Pismo Beach CA mice eat monarch butterflies, study finds


When you think of mice, you can imagine them munching on chunks of cheese, not eating monarch butterflies.

But that’s exactly what’s happening at Pismo State Beach Monarch Butterfly Grove, where University of Utah researchers found that harvest mice sometimes eat the colorful insects who migrate there every year, according to a recent study published in the journal Ecology.

“One of the things it really highlights is that this is a butterfly, an insect that we all know so well, that we all know. Almost every school kid in California knows about its life cycle and toxins, and it’s in everyone’s backyard. And yet here is an interaction we had no idea existed, ”said Sara Weinstein, the postdoctoral researcher who helped write the study.

Harvest mouse finds dead monarch butterfly reared in lab Researchers at the University of Utah have planted at Pismo State Beach Monarch Butterfly Grove. A 2020 study determined that grove mice sometimes included butterflies in their diet. Sara weinstein University of Utah

How researchers studied monarch-eating mice at Pismo Beach

Weinstein had researched mammals that eat poisonous diets and was interested in the study of animals that eat monarch butterflies. As caterpillars, monarchs eating plants with toxins, and the substances build up in their body and make them less appetizing to eat like butterflies, according to a University of Utah statement on the study.

Weinstein, who grew up in Santa Barbara, first considered studying mouse-monarch interactions in Mexico, which has even larger butterfly populations than California. But she ultimately decided to study the monarch grove at Pismo Beach, which can attract thousands of butterflies during the wintering season.

“Would that be great? Weinstein asked. “What if this interaction exists in our own garden and it just hasn’t been documented yet?” “

Weinstein and his team first visited the Pismo Beach grove in February 2020. Researchers first caught mice to find out which types live in the area. They also collected mouse droppings for DNA testing, which would provide information about their diet.

The team found a sample that contained monarch DNA. However, there weren’t so many butterflies around the end of winter when they visited, so Weinstein decided to come back around Thanksgiving.

But biologists only counted around 200 monarchs in the Pismo Beach grove from mid-November to early December 2020 – a significant decline that raised serious concerns about the species’ survival.

The Xerces Society for Invertebrate Conservation said wildfires, temperature changes, increased use of pesticides and an “unusually high number of reports of captive-bred non-migratory monarchs” were some of the factors behind the low numbers. .

Weinstein and his team moved forward, however, even with fewer monarchs. They chose to plant dead moths raised in the lab to see if they could capture videos of mice eating the monarchs.

Weinstein set up the monarchs under camera traps and, of course, got pictures of mice gnawing butterflies. She also briefly captured mice and kept them in a container with dead butterflies to see how they would interact.

University of Utah researcher Sara Weinstein is collecting data on wild mice trapped at Pismo State Beach Monarch Butterfly Grove. Weinstein visited the grove to find out if mice eat the monarch butterflies that migrate there each winter. Sara weinstein University of Utah

Why do mice eat butterflies?

Weinstein said mice and other animals may be more omnivorous than most people realize. They probably brave monarch toxins because the insect bodies contain nutrients that are beneficial to mice.

Most of the toxins are also found in the wings, not the body, so mice can avoid the most toxic area when eating. Some rodents also have genetic mutations that make them less susceptible to toxins than other animals or humans.

“You can bet that, like most insects, the body has all of this fat stored,” Weinstein said. “So for a mouse, if you can handle the toxins and avoid the wing, there’s quite a bit of nutrition in that body.”

She said monarch-eating mice are not a factor in the recent decline in the migrant population. Mice likely eat monarchs that fall from trees and to the ground, especially during storms. Butterflies can be too cold to fly away, making them easier for mice to eat, Weinstein said.

The migrating monarch population rebounded in 2021, when biologists counted more than 22,000 butterflies in the Pismo Beach grove. They told The Tribune in November that the rebound was “a bit unbelievable” and “a sign of hope.”

Despite this, the number of migrations in the grove has been low over the past five years, so biologists were cautiously optimistic about the recent turnaround.

Weinstein said that the fact that a mostly unexplored dietary relationship exists between monarchs and mice shows “how little we know about food webs, in general, and interactions with insects and their predators or scavengers. “.

“When you start to extrapolate that to the number of insect species and how little we know about all of them, I think it really highlights the need for more fundamental research to understand these interactions,” Weinstein said. “Especially given the reports of the decline of insect species around the world, especially flying insects.”

This story was originally published January 10, 2022 11:19 a.m.

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Lindsey Holden writes about housing, the San Luis Obispo County government and everything in between for The Tribune in San Luis Obispo. She became a writer in 2016 after working for the Rockford Register Star in Illinois. Lindsey is originally from California and raised in the Midwest and graduated from DePaul and Northwestern Universities.


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