North American Monarch Research Articles

Breeding season temporal and spatial trends in continental-scale migration of the monarch butterfly.

The monarch butterfly (Danaus plexippus) is a vagile species that undertakes an annual, multi-generational migration across North America. The abundance of this species at both eastern and western overwintering sites in Central Mexico and California indicates a population decline. Success of continental-scale conservation programs for a migratory species depends on providing, maintaining, and protecting habitats at appropriate temporal and spatial scales. Here, dynamics of monarch continental-scale migration and gene flow were obtained by combined stable isotope, morphological, and genetic analyses. These analyses were applied to temporal monarch samples collected from May to September during 2016-2021 at locations in Iowa, USA and spatial collections from Pennsylvania, Delaware, Iowa, Ohio, Nevada, Idaho, Hawaii, 3 Australian locations during July and August 2016, and Texas in April 2021. Evidence of seasonal multi-generational migration was obtained through δ2H analyses of spatial collections, which was corroborated by decreased wing hue (a morphological marker for non-migratory individuals). In Iowa, 10-15% of monarchs represented migrants from southern areas throughout the breeding season and 6% were migrants from the North in midsummer. Limited sequence variation detected across the mitochondrial genome impacted the capability to detect significant population genetic variation in our North American samples. However, 2 novel substitutions were identified and predicted to be fixed among Australia samples, contributing to intercontinental differentiation from counterparts in North America. Our assessment of temporal and spatial population dynamics across the North American monarch breeding range provides insight into continental-scale migration and previously undetected mitochondrial DNA variation among globally distributed populations.

Migration of Eastern North American monarch butterflies via the South-east and the Atlantic: evidence from stable isotopes, thin layer chromatography, DNA and phenotype

Abstract Monarch butterflies, Danaus plexippus (L.), from eastern North America are well known for their incredible autumn migration to Mexico; however, not all monarchs follow this route. There is evidence of monarchs overwintering and reproducing in Florida, arriving to insular and continental Caribbean, and roosting by the thousands in the Yucatán, Mexico. This work aims to present evidence that these monarchs are part of two current migratory routes, that we consider as the south-eastern and the Atlantic routes, routes that were probably more prominent in the past. Monarchs were collected for 12 months in south Florida, for 4 years in Cuba in November and once in March, in the Yucatán and Guatemala at different times, and once in northern Venezuela. We used two independent techniques, stable hydrogen isotope (δ2H) analyses of the wings and/or thin layer chromatography (TLC), to trace the monarch’s natal grounds. We analysed the DNA of monarchs collected in St. Marks in the Florida Panhandle, Cuba and Guatemala, and compared those data against previously-analysed DNA data from monarchs in the Americas to characterize their genetic structure and to assess the possible movement and presence of North American monarchs and/or their alleles outside the USA. Our results support the existence of south-eastern and Atlantic migratory routes. TLC, isotope and DNA analyses showed the arrival of likely North American monarchs in Cuba, Yucatán, Guatemala, Venezuela and other areas of the Americas. North American monarchs found in these areas have different natal grounds, phenotypic traits and DNA signature than Mexican migrants. Monarchs from the south-eastern route mostly originated in the south-east USA and fed on local Asclepias spp., such as Asclepias viridis, Asclepias humistrata, Asclepias perennis and Asclepias asperula. Butterflies from this migratory route move east, enter the Florida Peninsula, pass to Cuba, fly to the Yucatán and then to Guatemala where they appear to overwinter in the high mountains of Guatemala where Abies guatemalensis occurs. Monarchs that are part of the Atlantic route move east of the Appalachians, enter the Florida Peninsula, and from there pass to the insular and continental Caribbean. The main host plants for the Atlantic monarchs are A. perennis and A. humistrata; in contrast, Mexican monarchs mainly feed on Asclepias syriaca. Some monarchs from these two proposed migratory routes, south-eastern and Atlantic, will stay in the places where they travelled and others will return via Florida and Mexico. We propose a scenario for how the different migratory routes evolved.

Seasonal plasticity in morphology and metabolism differs between migratory North American and resident Costa Rican monarch butterflies.

