Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2019 Feb 19;116(8):3006-3011.
doi: 10.1073/pnas.1811437116. Epub 2019 Feb 5.

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

J H Boyle  1 , H J Dalgleish  1 , J R Puzey  2
Affiliations

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

J H Boyle et al. Proc Natl Acad Sci U S A. .

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.

Keywords: genetically modified crops; herbarium; milkweed; monarch butterfly; specimen records.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Museum specimens reveal long-term trends in monarchs and milkweed. (A) Green points show annual abundance for milkweed spp.; orange points show annual abundance for monarchs; and lines and shading indicate smoothed mean and 95% confidence intervals, calculated using the Loess smoothing method implemented in ggplot2 (19), with the default smoothing span. Green and orange vertical lines indicate the approximate beginning of the decline for milkweed and monarchs, respectively. The blue vertical line indicates the point at which half of all corn, soybeans, and cotton are herbicide resistant (HR) GM varieties. (B) Indicates (1) the discovery of the monarch overwintering grounds in Mexico; (2) the introduction of GM crops; (3) the winter population census at the Mexican overwintering grounds (20); (4) the summer NABA census of adults (available from ref. 7); (5) the summer MLMP census of eggs and larvae (available from ref. 5); and (6) the summer census of Iowa A. syriaca abundance (available from ref. 5).
Fig. 2.
Fig. 2.
Genus-level milkweed decline over the 20th century is recapitulated in the 10 most common Asclepias species. The total number of specimens collected is shown next to each species. Points indicate abundance for each year, and lines and shading indicate smoothed mean and 95% confidence intervals. Smoothing was done using the Loess smoothing method implemented in ggplot2 (19), with the default smoothing span. Because different species have different ranges, the abundances for each species do not add up to the abundance for the genus as a whole.
See this image and copyright information in PMC

Comment in

References

    1. Urquhart FA, Urquhart NR. Autumnal migration routes of the eastern population of the monarch butterfly (Danaus p. plexippus L.; Danaidae; Lepidoptera) in North America to the overwintering site in the Neovolcanic Plateau of Mexico. Can J Zool. 1978;56:1759–1764.
    1. Brower LP. Understanding and misunderstanding the migration of the monarch butterfly (Nymphalidae) in North America: 1857-1995. J Lepid Soc. 1995;49:304–385.
    1. Pleasants JM. Milkweed restoration in the Midwest for monarch butterfly recovery: Estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population. Insect Conserv Divers. 2017;10:42–53.
    1. Vidal O, Rendón-Salinas E. Dynamics and trends of overwintering colonies of the monarch butterfly in Mexico. Biol Conserv. 2014;180:165–175.
    1. Pleasants JM, et al. Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects. PLoS One. 2017;12:e0181245. - PMC - PubMed

Publication types

LinkOut - more resources