If the joy of seeing butterflies seems increasingly rare these days, it isn’t your imagination.
From 2000 to 2020, the number of butterflies fell by 22% across the continental United States. That’s 1 in 5 butterflies lost. The findings are from an analysis just published in the journal Science by the U.S. Geological Survey’s Powell Center Status of Butterflies of the United States Working Group, which I am involved in.
We found declines in just about every region of the continental U.S. and across almost all butterfly species.
Overall, nearly one-third of the 342 butterfly species we were able to study declined by more than half. Twenty-two species fell by more than 90%. Only nine actually increased in numbers.
West Coast lady butterflies range across the western U.S., but their numbers have dropped by 80% in two decades.
Renee Las Vegas/Wikimedia Commons, CC BY
Some species’ numbers are dropping faster than others. The West Coast lady, a fairly widespread species across the western U.S., dropped by 80% in 20 years. Given everything we know about its biology, it should be doing fine – it has a wide range and feeds on a variety of plants. Yet, its numbers are absolutely tanking across its range.
Why care about butterflies?
Butterflies are beautiful. They inspire people, from art to literature and poetry. They deserve to exist simply for the sake of existing. They are also important for ecosystem function.
Butterflies are pollinators, picking up pollen on their legs and bodies as they feed on nectar from one flower and carrying it to the next. In their caterpillar stage, they also play an important role as herbivores, keeping plant growth in check.
A pipevine swallowtail caterpillar munches on leaves at Brookside Gardens in Wheaton, Md. Herbivores help keep plant growth in check.
Judy Gallagher/Wikimedia Commons, CC BY
Butterflies can also serve as an indicator species that can warn of threats and trends in other insects. Because humans are fond of butterflies, it’s easy to get volunteers to participate in surveys to count them.
The annual North American Butterfly Association Fourth of July Count is an example and one we used in the analysis. The same kind of nationwide monitoring by amateur naturalists doesn’t exist for less charismatic insects such as walking sticks.
What’s causing butterflies to decline?
Butterfly populations can decline for a number of reasons. Habitat loss, insecticides, rising temperatures and drying landscapes can all harm these fragile insects.
A study published in 2024 found that a change in insecticide use was a major factor in driving butterfly declines in the Midwest over 17 years. The authors, many of whom were also part of the current study, noted that the drop coincided with a shift to using seeds with prophylactic insecticides, rather than only spraying crops after an infestation.
The Southwest…