Environmental heterogeneity in temperate latitudes is expected to maintain seasonally plastic life-history strategies that include the tuning of morphologies and metabolism that support overwintering. For species that have expanded their ranges into tropical latitudes, it is unclear the extent to which the capacity for plasticity will be maintained or will erode with disuse. The migratory generations of the North American (NA) monarch butterfly Danaus plexippus lead distinctly different lives from their summer generation NA parents and their tropical descendants living in Costa Rica (CR). NA migratory monarchs postpone reproduction, travel thousands of kilometers south to overwinter in Mexico, and subsist on little food for months. Whether recently dispersed populations of monarchs such as those in Costa Rica, which are no longer subject to selection imposed by migration, retain ancestral seasonal plasticity is unclear. To investigate the differences in seasonal plasticity, we reared the NA and CR monarchs in summer and autumn in Illinois, USA, and measured the seasonal reaction norms for aspects of morphology and metabolism related to flight. NA monarchs were seasonally plastic in forewing and thorax size, increasing wing area and thorax to body mass ratio in autumn. While CR monarchs increased thorax mass in autumn, they did not increase the area of the forewing. NA monarchs maintained similar resting and maximal flight metabolic rates across seasons. However, CR monarchs had elevated metabolic rates in autumn. Our findings suggest that the recent expansion of monarchs into habitats that support year-round breeding may be accompanied by (1) the loss of some aspects of morphological plasticity as well as (2) the underlying physiological mechanisms that maintain metabolic homeostasis in the face of temperature heterogeneity.

Dynamics of stored lipids in fall migratory monarch butterflies (Danaus plexippus): Nectaring in northern Mexico allows recovery from droughts at higher latitudes.

The eastern population of the North American monarch butterfly (Danaus plexippus) overwinters from November through March in the high-altitude (3000m+) forests of central Mexico during which time they rely largely on stored lipids. These are acquired during larval development and the conversion of sugars from floral nectar by adults. We sampled fall migrant monarchs from southern Canada through the migratory route to two overwintering sites in 2019 (n = 10 locations), 2020 (n = 8 locations) and 2021 (n = 7 locations). Moderate to extreme droughts along the migratory route were expected to result in low lipid levels in overwintering monarchs but our analysis of lipid levels of monarchs collected at overwintering sites indicated that in all years most had high levels of lipids prior to winter. Clearly, a significant proportion of lipids were consistently acquired in Mexico during the last portion of the migration. Drought conditions in Oklahoma, Texas and northern Mexico in 2019 resulted in the lowest levels of lipid mass and wing loading observed in that year but with higher levels at locations southward in Mexico to the overwintering sites. Compared with 2019, lipid levels increased during the 2020 and 2021 fall migrations but were again higher during the Mexican portion of the migration than for Oklahoma and Texas samples, emphasizing a recovery of lipids as monarchs advanced toward the overwintering locations. In all 3years, body water was highest during the Canada-USA phase of migration but then declined during the nectar foraging phase in Mexico before recovering again at the overwintering sites. The increase in mass and lipids from those in Texas to the overwintering sites in Mexico indicates that nectar availability in Mexico can compensate for poor conditions experienced further north. Our work emphasizes the need to maintain the floral and therefore nectar resources that fuel both the migration and storage of lipids throughout the entire migratory route.

Monarch Butterfly Ecology, Behavior, and Vulnerabilities in North Central United States Agricultural Landscapes.

The North American monarch butterfly (Danaus plexippus) is a candidate species for listing under the Endangered Species Act. Multiple factors are associated with the decline in the eastern population, including the loss of breeding and foraging habitat and pesticide use. Establishing habitat in agricultural landscapes of the North Central region of the United States is critical to increasing reproduction during the summer. We integrated spatially explicit modeling with empirical movement ecology and pesticide toxicology studies to simulate population outcomes for different habitat establishment scenarios. Because of their mobility, we conclude that breeding monarchs in the North Central states should be resilient to pesticide use and habitat fragmentation. Consequently, we predict that adult monarch recruitment can be enhanced even if new habitat is established near pesticide-treated crop fields. Our research has improved the understanding of monarch population dynamics at the landscape scale by examining the interactions among monarch movement ecology, habitat fragmentation, and pesticide use.

