Monarch Conservation Research Articles

Milkweed and floral resource availability for monarch butterflies (Danaus plexippus) in the United States

The global decline of pollinators, particularly insects, underscores the importance of enhanced monitoring of their populations and habitats. However, monitoring some pollinator habitat is challenging due to widespread species distributions and shifts in habitat requirements through seasons and life stages. The monarch butterfly (Danaus plexippus), a migratory insect pollinator that breeds widely throughout North America, presents a unique case study for testing a sampling framework to overcome these challenges. Monarchs exhibit discrete resource needs across life stages (e.g., larval requirement for milkweed, adult requirement for floral nectar), utilizing many land use types across their extensive geographic range during breeding and migration seasons. The Integrated Monarch Monitoring Program (IMMP) uses a standardized protocol with a generalized random tessellation stratified (GRTS) sampling design to gather spatially balanced and ecologically representative information on monarch habitats within the United States. The IMMP is applicable to various land use types and habitats used by breeding monarchs and may be extended to sites outside of the GRTS design to collect data on non-random sites of interest, such as legacy or conservation sites. Additionally, the IMMP’s modular design and publicly available training allows for broad participation, including involvement from community scientists. Here, we summarize habitat metrics (milkweed and floral resources) across 1,233 sites covering much of the monarch’s breeding range. We examine variation in milkweed density and floral resource availability on probabilistic (random) and non-probabilistic (convenience) samples and among land use types (site types). Additionally, we assess resource availability within core geographies for monarch breeding and migration, specifically within the U.S. Fish and Wildlife Service’s Monarch Conservation Units (western, northern, and southern United States). Milkweed density, floral frequency, and floral richness were higher on non-random sites and in the North region. Among site types, milkweed density was highest on Rights-of-Way and Unclassified Grassland, while floral frequency was lowest on Rights-of-Way. The IMMP represents the first field-based habitat monitoring program of this scale for monarchs, yielding a robust dataset on monarchs and their habitats across their breeding range and offering a framework for surveying the habitat of insect species with diverse habitat requirements or widespread distributions.

Challenges creating monarch butterfly management strategies for electric power companies in the United States

Returning monarch butterflies (Danaus plexippus) to sustainable levels of abundance will require an array of contributors to protect and restore habitat over broad areas. Due to the diversity and scale of land managed by electric power companies across the monarch range, plus an additional 32 million hectares needed for new solar arrays by 2050 to meet renewable energy goals, the industry may have potential to contribute to monarch conservation. However, it is challenging to clearly understand an individual company’s potential for monarch conservation because of the scale and distribution of their specific land assets (ranging from 4,800 to 240,000 hectares in this study alone), the complexity of monarch science, and the lack of a science-based approach for evaluating large land assets for monarch habitat. With monarchs potentially being protected under the United States Endangered Species Act in the future and thereby limiting land management approaches, there is interest from electric power companies to understand how their lands relate to monarchs. In collaboration with companies, we developed a GIS-based model to identify company landholdings that contain high-quality monarch habitat and applied the model to specific landholdings of eight power companies in the United States. We then facilitated discussions with company teams to balance conservation goals, corporate risk, and social opinion. This paper describes non-confidential results for developing a national GIS-based monarch habitat model and applying it to electric power companies who are considering monarch conservation while simultaneously transitioning to a new clean energy future. The model and applied experience may be useful for other organizations working across large landscapes to manage monarchs.

Aphid infestations reduce monarch butterfly colonization, herbivory, and growth on ornamental milkweed.

Anthropogenic disturbance is driving global biodiversity loss, including the monarch butterfly (Danaus plexippus), a dietary specialist of milkweed. In response, ornamental milkweed plantings are increasingly common in urbanized landscapes, and recent evidence indicates they have conservation value for monarch butterflies. Unfortunately, sap-feeding insect herbivores, including the oleander aphid (Aphis nerii), frequently reach high densities on plants in nursery settings and urbanized landscapes. Aphid-infested milkweed may inhibit monarch conservation efforts by reducing host plant quality and inducing plant defenses. To test this, we evaluated the effects of oleander aphid infestation on monarch oviposition, larval performance, and plant traits using tropical milkweed (Asclepias curassavica), the most common commercially available milkweed species in the southern U.S. We quantified monarch oviposition preference, larval herbivory, larval weight, and plant characteristics on aphid-free and aphid-infested milkweed. Monarch butterflies deposited three times more eggs on aphid-free versus aphid-infested milkweed. Similarly, larvae fed aphid-free milkweed consumed and weighed twice as much as larvae fed aphid-infested milkweed. Aphid-free milkweed had higher total dry leaf biomass and nitrogen content than aphid-infested milkweed. Our results indicate that oleander aphid infestations can have indirect negative impacts on urban monarch conservation efforts and highlight the need for effective Lepidoptera-friendly integrated pest management tactics for ornamental plants.

