Monarch butterfly and milkweed declines substantially predate the use of genetically modified crops

Abstract

Monarch butterfly (Danaus plexippus) decline over the past 25 years has received considerable public and scientific attention, in large part because its decline, and that of its milkweed (Asclepias spp.) host plant, have been linked to genetically modified (GM) crops and associated herbicide use. Here, we use museum and herbaria specimens to extend our knowledge of the dynamics of both monarchs and milkweeds in the United States to more than a century, from 1900 to 2016. We show that both monarchs and milkweeds increased during the early 20th century and that recent declines are actually part of a much longer-term decline in both monarchs and milkweed beginning around 1950. Herbicide-resistant crops, therefore, are clearly not the only culprit and, likely, not even the primary culprit: Not only did monarch and milkweed declines begin decades before GM crops were introduced, but other variables, particularly a decline in the number of farms, predict common milkweed trends more strongly over the period studied here.

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

“…During the past half-century, the eastern monarch population has declined by~80% (Semmens et al 2016), and the western population has declined by~95% (Schultz et al 2017), with an additional drop in 2018 to <1% of historic size (Pelton et al 2019). Several recent studies (Thogmartin et al 2017b, Boyle et al 2019) have evaluated the potential contributions of different factors to population dynamics of the eastern monarch population. Here, we evaluate the importance of different aspects of climate and land use change for the western monarch population, with special emphasis on how we make inferences about the importance of these correlated variables, and what actions might mitigate these threats in the short and long term.…”

The demographic effects of functional traits: an integral projection model approach reveals population‐level consequences of reproduction‐defence trade‐offs

“…We extend the Trait Probability Density approach of Carmona et al (2016) within an IPM framework to understand the influence of trait variation among individuals on the population dynamics of common milkweed (Asclepias syriaca). Common milkweed has gained public attention because its decline is implicated as a contributing factor in the decline of monarch butterflies (Pleasants & Oberhauser 2013;Flockhart et al 2015;Boyle et al 2019). We have developed spectroscopic tools for milkweed (Couture et al 2013(Couture et al , 2015 to nondestructively measure a suite of key functional traits, including leaf nutritional quality (nitrogen, carbon), structural composition (fibre, lignin, cellulose), morphology (leaf mass per area [LMA]) and secondary metabolites (cardenolides).…”

Applying Machine Learning to Investigate Long Term Insect-Plant Interactions Preserved on Digitized Herbarium Specimens

“…We show here that not only could herbaria offer a parallel resource for examining modern changes in herbivory, but also that we may be able to use automated techniques to extract damage types from specimens, allowing for the possibility of "big data" extraction. This is important because some limited data that are available on insect abundances (Boyle et al, 2019;Wepprich et al, 2019) and biomass (Hallmann et al, 2017;Lister and Garcia, 2018) over time suggest that insects are in decline in the Anthropocene.…”