- Author: Kathy Keatley Garvey
Butterfly and monarch scientists will discuss their expertise at the Bohart Museum of Entomology open house on "Monarchs," set Saturday, Nov. 4 from 1 to 4 p.m.
The event, free and family friendly, will be held in Room 1124 of the Academic Surge Building, Crocker Lane. This is an opportunity for attendees to ask questions about monarchs (Danaus plexippus) and native vs. non-native milkweed, among other topics.
The scientists will include:
- UC Davis distinguished professor emeritus Art Shapiro of the Department of Evolution and Ecology, who has studied butterfly populations in central California since 1972 and maintains a research website, Art's Butterfly World.
- UC Davis emeritus professor Hugh Dingle, a worldwide authority on animal migration, including monarchs. He is the author of Migration: The Biology of Life on the Move (Oxford University Press), a sequel to the first edition published in 1996. See news story on the UC Davis Entomology and Nematology website.
- UC Davis professor Louie Yang, who does research on monarchs. Due to parental duties, he may be able to attend only the last part of the open house. See news story about his work.
- UC Davis professor Elizabeth Crone of the Department of Evolution and Ecology, formerly of Tufts University, who researches monarchs. See news story about the declining monarch population on the UC Davis Entomology and Nematology website.
- UC Davis postdoctoral fellow Aramee Diethelm of the Elizabeth Crone lab. She holds a doctorate from the University of Nevada, Reno. Both her Ph.D. and postdoctoral work are on monarch butterflies. As a doctoral student, she investigated the phytochemical landscape of milkweed (Asclepias) species across northern Nevada and the effects of this variation on western monarch (Danaus plexippus) butterfly performance. See her research posted on Google Scholar, and her blog on "Drought Influences Monarch Host Plant Selection."
Shapiro points out that the monarch "is NOT a focal species in my research and I am NOT a monarch expert. On the other hand, I have a unique breeding-season census data set starting in 1999. The only other census data are for the overwintering roosts on the coast. It has become apparent that the two data sets do not always agree." Shapiro said he'd talk briefly about this at the open house.
"The iconic black and orange monarch butterfly is known for its astonishing long-distance annual migration and reliance on milkweed as its obligate larval host plant," according to a post on the California Department of Fish and Wildlife (CDFW). "Though genetically similar, there are two subpopulations of monarchs in North America, with the eastern population overwintering in Mexico and breeding in the midwestern states, and the western population overwintering in coastal California and fanning out across the west from Arizona to Idaho. Outside the U.S., there are at least 74 known populations of resident, non-migratory monarchs that have established around the world in the past 200 years, all with origins in North America (Nial et al. 2019)."
"Both North American migratory populations have declined over the past twenty years due to a suite of interrelated factors including habitat loss in breeding and overwintering sites, habitat degradation, disease, pesticide exposure, and climate change," CDFW says. "Recently the western population has experienced dramatic swings, for a low of less than 2,000 in 2020-21 to over 200,000 in 2021-22 (Xerces Society Western Monarch Count). While it is unclear which of the many factors are driving these dynamics, insect population commonly fluctuate from year to year. The overall downward trend remains concerning, particularly if the threats are not ameliorated. Though more research is needed, a stable population for western monarchs is likely closer to the historic averages in the 1980's, which are estimated to have ranged between one to four million overwintering butterflies."
"n 2014, monarchs were petitioned to be listed under the federal Endangered Species Act. In December 2020, the U.S. Fish and Wildlife Service found that listing was warranted but precluded by other listing actions on its National Priority List. The monarch is currently slated to be listed in 2024."
The monarch population is in trouble. Says the Xerces Society for Invertebrate Conservation in a post on its website: "In the 1990s, hundreds of millions of monarchs made the epic flight each fall from the northern plains of the U.S. and Canada to sites in the oyamel fir forests in central Mexico, and more than a million monarchs overwintered in forested groves on the California coast. Now, researchers and community scientists estimate that only a fraction of the population remains—a decline of approximately 70% has been seen in central Mexico and a decline of >90% has been seen in California."
Resources/Further Reading:
- Monarch Butterfly, California Department of Fish and Wildlife
- Research Permits, CDFW
- Western Monarch Mllkweed Mapper
- Integrated Monarch Monitoring Program, Monarch Joint Venture
- Western Monarch Count, Xerces Society for Invertebrate Conservation
- Monarch Butterfly Conservation, Xerces Society
- Spreading Milkweed, Not Myths, U.S. Fish and Wildlife Service
- Habitat Corridor Project, Views on milkweed
- Tropical Milkweed Doesn't Deserve the Bad Rap, Bug Squad blog, views on the ban of tropical milkweed by UC Davis emeriti professors Art Shapiro and Hugh Dingle, and Washington State University entomologist David James, who studies migratory monarchs.
- Author: Kathy Keatley Garvey
“This study collected a high-resolution temporal dataset on milkweed-monarch interactions during the last three years prior to the precipitous single-year population decline of western monarchs in 2018,” said community ecologist Louie Yang, a professor in the Department of Entomology and Nematology.
Yang organized and led a 135-member team, all co-authors of the paper, “Different Factors Limit Early- and Late-Season Windows of Opportunity for Monarch Development,” published in the journal Ecology and Evolution. (This document is open access at https://bit.ly/3volFaI.)
From 2015 through 2017, the team monitored the interactions of monarchs, Danaus plexippus, on narrow-leaved milkweed, Asclepias fascicularis, planted in December 2013on city-owned property adjacent to the North Davis irrigation channel.