Lack of evidence for a fine-scale magnetic map sense for fall migratory Eastern North American monarch butterflies (Danaus plexippus).

How first-time animal migrants find specific destinations remains an intriguing ecological question. Migratory marine species use geomagnetic map cues acquired as juveniles to aide long-distance migration, but less is known for long-distance migrants in other taxa. We test the hypothesis that naïve Eastern North American fall migratory monarch butterflies (Danaus plexippus), a species that possesses a magnetic sense, locate their overwintering sites in Central Mexico using inherited geomagnetic map cues. We examined whether overwintering locations and the abundance of monarchs changed with the natural shift of Earth's magnetic field from 2004 to 2018. We found that migratory monarchs continued to overwinter at established sites in similar abundance despite significant shifts in the geomagnetic field, which is inconsistent with monarchs using fine-scale geomagnetic map cues to find overwintering sites. It is more likely that monarchs use geomagnetic cues to assess migratory direction rather than location and use other cues to locate overwintering sites.

Flying on empty: reduced mitochondrial function and flight capacity in food-deprived monarch butterflies.

Mitochondrial function is fundamental to organismal performance, health and fitness - especially during energetically challenging events, such as migration. With this investigation, we evaluated mitochondrial sensitivity to ecologically relevant stressors. We focused on an iconic migrant, the North American monarch butterfly (Danaus plexippus), and examined the effects of two stressors: 7days of food deprivation and infection by the protozoan parasite Ophryocystis elektroscirrha (known to reduce survival and flight performance). We measured whole-animal resting metabolic rate (RMR) and peak flight metabolic rate, and mitochondrial respiration of isolated mitochondria from the flight muscles. Food deprivation reduced mass-independent RMR and peak flight metabolic rate, whereas infection did not. Fed monarchs used mainly lipids in flight (respiratory quotient 0.73), but the respiratory quotient dropped in food-deprived individuals, possibly indicating switching to alternative energy sources, such as ketone bodies. Food deprivation decreased mitochondrial maximum oxygen consumption but not basal respiration, resulting in lower respiratory control ratio (RCR). Furthermore, food deprivation decreased mitochondrial complex III activity, but increased complex IV activity. Infection did not result in any changes in these mitochondrial variables. Mitochondrial maximum respiration rate correlated positively with mass-independent RMR and flight metabolic rate, suggesting a link between mitochondria and whole-animal performance. In conclusion, low food availability negatively affects mitochondrial function and flight performance, with potential implications for migration, fitness and population dynamics. Although previous studies have reported poor flight performance in infected monarchs, we found no differences in physiological performance, suggesting that reduced flight capacity may be due to structural differences or low energy stores.

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies.

Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. It is well documented that, among migratory populations, numbers of overwintering monarchs have been falling across several decades, but trends among breeding monarchs are less clear. Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association's annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. Negative impacts of agricultural glyphosate use appeared to be counterbalanced by positive effects of annual temperature, particularly in the US Midwest. Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction. We suggest that density‐dependent reproductive compensation when lower numbers arrive each spring is currently able to maintain relatively stable breeding monarch numbers. However, we caution against complacency since accelerating climate change may bring growing threats. In addition, increases of summer monarchs in some regions, especially in California and in the south, may reflect replacement of migratory with resident populations. Nonetheless, it is perhaps reassuring that ubiquitous downward trends in summer monarch abundance are not evident.