Change in monarch winter abundance over the past decade: A Red List perspective

Abstract Assessing invertebrate species for the IUCN Red List under Criterion A frequently involves fitting an appropriate statistical model to available abundance data and calculating a 10‐year change (TYC) estimate from predicted abundances. When the rate of change has not been constant across the entire time series, models that accommodate variable change rates are recommended. The monarch butterfly (Danaus plexippus) was recently added to the IUCN Red List (A2ab Endangered) based on analysis of data on winter abundances in Mexico and the western USA between 1993 and 2020. TYC estimates in the monarch assessment came from models that assumed constant change rates. We conducted a Bayesian analysis of the same data using models that accommodated variable change rates and used those models to compute TYC estimates. Our results suggested that monarch population change rates have not been constant over the study period. The analysis yielded a model‐averaged TYC estimate of +5.23%, which was not statistically distinguishable from 0% and was considerably different from values of −46% and −72% in the assessment. The Bayesian posterior probability of a TYC value below −30% (A2ab Vulnerable) was 0.15 and that of a TYC value below −50% (A2ab Endangered) was 0.03. We recommend that researchers evaluating monarch conservation status consider using abundance models with variable change rates, as models with constant change rates may not accurately predict the trajectory of monarch abundances into the future.

The benefits of big-team science for conservation: Lessons learned from trinational monarch butterfly collaborations

Many pressing conservation issues are complex problems caused by multiple social and environmental drivers; their resolution is aided by interdisciplinary teams of scientists, decision makers, and stakeholders working together. In these situations, how do we generate science to effectively guide conservation (resource management and policy) decisions? This paper describes elements of successful big-team science in conservation, as well as shortcomings and lessons learned, based on our work with the monarch butterfly (Danaus plexippus) in North America. We summarize literature on effective science teams, extracting information about elements of success, effective implementation approaches, and barriers or pitfalls. We then describe recent and ongoing conservation science for the monarch butterfly in North America. We focus primarily on the activities of the Monarch Conservation Science Partnership–an international collaboration of interdisciplinary scientists, policy experts and natural resource managers spanning government, non-governmental and academic institutions—which developed science to inform imperilment status, recovery options, and monitoring strategies. We couch these science efforts in the adaptative management framework of Strategic Habitat Conservation, the business model for conservation employed by the US Fish and Wildlife Service to inform decision-making needs identified by stakeholders from Canada, the United States, and Mexico. We conclude with elements critical to effective big-team conservation science, discuss why science teams focused on applied conservation problems are unique relative to science teams focusing on traditional or theoretical research, and list benefits of big team science in conservation.