“This study has three key findings,” the UC Davis professor said. “First, we documented early and late seasonal windows of opportunity in the wild, migratory western monarch population. Second, our data suggest that early and late seasonal windows were constrained by different factors. Third, climatic and microclimatic variation had a strong effect on the timing and importance of multiple factors affecting monarch development. Broadly, we hope that this study contributes to a more temporally detailed understanding of the complex factors that contribute to year-to-year variation in monarch breeding success.”
Feared on its way to extinction, the migratory monarch is now on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species as Endangered—threatened by habitat destruction and climate change. Statistics show that the overwintering population of western monarchs along coastal California has declined by more than 99 percent since the 1980s, according to the Xerces Society for Invertebrate Conservation.
The UC Davis-based team set out to answer three questions: (1) How do the developmental prospects of monarchs vary in time, within- and across years? (2) How do the combined effects of bottom-up, top-down, and abiotic factors interact with seasonal variation in monarch density to constrain the timing and extent of seasonal windows of opportunity? and (3) How do climatic variation and microhabitat heterogeneity affect these constraints?
The results showed that different combinations of factors constrained the early- and late-season windows of opportunity for monarch recruitment. “Early-season windows of opportunity were characterized by high egg densities and low survival on a select subset of host plants, consistent with the hypothesis that early-spring migrant female monarchs select earlier-emerging plants to balance a seasonal trade-off between increasing host plant quantity and decreasing host plant quality,” the abstract relates. “Late-season windows of opportunity were coincident with the initiation of host plant senescence, and caterpillar success was negatively correlated with heatwave exposure, consistent with the hypothesis that late-season windows were constrained by plant defense traits and thermal stress.”
The researchers also noted:
- “Throughout this study, climatic and microclimatic variations played a foundational role in the timing and success of monarch developmental windows by affecting bottom-up, top-down, and abiotic limitations. More exposed microclimates were associated with higher developmental success during cooler conditions, and more shaded microclimates were associated with higher developmental success during warmer conditions, suggesting that habitat heterogeneity could buffer the effects of climatic variation.”
- “Together, these findings show an important dimension of seasonal change in milkweed-monarch interactions and illustrate how different biotic and abiotic factors can limit the developmental success of monarchs across the breeding season. These results also suggest the potential for seasonal sequences of favorable or unfavorable conditions across the breeding range to strongly affect monarch population dynamics.”
Yang and his team planted 318 narrow-leaved milkweed adjacent to the seasonal irrigation channel, which carries runoff water with a “seasonal pattern of generally increased flow during summer irrigation periods and immediately following winter precipitation events. As a result, this site combines several elements representative of the California Central Valley at a landscape scale.” The Davis site typifies a “Mediterranean pattern of cool, wet winters and hot dry summers.”
The researchers recorded daily temperatures and precipitation in one dataset, and in a second dataset, sub-hourly temperature observations, approximately every 20 minutes. They defined the “early season” as days 90–180 (approximately the end of March to the end of June) and the late season as days 180–270 (approximately the end of June to the end of September) each year.”
They measured and recorded the milkweed growth and leaf area removal by herbivores, and counted and measured the eggs and larvae. They also gathered information on the predator and herbivore community.
MMMILC Project. Participants in the Monitoring Milkweed–Monarch Interactions for Learning and Conservation (MMMILC) Project, directed by Yang, collected most of the observations. Yang provided hands-on, in-person training in milkweed-monarch biology, data collection, and data entry protocols, partnering with the Environmental Science internship program led by Eric Bastin at Davis Senior High School and the Growing Green internship program led by Karen Swan at the Center for Land-based Learning, Woodland.
“We documented 674 weekly observations of monarch eggs and 997 weekly observations of monarch caterpillars across the three years of this study,” the researchers wrote. “Monarchs were most numerous in 2016. We observed 2.7 times as many monarch eggs in 2016 as in 2015 and 2.2 times as many as in 2017. We observed 3.0 times as many caterpillars in 2016 as in 2015, and 2.5 times as many as in 2017. Separated by year and normalized by the total number of emerged plants each year, we observed 137 eggs and 193 caterpillars (0.49 egg and 0.69 caterpillar observations per plant) in 2015, 369 eggs and 576 caterpillars (1.55 egg and 2.42 caterpillar observations per plant) in 2016 and 168 eggs and 226 caterpillars (0.74 egg and 1.0 caterpillar observations per plant) in 2017.
Among their research findings:
- The early and late monarch developmental periods were generally warmer in 2017 than in the two previous years.
- The number of surviving emerged plants declined over the 3-year study, from 281 (88.3 percent) in 2015, to 238 (75 percent) in 2016 to 226 (71 percent) in 2017. However, an increasing proportion of the surviving plants attained a total stem length exceeding 50 cm across these same years: 137 (49 percent of 281) in 2015, 144 (61 percent of 238) in 2016, and 175 (77 percent of 226).
- The growth of milkweeds changed dramatically in 2017 following the rainy winter of 2016–2017. Milkweeds in 2017 attained sizes (maximum weekly mean total stem lengths) that were 70 percent larger than in 2015, and 64 percent larger than in 2016, and the variance of the plant size distribution also increased.
- Milkweed emerged earliest in 2016 (mean emergence day 110) and nearly four weeks later in 2017 (mean emergence day 137).
Unfortunately, a City of Davis maintenance crew unintentionally mowed the site on May 5, 2017, “damaging several plants in this population. However, most plants in the population were below the height of the mower blades at this point in the growing season.”
Today the milkweed population at the North Davis Channel is being maintained by the City of Davis and dedicated citizens, including Larry Snyder, who documented the project in photographs. “We aren't monitoring there intensively, but we've seen monarch eggs, caterpillars and adults there this year,” Yang said.
More monarch projects from the Louie Yang lab are pending. “The next paper in press represents research done several years ago and is focused on the timing of herbivory and its effects on flowering,” he said. “We are studying several California milkweed species.”