Project ButterBike, or Migrating with Monarchs

Dykman, Sara. 2021. Bicycling with Butterflies: My 10,201-Mile Journey Following the Monarch Migration. Timber Press, Portland, Oregon, USA. The Eastern North American monarch migration, spanning multiple countries and thousands of miles, has long fascinated ecologists, evolutionary biologists, and people of all backgrounds and ages alike. In Bicycling with Butterflies: My 10,201-mile Journey Following the Monarch Migration, Sara Dykman chronicles her expedition across Mexico, the United States, and Canada, cycling along monarch migration routes for nine months. Dykman is a gifted science communicator; she expertly walks readers through phenomena ranging from monarch physiology to coevolution to metapopulation dynamics, using language that is accessible to the general public but still informative to scientists. In this way, it is reminiscent of texts like Anurag Agrawal’s Monarchs and Milkweed (2017) and Sharman Apt Russell’s An Obsession with Butterflies (2004), but it stands out in that each phenomenon is vividly illustrated with Dykman’s personal anecdotes against the backdrop of her epic trip. The book is also an encyclopedia of the people who work with and advocate for monarch butterflies. Dykman details her interactions with guides to the sanctuaries in Mexico, Monarch Watch conservation specialists, “monarch celebrities,” and scientists alike, embodying them with glimpses into their conservation-focused backyards and homes that serve as waypoints throughout her journey. The practical details of her trip are also interesting. The themes of living on the road and cobbling together equipment may seem familiar to field biologists, and the logistics of undertaking such a long bicycle tour alone are fascinating. It is also where a missed opportunity for photographs stands out; as she describes biking along the Mississippi River, the Trans-Canada Highway, and the many other locales, some pictures would have been helpful, especially alongside her vivid descriptions of the ecological and cultural communities associated with each place. The map at the beginning of the book anchors each point of her route to the timing of each chapter effectively, but detailed pictures embedded in the chapters would have added extra clarity. Dykman’s perspective is an interesting one, as a person so captivated by monarch butterflies that she was motivated to bike the entirety of their migration route. In every chapter, her concern for the monarchs and wildlife in general is palpable as she stops to talk to passers-by about the environmental crisis and teach them when best to mow lawns to avoid killing insects, or to pick butterflies and turtles off the road. The gravity with which Dykman approaches these sections can seem a little heavy-handed in the bleakness of her outlook; although she is clearly earnest in trying to communicate the urgency of the situation to her readers, I imagine that most of her audience will already share her sentiments. In places, she also personifies the monarchs, which sometimes seems out of place besides the factually oriented sections on biology. In general, Bicycling with Butterflies is a worthwhile read, with pertinent pieces of information for everyone including cyclists, travelers, and people interested in ecology or the natural world. It serves both as an intriguing ecologist’s travel journal and as a thoughtfully presented compendium of monarch-related information.

Parasite dynamics in North American monarchs predicted by host density and seasonal migratory culling.

Insect-pathogen dynamics can show seasonal and inter-annual variations that covary with fluctuations in insect abundance and climate. Long-term analyses are especially needed to track parasite dynamics in migratory insects, in part because their vast habitat ranges and high mobility might dampen local effects of density and climate on infection prevalence. Monarch butterflies Danaus plexippus are commonly infected with the protozoan Ophryocystis elektroscirrha (OE). Because this parasite lowers monarch survival and flight performance, and because migratory monarchs have experienced declines in recent decades, it is important to understand the patterns and drivers of infection. Here we compiled data on OE infection spanning 50years, from wild monarchs sampled in the United States, Canada and Mexico during summer breeding, fall migrating and overwintering periods. We examined eastern versus western North American monarchs separately, to ask how abundance estimates, resource availability, climate and breeding season length impact infection trends. We further assessed the intensity of migratory culling, which occurs when infected individuals are removed from the population during migration. Average infection prevalence was four times higher in western compared to eastern subpopulations. In eastern North America, the proportion of infected monarchs increased threefold since the mid-2000s. In the western region, the proportion of infected monarchs declined sharply from 2000 to 2015, and increased thereafter. For both eastern and western subpopulations, years with greater summer adult abundance predicted greater infection prevalence, indicating that transmission increases with host breeding density. Environmental variables (temperature and NDVI) were not associated with changes in the proportion of infected adults. We found evidence for migratory culling of infected butterflies, based on declines in parasitism during fall migration. We estimated that tens of millions fewer monarchs reach overwintering sites in Mexico as a result of OE, highlighting the need to consider the parasite as a potential threat to the monarch population. Increases in infection among eastern North American monarchs post-2002 suggest that changes to the host's ecology or environment have intensified parasite transmission. Further work is needed to examine the degree to which human practices, such as mass caterpillar rearing and the widespread planting of exotic milkweed, have contributed to this trend.