Investing in monarch conservation: understanding private funding dynamics

Non-profit environmental organizations (NGOs) rely heavily on external donors to fulfill their mandates. However, forecasting donations for long-term planning is an elusive task at best. The non-compulsory nature of donation requires NGOs to understand how donors’ attention and funding allocations change over time as conservation scenarios change and incorporate these insights into their budgeting plans. We hypothesize that an NGO can hinder its capacity to reach its conservation goals by neglecting its donor-NGO-natural system (DNNS), which is reactive to the socio-ecological context. To test our hypothesis, we compared the ecological outcomes derived from a budgeting strategy assuming donors have a fixed willingness to pay throughout the program (open-loop) against the reality that donor preferences change over time (closed-loop) based on the evolving ecological context, partly driven by the program’s actions. Our analysis was performed using two different willingness to pay (WTP) behavioural models, one representing donors informed about the success of the program supported (GPI), and another without such information (GPI), evidencing how the underlying assumptions about the target donors can radically change the organization’s fundraising strategy. Next, we used our closed-loop approach to estimate NGO’s optimal yearly donation requests to achieve a conservation target. Finally, we tested the consequences of presuming an incorrect WTP behavioural model while estimating optimal yearly donation requests by applying the optimization results from the previous step into a model parameterized with a different behavioural model. Our model was created by coupling a discrete choice experiment (DCE) and a systems dynamics model, developing a coupled social-ecological model of the eastern Monarch butterfly (Danaus plexippus), a charismatic long-distant migrant butterfly that has dwindled in numbers across North America mainly due to the increases in GMO agriculture. Our results showed a significant difference in donations received and ecological outcome forecasted by an open-loop model and the actual numbers obtained by the more real-life, closed-loop model, highlighting the importance of accounting for human behaviour during the planning phase of a long-term conservation strategy. Next, when we used our closed-loop to estimate optimal donation requests, the conservation objectives and funds raised were consistently and efficiently achieved, regardless of the underlying behavioural WTP model. We also designed novel visual tools from the behaviour WTP model exploration to bridge the gap between science insights obtained from DCEs and decision-making. However, when we used closed-loop optimal donation requests obtained from one WTP behaviour model into a simulation parameterized with different WTP behavioural models, considerable ecological and financial targets deviations arose. These deviations highlight the importance of acknowledging the dynamic nature of donor’s behaviour and the need to thoroughly characterize such behaviour. Finally, we introduce a novel forecasting tool that conservation managers will have at their disposal to improve the accuracy of their budget forecasting and, ultimately, increase the program’s success rate.

Tracking the Migration of the Monarch Butterflies with the World's Smallest Computer

Each fall, millions of monarch butterflies across the U.S. and Canada migrate up to 4,000 km to overwinter in the same cluster of mountaintops in central Mexico. In spring, these migrants mate and remigrate northwards to repopulate their northern breeding territory over 2-4 partially overlapping generations. Because each migrant monarch completes only part of this round trip and does not return to the overwintering site, this navigational task cannot be learned from the prior generation. The number of monarchs completing the journey has dramatically declined in the past decades, coincident with the decreased availability of their milkweed host plant. The U.S., Mexico, and Canada have invested tremendous resources into monarch conservation efforts, including enacting specific policy initiatives, public outreach programs, and habitat protection and restoration projects. The US invested over $11 million between 2015-2017 alone [1]. Developing a tracking technology for monarch can be a key in these efforts, providing, for instance, detailed understanding of habitat use during migratory flight and dependence on weather conditions. Furthermore, it can significantly benefit animal research, and agricultural and environmental science.

Distribution and phenology of monarch butterfly larvae and their milkweed hosts in the South Central US

South Central US milkweeds (Asclepias) are critical adult nectar and larval food resources for producing the first spring and last summer/fall generations of declining eastern migratory monarch butterflies (Danaus plexippus). This study addresses multiple gaps in assessment of monarch conservation priorities for the South Central US through analyses of monarch larval host selectivity, phenology, and spatial density, as well as the phenology, niche modeled distribution, and land cover selectivity of important milkweed hosts. Results are synthesized to estimate seasonal milkweed resource areas. About 70% of monarch larval activity occurred from mid-March to mid-July (early season) and 30% from mid-August to late November (late season). Twenty-six wild milkweed (Apocynaceae) hosts were mapped, including four new records for North America. Important hosts included Asclepias a. ssp. capricornu, A. viridis, and A. oenotheroides, that were utilized more frequently during early season, and Asclepias latifolia, utilized more frequently during late season. Landscape host selectivity was positive for A. viridis and A. a. ssp. capricornu in late and early seasons, respectively, and negative for A. oenotheroides in late season. Milkweed land cover selectivity was positive for Developed-Open Space and Grassland Herbaceous, and negative for Cultivated Crops and Shrub/Scrub. Seasonal milkweed resource areas and larval spatial densities resolved interior and coastal corridors providing functional connectivity for monarch spring and fall migrations. A potential gap in milkweed resource areas was identified in South Texas. The novel merging of milkweed niche models with larval phenology, host selectivity, milkweed phenology, and land cover selectivity informs conservation assessment.

