Interpreting surveys to estimate the size of the monarch b

What this paper is about

Overwintering counts in Mexico fell while summer counts of adults and eggs in the Midwest did not, which led some to argue that migration mortality, rather than milkweed loss, is the problem.

This paper examines what those summer counts can actually measure. Counts per milkweed stem behave differently from counts per unit area when the amount of milkweed is itself changing.

Summarised here from the abstract below. Not the authors’ words.

Abstract

To assess the change in the size of the eastern North American monarch butterfly summer population, studies have used long-term data sets of counts of adult butterflies or eggs per milkweed stem. Despite the observed decline in the monarch population as measured at overwintering sites in Mexico, these studies found no decline in summer counts in the Midwest, the core of the summer breeding range, leading to a suggestion that the cause of the monarch population decline is not the loss of Midwest agricultural milkweeds but increased mortality during the fall migration. Using these counts to estimate population size, however, does not account for the shift of monarch activity from agricultural fields to non-agricultural sites over the past 20 years, as a result of the loss of agricultural milkweeds due to the near-ubiquitous use of glyphosate herbicides. We present the counter-hypotheses that the proportion of the monarch population present in non-agricultural habitats, where counts are made, has increased and that counts reflect both population size and the proportion of the population observed. We use data on the historical change in the proportion of milkweeds, and thus monarch activity, in agricultural fields and non-agricultural habitats to show why using counts can produce misleading conclusions about population size. We then separate out the shifting proportion effect from the counts to estimate the population size and show that these corrected summer monarch counts show a decline over time and are correlated with the size of the overwintering population. In addition, we present evidence against the hypothesis of increased mortality during migration. The milkweed limitation hypothesis for monarch decline remains supported and conservation efforts focusing on adding milkweeds to the landscape in the summer breeding region have a sound scientific basis.

Abstract reproduced from the published paper, which is licensed CC0. Copyright remains with the authors.

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APA

Milkweed USA. (2026). Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects (listing). Retrieved DATE, from https://milkweedusa.com/research/interpreting-surveys-to-estimate-the-size-of-the-monarch-2017/

MLA

"Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects (listing)." Milkweed USA, 2026, https://milkweedusa.com/research/interpreting-surveys-to-estimate-the-size-of-the-monarch-2017/. Accessed DATE.

Chicago

Milkweed USA. "Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects (listing)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/research/interpreting-surveys-to-estimate-the-size-of-the-monarch-2017/.

BibTeX

@misc{milkweedusa_researchinterpretingsurveyst,
  title  = {Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects (listing)},
  author = {{Milkweed USA}},
  year   = {2026},
  url    = {https://milkweedusa.com/research/interpreting-surveys-to-estimate-the-size-of-the-monarch-2017/},
  note   = {Accessed DATE}
}