Asclepiasdynamics on US rangelands: implications for conservation of monarch butterflies and other insects

AbstractThe genesis of this study is in response to the United States (US) Fish and Wildlife Service (USFWS) listing of the monarch butterfly (Danaus plexippus plexippus) on 17 December 2020 in the US Federal Register as a candidate species under the Endangered Species Act of 1973. Annual censuses have identified that the eastern and western North American monarch migratory populations have been generally declining over the last 20 yr due to a myriad of environmental factors. Monarch reproduction at the larval stage is dependent on the presence of milkweed (Asclepias) plant species. The United States Department of Agriculture‐Natural Resources Conservation Service (USDA‐NRCS) National Resource Inventory rangeland data set (~23,400 on‐site samples; 2032 sites with milkweed presence) was used to evaluate milkweed species densities, geolocations, and environmental gradients. Twenty‐two milkweed species were identified on rangelands across 17 sampled US western states, with seven species comprising 65.5% of milkweed frequency of occurrence. The most dominant milkweed species on non‐federal rangelands wereAsclepias viridis,A. syriaca,A. verticillata, andA. speciosa(constancy >10% where milkweed was present).Asclepias speciosawas the dominant species from the standpoint of total plant density for the data set, whereasA. viridiswas the most frequently occurring species. Total milkweed density estimates based on low, midpoint, and high estimates were 1.3, 4.1, and 6.9Bplants on 13.2 M ha. Seven US states (Kansas, Texas, Nebraska, South Dakota, North Dakota, Oklahoma, and Montana) contained 88.8% of the total estimated ha with milkweed presence. In the Central Great Plains, Northwestern Great Plains, Nebraska Sand Hills, and Flint Hills, Southwestern Tablelands, High Plains, Northwestern Glaciated Plains, and Cross Timbers, Omernik level III ecoregions contained 76.7% of the estimated milkweed plants. Milkweed species density was highest at latitude N35‐40 with decreasing populations toward south (N25‐30) and north (N45‐50) latitudes. Milkweed species densities were greatest at longitude W‐95‐100 and decreased toward the western US with lowest population numbers at W‐120‐125. Analysis of environmental variables showed milkweed species dominance on mollisols, non‐saline sites, neutral pH, well‐drained soils, loam and sandy loam soil textures, and sites with soil organic matter at 1.5–3%. Disturbance gradients and habitat dynamics relating to ecological condition and rangeland health differed among the dominant milkweed species identified in this study.

Genome assembly of Danaus chrysippus and comparison with the Monarch Danaus plexippus.

Milkweed butterflies in the genus Danaus are studied in a diverse range of research fields including the neurobiology of migration, biochemistry of plant detoxification, host–parasite interactions, evolution of sex chromosomes, and speciation. We have assembled a nearly chromosomal genome for Danaus chrysippus (known as the African Monarch, African Queen, and Plain Tiger) using long-read sequencing data. This species is of particular interest for the study of genome structural change and its consequences for evolution. Comparison with the genome of the North American Monarch Danaus plexippus reveals generally strong synteny but highlights 3 inversion differences. The 3 chromosomes involved were previously found to carry peaks of intraspecific differentiation in D. chrysippus in Africa, suggesting that these inversions may be polymorphic and associated with local adaptation. The D. chrysippus genome is over 40% larger than that of D. plexippus, and nearly all of the additional ∼100 Megabases of DNA comprises repeats. Future comparative genomic studies within this genus will shed light on the evolution of genome architecture.

Long-term monitoring indicates shifting fall migration timing in monarch butterflies (Danaus plexippus).

Eastern North American monarch butterflies (Danaus plexippus) embark on a yearly migration between summer breeding grounds in the northern United States and southern Canada and overwintering sites in central Mexico, traveling up to 4300km. This annual multi-generational migration cycle, like many seasonal natural phenomena, may be impacted by recent changes in climate and habitat. We investigated long-term trends in monarch abundance and fall migration timing over a 29-year period in Cape May, New Jersey, using data collected from daily population surveys designed to track patterns of occurrence during the fall migration period through Cape May (1 Sept-31 Oct). Between 1992 and 2020, the migration midpoint, average peak migration day and first peak migration day shifted between 16 and 19days later in the season, an average of approximately six days per decade. This observed shift in migration timing is correlated with increasing temperatures in September and October in northeastern monarch breeding grounds over the study period. Our data do not demonstrate a significant directional trend in monarch abundance over the study period, yet population data collected at overwintering sites in Mexico indicate a substantial decline over the same period. Further postponement of fall migration may lead to lower migration success and exacerbate the overall decline of this iconic species.