Native mycorrhizal fungi improve milkweed growth, latex, and establishment while some commercial fungi may inhibit them

AbstractArbuscular mycorrhizal (AM) fungi are root symbionts that can facilitate plant growth and influence plant communities by altering plant interactions with herbivores. Therefore, AM fungi could be critical for the conservation of certain rare plants and herbivores. For example, North American milkweed species are crucial hosts for monarch butterflies (Danaus plexippus). Understanding how mycorrhizal composition affects milkweeds will have direct impacts on the conservation and restoration of both increasingly threatened guilds. We present data from three studies on the effect of AM fungal composition on milkweed growth, latex production, and establishment. First, we grew seven milkweed species with and without a mixture of native mycorrhizal fungi. We assessed how important fungal composition is to milkweed growth and latex production by growing four milkweed species with seven fungal compositions, as single‐species inoculations with four native fungi, a mixture of native fungi, a single commercial fungus of presumably non‐native origin, and noninoculated controls. Finally, we assessed the field establishment of two milkweed species with and without native mycorrhizal inoculation. Milkweed species grew 98% larger and produced 82% more latex after inoculation with native mycorrhizae. Milkweeds were strongly affected by fungal composition; milkweeds were inhibited by commercial fungi (average of −14% growth) and showed variable but positive responses to native fungal species (average of +3% to +38% biomass). Finally, we found that restoration establishment was dependent on inoculation with native fungi and milkweed species. Overall, our findings indicate that some milkweed species (i.e., Asclepias syriaca and A. incarnata) are not responsive to mycorrhizal fungal presence or sensitive to mycorrhizal composition while others are, including endangered species (A. meadii) and species of high conservation value (A. tuberosa). We conclude that the reintroduction of native AM fungi could improve the establishment of desirable milkweed species and should be considered within strategies for plantings for monarch conservation.

The Role of Experiments in Monarch Butterfly Conservation: A Review of Recent Studies and Approaches

Monarch butterflies (Danaus plexippus) (Lepidoptera Danaidae Danaus plexippus (Linnaeus)) are an iconic species of conservation concern due to declines in the overwintering colonies over the past twenty years. Because of this downward trend in overwintering numbers in both California and Mexico, monarchs are currently considered 'warranted-but-precluded' for listing under the Endangered Species Act. Monarchs have a fascinating life history and have become a model system in chemical ecology, migration biology, and host-parasite interactions, but many aspects of monarch biology important for informing conservation practices remain unresolved. In this review, we focus on recent advances using experimental and genetic approaches that inform monarch conservation. In particular, we emphasize three areas of broad importance, which could have an immediate impact on monarch conservation efforts: 1) breeding habitat and host plant use, 2) natural enemies and exotic caterpillar food plants, and 3) the utility of genetic and genomic approaches for understanding monarch biology and informing ongoing conservation efforts. We also suggest future studies in these areas that could improve our understanding of monarch behavior and conservation.

High Survivorship of First-Generation Monarch Butterfly Eggs to Third Instar Associated with a Diverse Arthropod Community.

Simple SummaryThe eastern migratory population of the monarch butterfly has been the focus of extensive conservation efforts in recent years. However, there are gaps in our knowledge about the survival of first, or spring generation, monarchs in their core areas of Texas, Oklahoma, and Louisiana. This is important because the spring generation represents the first stage of annual recovery from overwinter mortality. It is, therefore, an important stage for monarch conservation efforts. This study showed that, in the context of a complex arthropod community in north Texas, first generation monarch survival was high. The study found that survival was not directly related to predators on the host plant, but was higher on host plants that harbored a greater number and variety of other, non-predatory arthropods. This is possibly because the presence of alternate, preferable prey enabled monarch eggs and larvae to be overlooked by predators. The implication is that, at least in the southern U.S., monarch conservation should consider strategies that promote diverse functional arthropod communities.Based on surveys of winter roost sites, the eastern migratory population of the monarch butterfly (Danaus plexippus) in North America appears to have declined in the last 20 years and this has prompted the implementation of numerous conservation strategies. However, there is little information on the survivorship of first-generation monarchs in the core area of occupancy in Texas, Oklahoma, and Louisiana where overwinter population recovery begins. The purpose of this study was to determine the survivorship of first-generation eggs to third instars at a site in north Texas and to evaluate host plant arthropods for their effect on survivorship. Survivorship to third instar averaged 13.4% and varied from 11.7% to 15.6% over three years. The host plants harbored 77 arthropod taxa, including 27 predatory taxa. Despite their abundance, neither predator abundance nor predator richness predicted monarch survival. However, host plants upon which monarchs survived often harbored higher numbers of non-predatory arthropod taxa and more individuals of non-predatory taxa. These results suggest that ecological processes may have buffered the effects of predators and improved monarch survival in our study. The creation of diverse functional arthropod communities should be considered for effective monarch conservation, particularly in southern latitudes.

