of butterflies lives as caterpillars for about six weeks. Then they form chrysalises and inhabit them, typically for another four weeks. Finally they emerge as butterflies.

It was common this summer to see a couple of tiger swallowtails fluttering around each other in circles. Some people say when that occurs, the butterflies are mating, but Miller said that’s not actually so. That kind of behavior can indicate a sort of turf war between two butterflies, or the insects might be courting. It’s unusual to spot butterflies mating, but when they are seen, they are connected tail to tail, usually on the ground, he said.

When temperatures start dropping, as they now have, the butterfly eggs stop hatching, and the whole process is put on hold, Miller said. He said he hasn’t seen any of the insects since about mid-September. Next year’s western tiger swallowtails will remain in their chrysalises, attached to small tree branches, through the winter.

With such a huge number of the butterflies this year, it’s probable that the number of chrysalises is also larger. The butterflies start to appear in the spring, sometimes as early as January.

Mattoon said it would be wonderful if many tiger swallowtails appear again next year, but there’s no way to predict whether that will happen.

Miller countered: “I’m very hopeful we’ll see a lot of tiger swallowtails next year, too.”