Monarch butterfly population decline in North America: identifying the threatening processes

Abstract

The monarch butterfly (Danaus plexippus) population in North America has sharply declined over the last two decades. Despite rising concern over the monarch butterfly's status, no comprehensive study of the factors driving this decline has been conducted. Using partial least-squares regressions and time-series analysis, we investigated climatic and habitat-related factors influencing monarch population size from 1993 to 2014. Potential threats included climatic factors, habitat loss (milkweed and overwinter forest), disease and agricultural insecticide use (neonicotinoids). While climatic factors, principally breeding season temperature, were important determinants of annual variation in abundance, our results indicated strong negative relationships between population size and habitat loss variables, principally glyphosate use, but also weaker negative effects from the loss of overwinter forest and breeding season use of neonicotinoids. Further declines in population size because of glyphosate application are not expected. Thus, if remaining threats to habitat are mitigated we expect climate-induced stochastic variation of the eastern migratory population of monarch butterfly around a relatively stationary population size.

Why are monarch butterflies declining in the West? Understanding the importance of multiple correlated drivers

“…Several recent studies (Thogmartin et al. , Boyle et al. ) have evaluated the potential contributions of different factors to population dynamics of the eastern monarch population.…”

“…Land use change has not previously been evaluated in relation to western monarchs, but is among the primary threats faced by eastern monarchs (Thogmartin et al. , Stenoien et al. ).…”

“…) and is a leading correlate of abundance (Thogmartin et al. ). In the breeding range, agricultural intensification and associated decline in milkweed ( Asclepias spp., monarch host plants), formerly common in agricultural landscapes, are also strongly associated with eastern monarch declines (Pleasants and Oberhauser , Boyle et al.…”

“…For instance, increased use of glyphosate explains 77% of annual variation in monarch abundance through time (Thogmartin et al. ). More generally, use of herbicides, herbicide‐resistant crops, and insecticides has increased throughout the United States in recent decades (Benbrook ).…”

Balancing sampling intensity against spatial coverage for a community science monitoring programme

“…A major cause of decline is thought to be loss of milkweed ( Asclepias spp. ), the obligate host plant of monarch larvae, from agricultural areas as a result of the adoption of glyphosate‐resistant crops (Pleasants & Oberhauser, ; Stenoien et al, ; Thogmartin, Wiederholt, et al, ). However, studies documenting milkweed declines have been limited in geographic scope and habitat coverage (Hartzler, ; Pleasants & Oberhauser, ; Zaya, Pearse, & Spyreas, ).…”