Assessing Role of Major Drivers in Recent Decline of Monarch Butterfly Population in North America

Abstract

Monarch (Danaus plexippus L.) is a long distance migratory species of butterfly in North America that has experienced population declines in recent years. Several interacting factors have been postulated to contribute to this decline. For example, reduced overwintering habitat in Mexico as a result of logging leading to deforestation and reduced breeding habitats in the United States. Decreased spring breeding populations and inclement weather patterns have also been proposed. Herbicides are commonly used in many agricultural production systems planted with genetically modified (GM) cultivars that are resistant to these chemicals. However, the non-target effects of these commonly used herbicides to the monarch host-plant, milkweed (Asclepias spp. L.), which commonly grows within GM planted agricultural fields has also been hypothesized to explain reduced breeding habitats in the midwestern United States. Currently, significant uncertainty regarding the dynamics of these multiple hypotheses and how their interaction may impact the eastern monarch butterfly population remains. Meanwhile, several efforts to help monarchs have been launched; for example, milkweed plantings and campaigns to minimize deforestation in overwintering habitats. In this context, this mini-review highlights different population decline hypotheses and conservation initiatives and discusses knowledge gaps in awareness and research efforts to mitigate the eastern monarch butterfly population decline.

Modeling Current and Future Potential Distributions of Milkweeds and the Monarch Butterfly in Idaho

“…Given climate variability, there will likely be years in which summer temperatures are above and below averages reported here. These model projections underscore the need to investigate western monarch population vital rates across all life stages (i.e., survival, individual growth, reproduction, recruitment) to better predict monarch response to changing climates and weather (Schultz et al, 2017;Belsky and Joshi, 2018).…”

“…Factors contributing to declines are increasingly understood and likely involve multiple biotic and abiotic stressors including climate change and loss and degradation of native milkweed (Asclepias spp. ), monarchs' obligate larval host plant (Flockhart et al, 2015;Jepsen et al, 2015;Inamine et al, 2016;Thogmartin et al, 2017;Belsky and Joshi, 2018). Changes in climate patterns, in particular, may both favor and hinder species through complex, seasonal relationships.…”

Multiscale seasonal factors drive the size of winter monarch colonies

“…1), with an 84% decrease from peak abundance in 1996 (10). The exact causes of the decline are debated, as a variety of stressors operating at different spatial scales and times of the life cycle threaten monarch butterflies (11). Loss of milkweed host plants during the breeding season due to increased herbicide (glyphosate) use in Midwestern US agricultural fields is hypothesized to be a primary cause of monarch declines (12)(13)(14)(15)(16)(17).…”

Pesticide contamination of milkweeds across the agricultural, urban, and open spaces of low elevation Northern California

“…In the eastern United States, many hypotheses have been proposed to explain the monarch decline, including loss of critical overwintering habitat, natural enemies, climate, and various pesticides, especially herbicides, that have reduced milkweed abundance ( Asclepias spp.) (Belsky and Joshi, 2018). In the west, monarch overwintering populations reached a historic low in 2018 (Pelton et al, 2019), and the causes appear to include loss of overwintering habitat, climate, and pesticides (Crone et al, 2019).…”