First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California
James DG, Schaefer MC, Krimmer Easton K and others. Insects, October 2021.
Read the paper Free to read · doi:10.3390/insects12100946
What this paper is about
The western overwintering count fell below 2,000 butterflies in 2020 and 2021. At the same time, breeding monarchs were being reported through winter in San Francisco.
This survey of Santa Clara County from January to June 2021 is the first population study of that winter breeding. A published comment disputing the interpretation is also in this list.
Summarised here from the abstract below. Not the authors’ words.
Abstract
The western North American monarch butterfly population assessed by counts of non-reproductive overwintering butterflies at coastal sites in California declined to less than 2000 in 2020/21. Simultaneously, reports of reproductive monarchs increased in San Francisco urban areas, perhaps representing a shift in overwintering strategy. To better understand monarch winter breeding in the Bay area, we studied adult and immature populations in Santa Clara County during January-June 2021. Adult monarchs were common with numbers ranging from 0.23-1.54/min during ~30 min weekly surveys at one site, with lowest numbers late April to mid-May. Eggs and larvae, primarily on ornamental milkweeds, were found on nearly all survey dates with lowest numbers mid-late April to mid-May. Levels of infection of adults by the parasite Ophryocystis elektroscirrha were consistently high during the study (69.3-77.5%). From 499 monarchs tagged post-eclosion, recovery rates of 19.2-23.6% occurred from releases in January-February and May-June but only 11.9-13.0% from March-April releases. Although distances were small, butterflies tagged in April were recovered from greater distances than other months. Tagged monarchs flew primarily north or east. There were reduced numbers of adult monarchs during late April-mid-May with some evidence of northerly and easterly emigration at the same time from tagged butterflies, suggesting some movement out of the South Bay area, perhaps representing spring migration. We conclude that monarchs can successfully breed and maintain populations on ornamental milkweeds during winter at urban sites in the South Bay of San Francisco and may still migrate during spring to remain part of the wider western population.
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). First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California (listing). Retrieved DATE, from https://milkweedusa.com/research/first-population-study-on-winter-breeding-monarch-butterflies-danaus-2021/
"First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California (listing)." Milkweed USA, 2026, https://milkweedusa.com/research/first-population-study-on-winter-breeding-monarch-butterflies-danaus-2021/. Accessed DATE.
Milkweed USA. "First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California (listing)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/research/first-population-study-on-winter-breeding-monarch-butterflies-danaus-2021/.
@misc{milkweedusa_researchfirstpopulationstudy,
title = {First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California (listing)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/research/first-population-study-on-winter-breeding-monarch-butterflies-danaus-2021/},
note = {Accessed DATE}
}