Oviposition Response of Monarch Butterfly (Lepidoptera: Nymphalidae) to Imidacloprid-Treated Milkweed.

Monarch butterfly (Danaus plexippus) populations have declined over the last two decades, attributable in part to declines in its larval host plant, milkweed (Asclepias spp.), across its breeding range. Conservation efforts in the United States call for restoration of 1.3 billion milkweed stems into the Midwestern landscape. Reaching this goal will require habitat establishment in marginal croplands, where there is a high potential for exposure to agrochemicals. Corn and soybean crops may be treated with neonicotinoid insecticides systemically or through foliar applications to provide protection against insect pests. Here, we investigate whether ovipositing monarchs discriminate against milkweed plants exposed to the neonicotinoid insecticide imidacloprid, either systemically or through foliar application. In our first experiment, we placed gravid females in enclosures containing a choice of two cut stems for oviposition: one in 15 ml of a 0.5 mg/ml aqueous solution of imidacloprid and one in 15 ml water. In a second experiment, females were given a choice of milkweed plants whose leaves were treated with 30 µl of a 0.825 mg/ml imidacloprid-surfactant solution or plants treated with surfactant alone. To evaluate oviposition preference, we counted and removed eggs from all plants daily for 3 d. We also collected video data on a subset of butterflies to evaluate landing behavior. Results indicate that neither systemic nor foliar treatment with imidacloprid influenced oviposition behavior in female monarchs. The implications of these findings for monarch conservation practices will be informed by the results of ongoing egg and larval toxicity studies.

Conservation risks and benefits of establishing monarch butterfly (Danaus plexippus) breeding habitats close to maize and soybean fields in the north central United States: A landscape-scale analysis of the impact of foliar insecticide on nonmigratory monarch butterfly populations.

Establishing habitat in agricultural landscapes of the north central United States is critical to reversing the decline of North America's eastern monarch butterfly (Danaus plexippus) population. Insecticide use could create population sinks and threaten recovery. Discouraging habitat establishment within a 38-m zone around crop fields is a suggested risk mitigation measure. In Story County, Iowa, United States, this mitigation would discourage habitat establishment in 84% of roadsides and 38% of noncrop land. It is unclear if the conservation benefits from establishing habitat close to crop fields outweigh suppression of population growth owing to insecticide exposure. Consequently, monarch conservation plans require spatially and temporally explicit landscape-scale assessments. Using an agent-based model that incorporates female monarch movement and egg laying, the number and location of eggs laid in Story County were simulated for four habitat scenarios: current condition, maximum new establishment, moderate establishment, and moderate establishment only outside a 38-m no-plant zone around crop fields. A demographic model incorporated mortality from natural causes and insecticide exposure to simulate adult monarch production over 10 years. Assuming no insecticide exposure, simulated adult production increased 24.7% and 9.3%, respectively, with maximum and moderate habitat establishment and no planting restrictions. A 3.5% increase was simulated assuming moderate habitat establishment with a 38-m planting restriction. Impacts on adult production were simulated for six representative insecticides registered for soybean aphid (Aphis glycines) management. Depending on the frequency of insecticide applications over a 10-year period, simulated production increased 8.2%-9.3%, assuming moderate habitat establishment with no planting restrictions. Results suggest that the benefits of establishing habitat close to crop fields outweigh the adverse effects of insecticide spray drift; that is, metapopulation extirpation is not a concern for monarchs. These findings are only applicable to species that move at spatial scales greater than the scale of potential spray-drift impacts. Integr Environ Assess Manag 2021;17:989-1002. © 2021 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Are eastern and western monarch butterflies distinct populations? A review of evidence for ecological, phenotypic, and genetic differentiation and implications for conservation

