Threats to Monarch Survival
According to the Xerces Society for Invertebrate Conservation, the overwintering population of western monarchs remains at approximately 5% of its size in the 1980s.
Xerces has conducted an annual Western Monarch Thanksgiving Count since 1997. For the count, community observers have observed and counted the monarchs at all of California’s overwintering sites. The decline in monarch overwintering is notable, with particularly steep drops between 1997 and 1998 and again after 2017. The number rebounded in 2021, rose in 2022, and returned to the 2021 level in 2023.
“Like most butterflies, monarchs are highly sensitive to weather and climate. They depend on environmental cues (temperature in particular) to trigger reproduction, migration, and hibernation. Their dependence on milkweed alone as a host plant is a further vulnerability, particularly as milkweed abundance is declining throughout the monarch range. They also face a decline in their overwinter habitat, and the effects of an increasing frequency of extreme weather events such as drought an severe storms, and extremes in hot and cold temperatures. ”
Habitat loss particularly affects specialist species like the western monarch, which requires nectar plants as well as milkweed when breeding and suitable roosting trees when overwintering.
Parasites live and multiply inside their hosts. Monarchs infected with OE, a common parasite, often die in the pupal stage, or emerge from the pupa with significant damage.
Many types of herbicides are harmful to butterflies and other pollinators, but one of the most problematic is the class of neonicotinoids, widely used in agriculture and landscaping.