Host Plant Species Mediates Impact of Neonicotinoid Exposure to Monarch Butterflies.

Simple SummaryNeonicotinoids are the most widely used insecticides in North America and many studies document the negative effects of neonicotinoids on bees. Monarch butterflies are famous for their long-distance migrations, and for their ability to sequester toxins from their milkweed host plants. The neonicotinoids imidacloprid and clothianidin were suggested to correlate with declines in North American monarchs. We examined how monarch development, survival, and flight were affected by exposure to neonicotinoids, and how these effects depend on milkweed host plant species that differ in their cardenolide toxins. Monarch survival and flight were unaffected by low and intermediate neonicotinoid doses. At the highest dose, neonicotinoids negatively affected monarch pupation and survival, for caterpillars that fed on the least toxic milkweed. Monarchs fed milkweed of intermediate toxicity experienced moderate negative effects of high insecticide doses. Monarchs fed the most toxic milkweed species had no negative consequences associated with neonicotinoid treatment. Our work shows that monarchs tolerate low neonicotinoid doses, but experience detrimental effects at higher doses, depending on milkweed species. To our knowledge, this is the first study to show that host plant species potentially reduce the residue of neonicotinoid insecticides on the leaf surface, and this phenomenon warrants further investigation.Neonicotinoids are the most widely used insecticides in North America. Numerous studies document the negative effects of neonicotinoids on bees, and it remains crucial to demonstrate if neonicotinoids affect other non-target insects, such as butterflies. Here we examine how two neonicotinoids (imidacloprid and clothianidin) affect the development, survival, and flight of monarch butterflies, and how these chemicals interact with the monarch’s milkweed host plant. We first fed caterpillars field-relevant low doses (0.075 and 0.225 ng/g) of neonicotinoids applied to milkweed leaves (Asclepias incarnata), and found no significant reductions in larval development rate, pre-adult survival, or adult flight performance. We next fed larvae higher neonicotinoid doses (4–70 ng/g) and reared them on milkweed species known to produce low, moderate, or high levels of secondary toxins (cardenolides). Monarchs exposed to the highest dose of clothianidin (51–70 ng/g) experienced pupal deformity, low survival to eclosion, smaller body size, and weaker adult grip strength. This effect was most evident for monarchs reared on the lowest cardenolide milkweed (A. incarnata), whereas monarchs reared on the high-cardenolide A. curassavica showed no significant reductions in any variable measured. Our results indicate that monarchs are tolerant to low doses of neonicotinoid, and that negative impacts of neonicotinoids depend on host plant type. Plant toxins may confer protective effects or leaf physical properties may affect chemical retention. Although neonicotinoid residues are ubiquitous on milkweeds in agricultural and ornamental settings, commonly encountered doses below 50 ng/g are unlikely to cause substantial declines in monarch survival or migratory performance.

Spiraling Flight Behavior May Integrate the Biological Compass Systems of Migratory North American Monarch Butterflies, (Danaus plexippus, L.)

Spiraling Flight Behavior May Integrate the Biological Compass Systems of Migratory North American Monarch Butterflies, (Danaus plexippus, L.)

Influence of habitat quality and resource density on breeding‐season female monarch butterfly Danaus plexippus movement and space use in north‐central USA agroecosystem landscapes