AbstractMonarch butterflies are a species of conservation priority due to declining overwintering populations in both eastern and western North America. Declines in western overwintering monarchs—more than 99.9% since monitoring began—are especially acute. However, the degree to which western monarchs are a distinct biological entity is uncertain. In this review, we focus on phenotypic and genetic differentiation between eastern and western monarchs, with the goal of informing researchers and policy‐makers who are interested in monarch conservation. Eastern and western monarchs occupy distinct environments and show some evidence for phenotypic differentiation, particularly for migration‐associated traits, though population genetic and genomic studies suggest that they are indistinguishable from one another. We suggest future studies that could improve our understanding of differences between eastern and western monarchs. We also discuss the concept of adaptive capacity in eastern and western monarchs as well as non‐migratory populations outside of the monarch's primary North American range. Finally, we discuss the prospect of completely losing migratory monarchs from western North America and what this entails for monarch conservation.

Assessing risk of Lymantria dispar L. invasion management to monarch butterflies (Danaus plexippus)

AbstractConflicting conservation goals that lead to management tradeoffs are not uncommon in conservation practice. In this study, we explore a potential conflict between the management of a destructive invasive insect pest, Lymantria dispar, and conservation of a widely declining iconic species, the monarch butterfly (Danaus plexippus). Specifically, we assess the risk of exposure of susceptible monarch larvae to aerial applications of Btk, a Lepidoptera‐specific larvicide used in L. dispar suppression and eradication. Our findings indicate minimal conflict between L. dispar management and monarch conservation, as spatial overlap between Btk aerial applications and monarch larvae was found to be marginal. Furthermore, the results of our study indicated specific actions management can take to further minimize potential conflict between these conservation goals. Our study could serve as a guide to other efforts to evaluate potential tradeoffs between the management of invasive species and the conservation of vulnerable species.

Clonal versus non-clonal milkweeds (Asclepias spp.) respond differently to stem damage, affecting oviposition by monarch butterflies.

BackgroundOviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies (Danaus plexippus) rely on milkweeds (Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies.Methodology/Principal FindingsWe first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca. We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa, and (2) A. verticillata and A. incarnata. Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35–40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20–30% each) but more eggs on regrown plants (40–60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance.Conclusions/SignificanceOverall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage.

Mixed-Species Gardens Increase Monarch Oviposition without Increasing Top-Down Predation.

Simple SummaryThe North American monarch butterfly is an iconic insect that has recently declined by over 80%, largely due to habitat loss. The primary approach to mitigate population declines is to plant milkweed, the primary host plant that monarch caterpillars feed and develop on. Recently, researchers have focused on optimizing monarch conservation habitats (i.e., milkweed plantings) in urban green spaces by studying habitat design and plant species selection. In many cases, as plant diversity increases, predatory and parasitic insect diversity increases and insect herbivore colonization and establishment decrease. We compared milkweed monocultures to a mixture of milkweed and other wildflower species to see what effects plant diversity have on monarchs and potential predators. We found that monarchs laid 22% more eggs on milkweed planted in mixed-species plots than milkweed in monoculture. We also found more predators in the mixed-species plantings, but this did not affect monarch disappearance rates. These results can be used to create evidence-based guidelines for monarch conservation habitats.Monarch butterfly populations have declined by over 80% in the last 20 years. Conservation efforts focus on the creation of milkweed habitats to mitigate this decline. Previous research has found monarchs lay more eggs per milkweed stem in urban gardens than natural habitats and recent work identified specific garden designs that make urban gardens more attractive to monarchs. Increasing plant diversity can reduce specialist insect herbivore colonization via bottom-up (e.g., plant) and top-down (e.g., predation) regulatory factors. Although this is beneficial for pest management efforts, it contradicts conservation efforts. In this study, we explored if adding multiple flowering species to garden-sized milkweed plantings affected monarch oviposition or top-down regulation of larvae. We compared monarch egg abundance, natural enemy abundance and richness, and biological control of monarch larvae in milkweed monocultures and milkweed mixed with four additional wildflower species. We found that monarchs laid 22% more eggs on sentinel milkweed plants in mixed-species plots with no effect of plant diversity on monarch survival. We also found higher natural enemy richness, wasp, and predatory bug abundance in the mixed-species plots and this did not translate to higher biological control rates. Our results provide more evidence that plant selection and habitat design are important for monarch conservation.

