Where are all the butterflies?
Many butterfly species you remember from your childhood simply aren’t around anymore or their numbers are dwindling. Nationally, we’ve seen a 20 percent decline in our butterfly population over the last 24 years, and that, coupled with the rising threat of extinction, is alarming.
So said UC Davis Distinguished Professor Emeritus Art Shapiro in a recent program on the national decline of butterflies. Speaking to the Vacaville Museum Guild, he attributed the decline to two primary causes: pesticide use--especially neonicotinoids--and climate change.
“I used to record 15 to 20 or more butterfly species in my garden in north-central Davis in the 1970s,” he said. “Now I am lucky to get eight. According to our many collaborators and correspondents across the country, my experience is being repeated from coast to coast. An old friend, British journalist Oliver Milman, asked me for a quotable quote for an article about our recent paper about these declines. I told him that at 79, I wasn’t sure whether I or my butterflies would go extinct first.”
Largest Butterfly Monitoring Set in North America
Shapiro has monitored the butterfly populations in central and northern California, encompassing some 115 species, since 1972. His data set, known as the largest butterfly monitoring data set in North America and one of the largest in the world, runs parallel to Interstate 80, with 10 sites distributed from the inner Coast Range, across the Sacramento Valley and the Sierra Nevada, to the western edge of the Great Basin. His sites span a wide range of climates and vegetations, from sea level to treeline at 9000 feet.
In the 1970s, Shapiro used to see the Great Copper, Large Marble and Field Cresent near his home: The Great Copper within a half-hour walk, the Large Marble within two blocks and the Field Crescent within a mile and a half. “They all became regionally extinct in 2005. By regionally extinct, I mean, it's gone from all of my study sites at low elevation in a wide geographic area, essentially from Chico to Stockton.”
“The California Ringlet held out a little longer; until 2007. The Common Sooty Wing; whose name is no longer accurate, used to breed on pigweed outside my building. Storer Hall, at UC Davis and I could find caterpillars 200 feet from my building. It's no longer found within the city limits of Davis. There's still a population in West Sacramento.”
“On the other hand, we have had some minor gains.” He called attention to the Umber Skipper, a species of cool, moist habitats. “It's been common in the Bay Area as far back as we have records. It has occurred in cool, moist habitats here in Vacaville, like Buck Avenue, all this time. But it has increased dramatically in Sacramento and Davis in the last few years. We have no idea why, because it's not getting cooler and damper. Yeah, really, what's with that?”
Turning to the Gulf Fritillary, Shapiro said neither the butterfly nor its host plant, passion vine or passion flower, is native. “This butterfly entered California about 1875 with the railroad from the east. It entered the Bay Area spontaneously about 1910 and has been present in the Bay Area ever since. When I came to California, it had a toehold in Sacramento and Davis gardens, but it's extremely frost sensitive, and in the ‘70s it got frozen out completely and went extinct in this whole area inland. It reappeared in eastern Sacramento County, about 2011 or 2012, and has spread like wildfire. It's now north to Redding and up to Grass Valley, so it has occupied a huge area very quickly, and seems to be very happy. So, it's a plus.”
Shapiro, who retired from UC Davis in July 2023 but continues his research, now monitors four sites--Suisun, West Sacramento, North Sacramento and Ranch Cordova. His former doctoral student Professor Matt Forister of University of Nevada, Reno, now heads the research group. They maintain a UC Davis website at https://butterfly.ucdavis.edu/
“Our research group is tied to a continent-wide network of butterfly-monitoring projects,” Shapiro said. He and Forister teamed with 31 other colleagues to produce a major report, “Rapid Butterfly Declines Across the United States During the 21stt Century,” published in the journal Science in March 2025.
“Altogether, we analyzed data on some 12.6 million individual butterflies extracted from roughly 77,000 surveys distributed over 35 monitoring projects, ours being one of the largest if not the largest,” Shapiro said. “The overall conclusion—a roughly 20 percent decline in butterfly numbers across the U.S. in just 24 years—was so shocking that it attracted widespread media attention. And well it should.”
