Despite high parasite loads, resident California monarch butterfly populations thrive in urban gardens with nonnative evergreen milkweed
The decline of western migratory monarchs is likely not driven by the increasing number of resident monarchs in California’s Bay Area, a pattern associated with warmer winters, the presence of nonnative milkweed and potentially elevated parasite prevalence.
“My intuition was that a lot of effort was being put into something that might not be as bad for monarch butterflies as people thought,” said Elizabeth Crone, a professor at UC Davis.
Western migratory monarchs (Danaus plexippus) are a distinct population that breeds west of the Rocky Mountains during the spring and summer in areas like California’s Central Valley, the Pacific Northwest and parts of the Intermountain West. In the fall, the last generation of the year migrates to the California coast, where they cluster in sheltered groves to overwinter. While this migratory population has plummeted in the past decade, more and more monarch butterflies have become year-round residents in coastal Californian cities.
During the start of the COVID-19 pandemic in 2020, people—and it seemed, monarchs—were on lockdown in the California Bay Area. That winter of 2020-2021, the lowest ever count of western monarch butterflies overwintered on the California coast, making up only 1% of the typical numbers. At the same time, people in places like the East and South Bay were spotting monarchs in their gardens.
These observations led Crone to lead a study published in Ecosphere to find out what was going on with both migratory and resident monarchs in the area. She also wondered if the migratory population was suffering from the effects of a parasitic protozoan, OE (Ophryocystis elektroscirrha), which is prominently found on nonnative milkweed. The infection can make it hard for migratory monarchs to complete their journeys and has been associated with high abundances of resident monarchs.
For a year, Crone and her colleagues tracked monarch numbers, life stages, disease levels and plant types across 15 routes in East Bay neighborhoods. They found that urban gardens support nonmigratory populations of monarch butterflies, which appear to persist year-round, feeding and breeding on both native and nonnative milkweed.
While tropical milkweed is often blamed for disrupting monarch migration and spreading disease, the research suggests that urban gardens may act more as stable habitats for resident monarchs than harmful traps.
“Among the community of people studying monarch butterflies, there are at least a handful of people who still strongly disagree with our conclusion that the tropical milkweed is not obviously bad for the migratory monarch populations,” Crone said. She also said that interactions may differ in other regions.
Crone said conservation groups or homeowners associations may not need to be overly concerned about whether individuals have tropical milkweed in their yards. Instead, they can spend more time encouraging people to plant diverse flower gardens that are good for nectar, for monarch butterflies and for other pollinators.
Her lab continues to examine four of the 15 sites from this study to examine yearly variation in the response. Future work aims to examine the potential impact of these urban gardens on other pollinators like bees and pipevine swallowtail (Battus philenor) butterflies.