Prevalence of monarch (Danaus plexippus) and queen (Danaus gilippus) butterflies in West Texas during the fall of 2018

BackgroundThe monarch butterfly (Danaus plexippus) is a conspicuous insect that has experienced a drastic population decline over the past two decades. While there are several factors contributing to dwindling monarch populations, habitat loss is considered the most significant threat to monarchs. In the United States, loss of milkweed, particularly in the Midwest, has greatly reduced the available breeding habitat of monarchs. This has led to extensive efforts to conserve and restore milkweed resources throughout the Midwest. Recently, these research and conservation efforts have been expanded to include other important areas along the monarch’s migratory path.ResultsDuring the fall of 2018, we conducted surveys of monarch eggs and larvae through West Texas. We documented monarch and queen butterfly (Danaus gilippus) reproduction throughout the region and used the proportion of monarch and queen larva to estimate the number of monarch eggs. Peak egg densities for monarchs were as high as 0.78 per milkweed ramet after correction for the presence of queens. Despite our observations encompassing only a limited sample across one season, the peak monarch egg densities we observed exceeded published reports from when monarch populations were higher.ConclusionsTo our knowledge, this is the first study to correct for the presence of queens when calculating the density of monarch eggs. This research also provides insight into monarch utilization of less well-known regions, such as West Texas, and highlights the need to expand the scope of monarch monitoring and conservation initiatives. While the importance of monarch research and conservation in the Midwest is unquestionable, more comprehensive efforts may identify new priorities in monarch conservation and lead to a more robust and effective overall strategy, particularly given the dynamic and rapidly changing global environment.

Milkweed (Asclepias syriaca) plant detection using mobile cameras

AbstractMilkweed (Asclepias spp.) are host plants of monarch butterflies (Danaus plexippus). It is important to detect milkweed plant locations to assess the status and trends of monarch habitat in support of monarch conservation programs. In this paper, we describe autonomous detection of milkweed plants using cameras mounted to vehicles. For detection, we used both aggregated channel features (ACF) for running the detectors on embedded computing platforms with central processing unit and faster region‐based convolutional neural network (Faster R‐CNN) with a ResNet architecture‐based detector that is suitable for graphics processing unit optimized processing. The ACF‐based model produced 0.89 mean average precision (mAP) on the training dataset and 0.29 mAP on the test dataset, whereas the ResNet‐based Faster R‐CNN model provided 0.98 mAP on training and 0.44 mAP on the test dataset. The detections were used to calculate approximate densities of milkweed plants in geo‐referenced locations based on global positioning system point correspondences of recorded images. Probability‐of‐count distributions are compared for the actual milkweed plant locations near roadsides. This is one of the first examples of using automated milkweed plant detection and density mapping using a vehicle‐mounted camera.

Assessing preference and survival of Danaus plexippus on two western species of Asclepias

Monarch butterfly (Danaus plexippus L.) migratory populations in North America have declined over the last 20 years. Milkweed (Asclepias, Apocynaceae) is the primary larval host plant for monarchs. The primary conservation strategy for monarchs has been to build habitat, with a focus on milkweed seeds and plugs. However, not all milkweed may offer the same benefits for monarch conservation, as monarchs may prefer some taxa over others, or taxa may vary in the nutrients and protective toxins they provide. Comparisons of milkweed suitability have been made for milkweed taxa in the midwestern and eastern United States (U.S.), but very little is known about milkweeds found in the southwestern U.S. We tested female egg-laying preferences and larval performance in captivity on two species of milkweed commonly available for habitat plantings in the Southwest: Arizona milkweed (Asclepias angustifolia Schweig.) and pineneedle milkweed (A. linaria Cav.). We tested four measures of larval performance: survival, duration, mass and adult forewing length. Monarchs were more likely to lay eggs on Arizona milkweed than pineneedle milkweed. Larvae had a higher survival rate and grew more rapidly on Arizona milkweed, and adults from larvae raised on Arizona milkweed were larger than those raised on pineneedle milkweed. In the field, monarch caterpillars were more likely to be found on Arizona milkweed. We recommend Arizona milkweed for monarch habitat conservation projects in the Southwest within the natural range of Arizona milkweed, but that D. plexippus preference and performance be tested for additional western milkweed species.