Monarch butterflies counted at near record-low

The butterfly species has had a volatile population over the past several years.

The monarch butterfly population spending the winter in the western United States has declined to its second-lowest level in nearly three decades. 

The big picture: The Xerces Society for Invertebrate Conservation has been monitoring the western overwinter populations of monarch butterflies for the past 28 years, with the latest count revealing a significant decrease. 

  • In 2024, only 9,119 monarchs were counted, a 96% decline from 2023 when 233,394 monarchs were recorded. This marks the second-lowest count since the survey began in 1997, with the lowest being 1,901 monarchs in 2020.

Driving the news: The decline has been attributed to multiple factors, including the disappearance of milkweed, which serves as the host plant for the butterflies’ caterpillars. 

  • The vanishing milkweed is a result of a combination of drought, wildfires, agriculture, and urban development. 
  • Additionally, remaining milkweed plants have been contaminated by pesticides, further threatening the monarch butterfly population. 
  • The drastic drop in the western monarch population in just one year has raised concerns. 
  • One contributing factor could be the impact of extreme heat, with temperatures reaching triple-digit levels in the western states. Monarchs suffer when exposed to temperatures above 100 degrees Fahrenheit (37.7 Celsius), and any temperatures exceeding 108 degrees Fahrenheit (42.2 degrees Celsius) are lethal to the insects.

What we’re watching: While it remains uncertain what the long-term impact of the population decline may be, experts suggest that the insects have the potential for exponential growth. After hitting a low of 1,901 butterflies in 2020, the population rebounded to 247,246 the following year and then to 335,479 monarchs, indicating the potential for recovery.

  • Recognizing the severity of the situation, the U.S. Fish and Wildlife Service announced plans to consider listing monarchs as threatened, a move that would provide protection for their habitat. The proposed listing would prohibit actions that could harm the species, such as killing or transporting them, and would safeguard specific overwintering sites in seven coastal California counties.
  • The proposal to list monarchs as threatened is currently open for public comment, with a decision expected by December. In addition, environmental organizations have been pushing for measures to address the impact of pesticides on insects like bees, moths, and butterflies, urging the Environmental Protection Agency to mandate testing for pesticide effects.
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