Abstract The eastern North American monarch butterfly is at risk of quasi‐extinction due, in part, to the loss of breeding habitat in agricultural landscapes of the USA Midwest. Because adult females are not patch residents, egg abundance and distribution across the landscape are a function of their perceptual range, flight directionality and flight step lengths. Conservation actions that account for habitat use in agricultural landscapes can enhance functional connectivity. Field‐captured females (n = 114) were released in a 64‐ha area containing restored prairies, grass‐dominated fields and crop fields in Floyd County, Iowa, USA, and two 1,000 m linear north–south sections of grass‐dominated roadside along secondary roads (~35 ha) with different proximity to prairie habitat in rural Story County, Iowa. Radio‐tagged or untagged monarchs were released in areas with high density, low density and zero density of forage and oviposition resources, as well as on habitat edges between high‐ and zero‐density habitats. Monarchs were observed for 1 hr. Radio‐tagged individuals that flew beyond visual detection were relocated using handheld radiotelemetry. Monarchs moved within and between habitat classes and typically performed upwind search behaviour. Monarchs successfully located resources, with some flying over 500 m to find high‐density areas, providing evidence that the monarch's perceptual distance is >100 m. Regardless of habitat class or field site, most step lengths were <50 m, and turn angles were directional. Large steps (≥50 m) crossing habitat boundaries occurred with approximately half of the monarchs, which may indicate initiation of long‐range searches for suitable habitat, consistent with their vagile behaviour. Establishing habitat patches 50 m apart in agricultural landscapes would facilitate efficient movement. Synthesis and applications. This study provides an extensive dataset of directly observed breeding‐season monarch butterflies to assess the utilization of agricultural landscapes. Documentation of step lengths >50 m in complex, agricultural landscapes would not have been possible without the aid of radiotelemetry. Results provide improved estimates of perceptual range and flight patterns within and between habitat patches that support models that simulate natural population dynamics to enable conservation planning at a landscape scale.

First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California.

Simple SummaryPopulations of western monarch butterflies, which formerly populated coastal overwintering sites in California in numbers exceeding one million, dwindled to less than 2000 in 2020/21. In the same winter, breeding populations of monarchs occurred commonly in the San Francisco Bay urban area for the first time. The urgent conservation need to understand this possible shift in overwintering strategy prompted this first study of the viability and ecology of monarch breeding populations in the South Bay urban area of San Francisco during January–June 2021. Adult monarchs along with eggs and larvae were common during winter and most of spring, utilizing non-native ornamental milkweed and multiple nectar sources from native and ornamental plants. Evidence from weekly counts and tagged butterflies indicated increased dispersal to the north and east during late April-mid-May, possibly representing spring migration to the Pacific Northwest and eastern California. Infection of reared adult monarchs with a protozoan parasite was high. Winter breeding of monarchs in the San Francisco urban area as an alternative and sustainable overwintering strategy for the western US population will likely depend on the continued presence of ornamental milkweeds, sustainable co-existence of monarchs and protozoan parasites, and successful migration to the greater western US during spring.The western North American monarch butterfly population assessed by counts of non-reproductive overwintering butterflies at coastal sites in California declined to less than 2000 in 2020/21. Simultaneously, reports of reproductive monarchs increased in San Francisco urban areas, perhaps representing a shift in overwintering strategy. To better understand monarch winter breeding in the Bay area, we studied adult and immature populations in Santa Clara County during January–June 2021. Adult monarchs were common with numbers ranging from 0.23–1.54/min during ~30 min weekly surveys at one site, with lowest numbers late April to mid-May. Eggs and larvae, primarily on ornamental milkweeds, were found on nearly all survey dates with lowest numbers mid-late April to mid-May. Levels of infection of adults by the parasite Ophryocystis elektroscirrha were consistently high during the study (69.3–77.5%). From 499 monarchs tagged post-eclosion, recovery rates of 19.2–23.6% occurred from releases in January-February and May-June but only 11.9–13.0% from March-April releases. Although distances were small, butterflies tagged in April were recovered from greater distances than other months. Tagged monarchs flew primarily north or east. There were reduced numbers of adult monarchs during late April-mid-May with some evidence of northerly and easterly emigration at the same time from tagged butterflies, suggesting some movement out of the South Bay area, perhaps representing spring migration. We conclude that monarchs can successfully breed and maintain populations on ornamental milkweeds during winter at urban sites in the South Bay of San Francisco and may still migrate during spring to remain part of the wider western population.

Population genomics reveals variable patterns of immune gene evolution in monarch butterflies (Danaus plexippus).

