First Population Study on Winter Breeding Monarch Butterflies, Danaus plexippus (Lepidoptera: Nymphalidae) in the Urban South Bay of San Francisco, California

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.

Dramatic Recent Declines in the Size of Monarch Butterfly (Danaus plexippus) Roosts During Fall Migration

“…Finally, year-round resident populations of monarchs appear to be increasingly common and growing along the western and southern edges of the species range (29). Monarch migration leads to a natural culling of monarchs weakened by OE infection or other flaws, and abandonment of migration by these year-round residents is associated with dramatically higher OE infection levels (34,35), smaller wing and body sizes (41), and weakened dispersal abilities (42). Interbreeding between migratory and non-migratory monarchs, in turn, likely dilutes genes associated with migratory ability (37).…”

Monarch Butterflies in Western North America: A Holistic Review of Population Trends, Ecology, Stressors, Resilience and Adaptation

“…A study of monarch winter breeding in the San Francisco Bay area during January-April 2021 showed that adults were common during February-March with numbers ranging from 0.23-1.54/min during ~30 min weekly surveys [69]. Eggs and larvae were abundant during the same period.…”

“…The extent and success of the first generation of eggs and larvae produced in March April by surviving overwintered females and females in winter-breeding populations in 2021 is unknown. This generation develops within California, and it is notable that the intensity of breeding at South Bay winter-breeding sites increased during March [69] Adults from this first spring generation migrate during May and June into far northern California, Oregon, and Washington. The number of monarchs seen in Oregon, Idaho, and Washington during migration in April-June 2021 reported to websites like iNaturalist was the same (25) as during the same period in 2020 [James, unpubl.…”

“…Warmer conditions may enable monarchs to breed during the winter in coastal areas north of San Francisco. Since the 1960s, we have seen a northward spread of monarch winter-breeding from San Diego to San Francisco [69,89]. A warming climate is also predicted to result in more frequent and intense winter storms in California [90], which will increase monarch mortality at traditional overwintering sites (but not necessarily at winter-breeding sites).…”

“…Aside from imidacloprid, we have no information on the possible sub-lethal effects of these other pesticides on monarchs. Urban areas are likely to be important breeding areas for monarchs in the west [69]. These are also areas where much pesticide use occurs in home gardens, parks, sports fields, golf courses, and other public and private spaces.…”