Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation

Monarch butterfly populations in western North America suffered a substantial decline, from millions of butterflies overwintering in California in the 1980s to less than 400,000 at the beginning of the 21st century. The introduction of neonicotinoid insecticides in the mid-1990s and their subsequent widespread use appears to be the most likely major factor behind this sudden decline. Habitat loss and unfavorable climates (high temperatures, aridity, and winter storms) have also played important and ongoing roles. These factors kept overwintering populations stable but below 300,000 during 2001-2017. Late winter storm mortality and consequent poor spring reproduction drove winter populations to less than 30,000 butterflies during 2018-2019. Record high temperatures in California during the fall of 2020 appeared to prematurely terminate monarch migration, resulting in the lowest overwintering population (1899) ever recorded. Many migrants formed winter-breeding populations in urban areas. Normal seasonal temperatures in the autumns of 2021 and 2022 enabled overwintering populations to return to around the 300,000 level, characteristic of the previous two decades. Natural enemies (predators, parasitoids, parasites, and pathogens) may be important regional or local drivers at times but they are a consistent and fundamental part of monarch ecology. Human interference (capture, rearing) likely has the least impact on monarch populations. The rearing of monarch caterpillars, particularly by children, is an important human link to nature that has positive ramifications for insect conservation beyond monarch butterflies and should be encouraged.

Roadside habitat: Boon or bane for pollinating insects?

Pollinators, which provide vital services to wild ecosystems and agricultural crops, are facing global declines and habitat loss. As undeveloped land becomes increasingly scarce, much focus has been directed recently to roadsides as potential target zones for providing floral resources to pollinators. Roadsides, however, are risky places for pollinators, with threats from vehicle collisions, toxic pollutants, mowing, herbicides, and more. Although these threats have been investigated, most studies have yet to quantify the costs and benefits of roadsides to pollinators and, therefore, do not address whether the costs outweigh the benefits for pollinator populations using roadside habitats. In this article, we address how, when, and under what conditions roadside habitats may benefit or harm pollinators, reviewing existing knowledge and recommending practical questions that managers and policymakers should consider when planning pollinator-focused roadside management.

Effects of Patch Size, Fragmentation, and Invasive Species on Plant and Lepidoptera Communities in Southern Texas

Habitat loss, fragmentation, and invasive species are major threats to biodiversity. In the Lower Rio Grande Valley (LRGV) of southern Texas, a conservation hotspot, few studies have examined how land use change and biotic disturbance influence biodiversity, particularly among Lepidoptera. We surveyed 24 habitat fragments on private lands in the LRGV and examined how patch size, edge to interior ratio (EIR), prevalence of invasive, exotic, and pest (IEP) plant species, and other environmental factors influenced plant and Lepidoptera communities within four habitat classes. Biotic disturbance was widespread and intense. IEP plants represented three of the four most common species in all but one habitat class; yet, classes largely had distinctive plant and Lepidoptera communities. Larger habitat patches had lower IEP prevalence but also lower plant richness and lower Lepidoptera richness and abundance. Conversely, patches with higher EIRs had greater IEP prevalence, plant richness, and Lepidoptera richness and abundance. IEP prevalence was negatively related to plant diversity and positively related to woody dominance, blooming plant abundance, and, surprisingly, both plant cover and richness. However, plant richness, abundance, and diversity were higher where a greater proportion of the plants were native. Lepidoptera diversity increased with plant cover, and Lepidoptera richness and abundance increased with plant richness. More individual Lepidoptera species were influenced by habitat attributes than by availability of resources such as host plants or nectar sources. Our results illustrate extensive landscape alteration and biotic disturbance and suggest that most regional habitats are at early successional stages and populated by a novel species pool heavy in IEP species; these factors must be considered together to develop effective and realistic management plans for the LRGV.

Local Decision-makers' Perspectives on Roadside Revegetation and Management in Iowa, USA

Environmental practitioners must understand those they collaborate with to implement programs that are both socially and ecologically effective. Practitioners who understand decision-makers' perspectives are better able to collaborate to lower political, financial, and cultural obstacles. In this study, we surveyed decision-makers involved with a voluntary environmental program in Iowa, USA. Iowa counties can choose to manage their roadside vegetation using an ecological approach, called integrated roadside vegetation management. Key decision-makers who decide whether a county has a roadside program are the county board of supervisors and the county conservation board. We used a mixed-mode design to survey the conservation board directors and chairs of the board of supervisors in each county. Our main goals were to understand the decision-makers' perceived benefits and barriers to having a roadside program in their counties, as well as the key factors influencing their decisions about roadside vegetation management. Safety, maintenance cost savings, and erosion control were the main factors that influenced decision-making, while pollinators and other wildlife received the least consideration. However, decision-makers in counties with a roadside vegetation manager were more influenced by pollinators and other wildlife compared to their counterparts in counties without a roadside vegetation manager. The main barriers to having a program include a lack of resources or other concerns being a higher priority. Emphasizing safety, cost savings, and erosion control benefits of roadside programs, and identifying ways to lower startup costs may increase buy-in with county decision-makers.

Traffic influences nutritional quality of roadside plants for monarch caterpillars

Roadside habitats are increasingly being targeted for restoration and conservation. Roadside habitats often exhibit altered soil and plant chemistry due to pollution from maintenance (e.g. de-icing salt), car deterioration, and exhaust. Roadside plants may attract animals due to elevated levels of sodium or nitrogen, but high concentrations of heavy metals and sodium can be toxic, potentially setting an ecological trap. In this study, we determine how roads influence the chemistry of common milkweed (Asclepias syriaca) as it is the primary roadside host plant for the declining monarch butterfly (Danaus plexippus) in the eastern United States. Even though road salt is applied during the winter, we detect enhanced sodium along roads the following growing season. Road salts increase soil sodium, which in turn elevates host-plant foliar sodium (occasionally to toxic levels in <10% of plants) and sodium content in monarch caterpillars feeding on these plants. Sodium levels of milkweed leaves are highest close to the edge of busy roads. Some heavy metals (lead, zinc) are also elevated in roadside soils or plants. Nitrogen content was affected by adjacent agricultural use, but not traffic volume or proximity to a road. Other potential road pollutants (e.g. nickel) were not elevated in soil or plants. Despite a clear signature of road pollution in the chemistry of milkweed, most plants are likely still suitable for developing monarchs. Nonetheless, restoration investments in snowy regions should prioritize sites with lower-traffic density that are further from the road edge to minimize toxic impacts of high sodium. To extend this research to other insects of conservation concern, future work should characterize the nutritional quality of nectar, pollen, and other species of host-plants in roadside habitats.