“Of 342 species tracked, 245 declined, 32 appeared stable, and 65 increased,” Shapiro related “The declines were so broadly distributed across taxonomic groups, climate and geography as to suggest very fundamental factors are driving them--factors transcending the purely local. One species, the Hermes Copper of southern California, appears to be in danger of global extinction. What about the faunas monitored on our transect?”
“Three formerly common and widespread species, the Field Crescent (Phyciodes campestris, also called P.pulchellus), Great Copper (Lycaena xanthoides), and Large Marble (Euchloe ausonides), have disappeared from the lower-elevation sites on our transect,” he said. “Their host plants (asters, docks, and mustards respectively) are as common as ever; docks and mustards are weedy introductions! Nor has there been any noticeable loss of habitat.”
“The disappearance of the Large Marble is particularly well-documented because I used to carry out research projects on it,” Shapiro said. “It had had two generations a year in the Sacramento Valley. The second annual generation disappeared a few years before the first. What could this mean? Several other once-common species are in conspicuous decline. Of particular concern are the Common Sooty-Wing (Pholisora catullus), mentioned before, and the Sandhill Skipper (Polites sabuleti), which breeds on Bermuda Grass, not exactly a rare plant! It is also in precipitous decline at our east-slope Sierran site, Sierra Valley. “
“But the decline in population numbers extends to such ubiquitous weedy species as the (introduced) Cabbage White (Pieris rapae) and the West Coast Lady (Vanessa annabella),” the .professor said, adding that “during most of the year I used to be able to collect West Coast Lady larvae from weedy mallows to use in lab cold-shock experiments. Today, I cannot find any at all!”
He went on to explain that in temperate latitudes, “most species confront a problem: how to get through the adverse season, that part of the year when resources, such as host plants and nectar sources, are unavailable and weather conditions are hostile to insect activities if not actually hostile to life. Most species have a well-defined life stage adapted to surviving the adverse season, typically in a physiological state of dormancy called diapause. Species with only one brood a year must always diapause, but those with two or more generations a year must decide whether or not to enter diapause, which is then called facultative.”
“The individual decision to diapause or not is controlled by environmental cues, specifically the photoperiod (length of daylight or photophase and darkness (scotophase) and temperature. The photoperiod cycle is fully determined by the calendar, but temperature is free to vary on various time frames. And it is varying : the frequency of temperature extremes is increasing ("noise" in statistician argot, but the temporal change in frequency of extremes is now showing a clear trend, or "signal!") while a more-or-less steady increase in temperatures, especially at night, is sending a very clear signal.”
Photoperiod and Temperature
“But we know that in determining facultative diapause, the two cues—photoperiod and temperature—are coupled: warmer temperatures require longer nights to induce dormancy,” he pointed out. “That is to say, a fine-tuned synched diapause-inducing system naturally-selected to match prevailing local or regional climates becomes maladapted given a photoperiod-temperature mismatch. This is probably the explanation for the very recent appearance of winter breeding by the monarch, which would normally be in facultative adult reproductive diapause. And it gets worse: for species like the cabbage white butterfly that diapause as pupae, the cue to break diapause and resume development is passing a threshold of accumulated chilling below a threshold temperature; under warming climates that threshold may never be passed, and the animal simply dies.”
In his talk, Shapiro said it’s important to distinguish between noise vs. signal in butterfly faunas. “When we're monitoring butterflies, the individual observations are noise, but we're also looking for signal, and it takes a big data set to do that.”
He mentioned that the title of the book, “The Signal and the Noise,” by popular media statistics guru Nate Silver, describes the challenge. “Statisticians are called upon to analyze and use the properties of data sets, made up of measurements repeated in space or time. Suppose we want to know if numbers of a certain species of butterfly at a particular location are rising, stable or falling. Our data set comprises a time series generated by a monitoring program using standardized methods to assure to the greatest extent possible that the data are properly comparable.”
“For butterflies, such protocols begin with the British Butterfly Monitoring Scheme implemented some 60 years ago; the standardized count methodology is called a Pollard Walk. For a given time series of standardized counts we are interested in sussing out actual trends (signal) distinguished from short-term variations of no predictive value (noise). This is comparable to trying to identify actual climate-change signals amid a welter of short-term fluctuations that we call weather.”
