Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation
James DG. Insects, January 2024.
Read the paper Free to read · doi:10.3390/insects15010040
What this paper is about
A review of the western monarch population, which fell from millions overwintering in California in the 1980s to under 400,000 by 2001.
The authors point to the introduction and spread of neonicotinoid insecticides from the mid-1990s as the most likely major cause, with habitat loss, heat, aridity and winter storms contributing.
Summarised here from the abstract below. Not the authors’ words.
Abstract
Monarch butterfly populations in western North America suffered a substantial decline, from millions of butterflies overwintering in California in the 1980s to less than 400,000 at the beginning of the 21st century. The introduction of neonicotinoid insecticides in the mid-1990s and their subsequent widespread use appears to be the most likely major factor behind this sudden decline. Habitat loss and unfavorable climates (high temperatures, aridity, and winter storms) have also played important and ongoing roles. These factors kept overwintering populations stable but below 300,000 during 2001-2017. Late winter storm mortality and consequent poor spring reproduction drove winter populations to less than 30,000 butterflies during 2018-2019. Record high temperatures in California during the fall of 2020 appeared to prematurely terminate monarch migration, resulting in the lowest overwintering population (1899) ever recorded. Many migrants formed winter-breeding populations in urban areas. Normal seasonal temperatures in the autumns of 2021 and 2022 enabled overwintering populations to return to around the 300,000 level, characteristic of the previous two decades. Natural enemies (predators, parasitoids, parasites, and pathogens) may be important regional or local drivers at times but they are a consistent and fundamental part of monarch ecology. Human interference (capture, rearing) likely has the least impact on monarch populations. The rearing of monarch caterpillars, particularly by children, is an important human link to nature that has positive ramifications for insect conservation beyond monarch butterflies and should be encouraged.
Abstract reproduced from the published paper, which is licensed CC BY. Copyright remains with the authors.
Key terms
Find the paper
Cite this page
This page is licensed CC BY 4.0. Quote it, adapt it and reuse it, including commercially, as long as you credit Milkweed USA and link back.
Milkweed USA. (2026). Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation (listing). Retrieved DATE, from https://milkweedusa.com/research/monarch-butterflies-in-western-north-america-a-holistic-review-2024/
"Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation (listing)." Milkweed USA, 2026, https://milkweedusa.com/research/monarch-butterflies-in-western-north-america-a-holistic-review-2024/. Accessed DATE.
Milkweed USA. "Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation (listing)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/research/monarch-butterflies-in-western-north-america-a-holistic-review-2024/.
@misc{milkweedusa_researchmonarchbutterfliesin,
title = {Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation (listing)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/research/monarch-butterflies-in-western-north-america-a-holistic-review-2024/},
note = {Accessed DATE}
}