What happened to the monarch butterflies? Pismo Beach numbers plunge 81 percent

If you’re planning to check out the monarch butterfly grove this winter, plan to see a lot fewer butterflies than usual.

According to California State Parks Interpreter Danielle Bronson, the Pismo State Beach Butterfly Grove recorded a record low number of butterflies wintering in the area this season. The results are similar to the rest of the state, which has also seen a marked decline in the fluttering population in the past year.

Volunteers from the Xerces Society for Invertebrate Conservation counted only 2,300 butterflies at the Pismo Beach grove at this season’s annual Thanksgiving Day tally.

That number was down roughly 81 percent from the 2017 count of 12,300.

“This is a record low for us,” Bronson wrote in an email to The Tribune.

It’s the same story across the state.

In the first Xerces Thanksgiving count in 1997, volunteers recorded more than 1.2 million butterflies across the state. Every year since then, the population has been less than 600,000, with most years at less than a quarter of the 1997 population.

In 2018, the statewide count recorded a mere 20,456 monarch butterflies, according to preliminary reports. That’s 1.7 percent of the 1997 population.

The Pismo Beach grove — historically one of the largest butterfly colonies in the state, according to Bronson — at one time hosted more than 230,000 butterflies a season.

Now that the population is about 1 percent of what it once was, longtime visitors are taking note of the decline, Bronson said.

“The ones that have been here in the past notice,” she wrote. “But we still get a lot of first-timers that are still in complete joy.”

The reasons for the decline are unclear.

Most, like Bronson, cite a loss of overwintering habitat to drought and human development as a factor. The spread of development and pesticide use has also endangered milkweed — an incredibly important part of the monarch butterfly life cycle, because it’s the only plant on which the butterflies will lay their eggs, and their only food source as caterpillars.

“The truth is it’s really unknown for sure as to why the decline,” Bronson said, noting there are ongoing studies into why the declines seem to be happening with both California and Mexico’s overwintering populations.

Though many might want to help out by planting milkweed in their yards — one of the most common suggestions for helping the population — Bronson cautioned people to do their research when choosing the type they plant.

She said some have unknowingly planted a non-native milkweed known as tropical milkweed “that could actually be harming the population instead of helping.”

“It doesn’t die back naturally in the winter like native milkweed, and a parasite that is speeding through the population is not naturally shed, continuing the cycle,” she said.

To help prevent this, Bronson recommended avoiding big-box stores that tend to sell tropical milkweed, and instead get native versions from a local native nursery where they can ensure you are planting the correct type.

People are also advised to not plant any milkweed — regardless of variety — within 10 miles of the coast because it “could have adverse effects with the overwintering populations,” Bronson said.

For more information on monarch butterfly conservation and what you can do to help, visit xerces.org.

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Kaytlyn Leslie writes about business and development for The San Luis Obispo Tribune. Hailing from Nipomo, she also covers city governments and happenings in the South County region, including Arroyo Grande, Pismo Beach and Grover Beach. She joined The Tribune in 2013 after graduating from Cal Poly with her journalism degree.