He called attention to an article, “Study Shows 50% Decline in Butterfly Populations Across The European Union,” printed May 26, 2003 in Beyond Pesticides. “Grassland butterflies have undergone a huge overall decrease in numbers,” Shapiro quoted from the article. “Their populations declined by almost 50% from 1990 to 2011” across the EU member states….Intensification of farming is the major culprit for grassland butterflies, and climate, especially heat waves and drought, is close behind. Industrial farming not only destroys habitat, but it uses poisonous chemicals as well.”
“Okay, what is going on?” Shapiro asked. “Why are butterflies in decline throughout the North Temperate zone? We are assuming something of the sort is happening in China and Japan, in Korea, also, but there aren't long term monitoring data available to say.”
“Where we do have long term monitoring data, two things are very apparent. One is there is a long-term trend in butterfly decline, and the other is that it is closely matched with a couple of aspects of climate change. One of them is a more or less clear signal in increasing nighttime temperatures, not so much day, but night.”
“The other is a long-term trend in the increasing frequency of climatic weather extremes. That is, the noise is getting noisier, and that creates a signal, follow me? Climate change is clearly implicated in butterfly decline. We see low elevation species migrating uphill. We see an increasing frequency of records of low elevation species at high elevations and mountains, from the Sierra Nevada, locally in the Rockies in the East. Unfortunately for them, plants can't fly. …So the plants on which butterflies depend may be much slower, and that may prevent the establishment of populations of butterflies at higher elevations, leading to their extinction.”
Turning to neonicintoids (an insecticide family chemically similar to nicotine and which disrupt insect nervous function, leading to paralysis and death), he commented that “the highest concentrations of nicotine that I know are in garden petunias.” They feel sticky. “That's actually nicotine being exuded from glandular hairs or trichomes on the plant. Why is nicotine there? Nicotine is an insecticide, folks. It's a chemical defense against herbivory. Neonics are synthetic insecticides in which lab scientists have decided to improve on the properties of nicotine and make it even deadlier. And so we have a whole family of neonicotinoid insecticides, which came into widespread use in the 1990s both here and in the UK. Marked butterfly declines quickly followed. We had our first sign of that kind of trouble here in this part of California in 1999 when 14 species of butterflies is widely different, habitats declined simultaneously, followed by a gradual drift, culminating in multiple extinctions in 2005.”
What Are the Take-Homes?
"What are the take-homes?" he asked
- “Number one, the only way you can say with confidence that something is happening is to have the combination of reliable data and reliable methods for analyzing and interpreting those data. And because we now have both, we know that whether or not people were correct in their perceptions that there are fewer butterflies than when they were kids, we know now that it has to be true.”
- “The second take-home is that butterflies are an indicator of the overall health of the ecosystem. It isn't to say that the ecosystem depends on the welfare of butterflies. If butterflies went extinct tomorrow, it isn't as if all life would quickly come to an end. But if we are concerned about the future of the planet, we have to take seriously when one of the things that's the easiest to monitor is advertising to us something is very gravely wrong.”
- Third, we have only indirect approaches to deal with climate change, but in order to apply those, we have to be willing to accept that it's happening. I'm not here to talk politics, but I will say that it is a terrible, frightening scandal--the people who purport to be our leaders are rejecting the validity of an overwhelming body of scientific evidence. Climate change is real, folks.” He said he taught biogeography and community ecology for 52 years, and he hoped that the thousands of his students “came out of my courses, I hope, thoroughly convinced that climate is always changing, but it's changing more rapidly than it normally does, now, and we are deeply involved in the causation of that.”
"We can do something about neonics," Shapiro said. "They've been largely outlawed in Europe because of their toxicity in honey bees and other things. There are some moves in that direction in this country. too. For some things, it's going to be too late. "But if we last long enough as individuals and as a civilization, maybe we will be able to see how this thing turns out. I'm not making any predictions. Let's just say I'm scared.”
(Editor's Note: UC Davis Distinguished Professor Emeritus Art Shapiro is a Fellow of the American Association for the Advancement of Science, the California Academy of Sciences, the Royal Entomological Society of London, and the Explorers Club. He is a recipient of the UC Davis Academic Senate's Distinguished Teaching Award and the Outstanding Adviser Award. See more biographical information here)