Humoral and cellular immune responses provide animals with major defences against harmful pathogens. While it is often assumed that immune genes undergo rapid diversifying selection, this assumption has not been tested in many species. Moreover, it is likely that different classes of immune genes experience different levels of evolutionary constraint, resulting in varying selection patterns. We examined the evolutionary patterns for a set of 91 canonical immune genes of North American monarch butterflies (Danaus plexippus), using as an outgroup the closely related soldier butterfly (Danaus eresimus). As a comparison to these immune genes, we selected a set of control genes that were paired with each immune for approximate size and genomic location. As a whole, these immune genes had a significant but modest reduction in Tajima's D relative to paired-control genes, but otherwise did not show distinct patterns of population genetic variation or evolutionary rates. When further partitioning these immune genes into four functional classes (recognition, signalling, modulation, and effector), we found distinct differences among these groups. Relative to control genes, recognition genes exhibit increased nonsynonymous diversity and divergence, suggesting reduced constraints on evolution, and supporting the notion that coevolution with pathogens results in diversifying selection. In contrast, signalling genes showed an opposite pattern of reduced diversity and divergence, suggesting evolutionary constraints and conservation. Modulator and effector genes showed no statistical differences from controls. These results are consistent with patterns found in immune genes in fruit flies and Pieris butterflies, suggesting that consistent selective pressures on different classes of immune genes broadly govern the evolution of innate immunity among insects.

Estimating Perceptual Range of Female Monarch Butterflies (Danaus plexippus) to Potted Vegetative Common Milkweed (Asclepias syriaca) and Blooming Nectar Resources.

Habitat loss in the summer breeding range contributes to eastern North American monarch (Lepidoptera: Nymphalidae) population decline. Habitat restoration efforts include increasing native prairie plants for adult forage and milkweed (Asclepias spp.) for oviposition and larval development. As the monarch is a vagile species, habitat establishment at a grain that matches the monarch perceptual range will facilitate efficient movement, decrease fitness costs of dispersal, and increase oviposition. We released 188 experimental monarch females 5, 25, 50, and 75 m downwind from potted milkweed and blooming forbs in 4-32 ha sod fields. Perceptual range was estimated from monarchs that flew towards and landed on the milkweed and forbs. Flight patterns of 49 non-experimental monarchs that landed on the resources were also observed. In our experimental, resource-devoid setting, wind-facilitated movement occurred most frequently. Monarchs performed direct displacement as evidenced by shallow turn angles and similarity of Euclidian and total distances traveled. We hypothesize similar monarch flight behavior when traveling over other resource-devoid areas, such as crop fields. Although the majority of experimental monarchs flew downwind, eight experimental and 49 non-experimental monarchs were observed flying upwind toward, and landing on, the potted resources from distances ranging from 3 to 125 m (mean = 30.98 m, median = 25 m, mode = 25 m). A conservative estimate of the perceptual range is 125 m, as longer distances cannot be precluded; however, the majority of observations were ≤50 m. Our findings suggest establishing habitat patches ~ 50 m apart would create functional connectivity across fragmented agricultural landscapes.

Pyrethroid Exposure Reduces Growth and Development of Monarch Butterfly (Lepidoptera: Nymphalidae) Caterpillars.

Insecticide exposure has been identified as a contributing stressor to the decline in the North American monarch butterfly Danaus plexippus L. (Lepidoptera: Nymphalidae) population. Monarch toxicity data are currently limited and available data focuses on lethal endpoints. This study examined the 72-h toxicity of two pyrethroid insecticides, bifenthrin and β-cyfluthrin, and their effects on growth and diet consumption. The toxicity of bifenthrin to caterpillars was lower than β-cyfluthrin after 72 h. Survival was the most sensitive endpoint for bifenthrin, but diet consumption and caterpillar growth were significantly reduced at sublethal levels of β-cyfluthrin. Using AgDRIFT spray drift assessment, the aerial application of bifenthrin or β-cyfluthrin is predicted to pose the greatest risk to fifth-instar caterpillars, with lethal insecticide deposition up to 28 m for bifenthrin and up to 23 m for β-cyfluthrin from treated edges of fields. Low boom ground applications are predicted to reduce distances of lethal insecticide exposure to 2 m from the treated field edge for bifenthrin and β-cyfluthrin. Growth and survival of fifth-instar monarch caterpillars developing within the margins of a treated field may be significantly impacted following foliar applications of bifenthrin or β-cyfluthrin. These findings provide evidence that pyrethroid insecticides commonly used for soybean pest control are a potential risk to monarch caterpillars in agricultural landscapes.