Every year in late July, I hike along Vista Trail in O’Neill Regional Park to observe two healthy populations of wild milkweed growing along the trail. Last year at this time, the woolly-pod milkweed (Asclepias eriocarpa) plants were in full flower and bustling with bumblebee activity and hungry caterpillars.
This year, I was surprised to see that the woolly-pod milkweed blossoms were dry and seed pods were already forming. And I didn’t see any monarch caterpillars on the plant.
Did last winter’s drought (only 5 inches of rain) and this summer’s heat cause the woolly-pod milkweed plants to set seed early? And is this year’s change in the milkweed blossoms detrimental to the monarch butterfly population?
A recent article in Popular Science magazine discussed findings from scientists who studied 25 years of data from more than 18,000 monarch counts to try to determine whether increased herbicide use, mortality during migration, or changes in climate might be most responsible for the decline in monarch numbers.
The magazine article summarized findings recently published in the July 19, 2021 scientific journal Nature Ecology & Evolution. The findings concluded that breeding season weather was nearly seven times more important than other factors in explaining the changes in summer monarch butterfly populations.
Michigan State University scientist Elise Zipkin and her team found that “monarchs tended to thrive when weather conditions stayed close to average in their spring breeding grounds, and when temperatures in the northern and southern parts of their summer habitat weren’t too cool or warm.”
The Nature Ecology & Evolution report concluded that “If observed changes in spring and summer climate continue, portions of the current breeding range may become inhospitable for monarchs. Results highlight the increasingly important contribution of a changing climate to insect decline.”
Although the study focused mainly on data from the eastern monarch population, it is likely the changing climate is also contributing to the precipitous decline of the western monarchs that inhabit our California landscapes.
I hope that we can learn from recent scientific studies to create change that will help support monarch butterfly populations. As I write this post on a hot summer afternoon, I see monarch butterflies in my yard. They are doing their best to cope with changing conditions. Let’s do what we can to help monarchs thrive.