Tuesday, April 30, 2019

2019 Painted Lady Migration in Utah

Ideal winter precipitation and other factors have triggered a massive northward migration of the painted lady butterfly (Vanessa cardui) into Utah and other western U.S. states from Mexico.   Previous painted lady mass migrations in the Western U.S. have been noted in 1972 and 1991.

About half the size of the monarch, these fast flying butterflies are everywhere throughout the state where they occasionally take a short break from their northward flight to nectar on dandelions and other flowers.  They also lay eggs on weeds such as cheeseweed in our yards and anywhere else weeds grow.  (Ironically, you may see fewer painted ladies if you have an immaculate garden, free of weeds.)   Because their caterpillars feed mainly on weeds such as cheeseweed, thistles, sunflowers, fiddleneck, and others, painted lady caterpillars pose no threat to agricultural crops.   

Of the tens of thousands of painted ladies passing through Utah's West Desert last Saturday, April 28, 2019, it was interesting to note that some were smaller, worn migrants from Mexico, whereas others were larger, fresher adults representing the next generation that have joined in the migration.

Similar to the monarch migration, the painted lady migration is an annual event that usually goes unnoticed by the public as numbers are substantially less than this year's extraordinary explosion. In fact, in some years, the painted lady northward migration has been so minuscule that, while doing field work, I have failed to see a single butterfly or caterpillar on its many host plants in during its most visible months of April and May.

Painted lady adult butterflies that fed locally on weeds and emerge as butterflies around late May into June continue their trek northward.   During the painted lady mass migration of 1991, well after the offspring of migrant painted ladies emerged, painted lady adult butterflies were difficult to locate in Northern Utah in June/July. This differs from monarchs where migratory monarchs will fly through Utah, lay eggs on milkweed, and continue northward.  However, about one month later, the offspring of those migratory monarchs generally remain local to breed and build their numbers with an additional generation or two leading up to their return migration to California starting around mid-September. (Eastern monarchs migrate to Michoacan, Mexico whereas western monarchs migrate to California.)

The painted ladies' ability to rebound from extremely low to unprecedented high numbers is truly extraordinary even by insect standards.  In order to survive extremely high mortality, most butterflies instinct towards self-conservation is evidenced by laying 300 or more eggs where roughly 97 percent perish due to predation, parasitism, drought, floods, fire, and many other natural factors.

Butterfly predators such as spiders, earwigs, wasps, and others help keep butterfly numbers in check.  However, on occasion, butterfly numbers have been known to temporarily explode based upon a combination of factors including, but not limited to, high winter precipitation in Mexico.  Several years' ago the pine white butterfly (Neophasia menapia) experienced a temporary population explosion and denuded thousands of pine trees in Malheur National Forest in Oregon.  Numbers have since returned to normal levels and the trees have recovered.

This is an unprecedented year for educators and school teachers to discuss the painted lady butterfly and their migratory story during their insect units where it is not difficult to find painted lady eggs and caterpillars between now roughly through the end of May anywhere in the state where weeds grow.

Basic instructions on how to raise painted ladies can be found on my raising butterflies' page here. Social media community observations on the painted lady migration (and other Utah butterflies) can be found by joining the Utah Butterfly Field Trips Facebook page.   Information on Northern Utah Monarch conservation can be found on Monarchs of the Wasatch Front and Monarchs of Bridgerland Facebook pages.  

For more information about this article, please contact Todd Stout at todd@raisingbutterflies.org