Butterflies are Coming to Town
Everyone’s talking about the butterflies that have been fluttering through California. You might have seen several painted ladies at a time, hurrying along in a northeasterly direction. They generally fly 6 to 12 feet above the ground and pass over an obstacle rather than navigate around it. You may even see them stop to sip some nectar from flowering plants.
Why so many?
“We had one of the wettest years in Southern California, characteristic of an El Nino year,” said Arthur Shapiro, butterfly expert and professor of ecology and evolution at the University of California, Davis. And the wet weather has led to a surge of desert blooms of host plants for the caterpillars, said Shapiro. The plants have provided plenty of fuel for the butterflies’ migration, which the caterpillars had stored up during the winter.
Painted ladies, their scientific name is Vanessa Cardui, overwinter in the desert along the US-Mexican border. They begin to migrate as early as late January and as late as mid-April, depending upon the climate conditions and food availability. During a drought year, the butterflies tend to migrate early because their food is scarce.
On the other hand, “the colder and wetter the climate, the later they migrate,” said Shapiro.
Last winter, heavy rains led to a burst of food plants for the caterpillars. As a result, more food allowed more caterpillars to survive, said Shapiro.
“April 11 was the butterflies’ one month anniversary. It’s unusual for them to keep coming for a month,” said Shapiro. “Just yesterday 175 painted ladies per minute were spotted in Solano County.” Shapiro has also received reports of billions of butterflies near Death Valley and San Fernando Valley. And more are expected to reach the Central Valley over the next few weeks.
Painted lady butterflies migrate from the southeast to the northwest in search of food and breeding grounds. Their journey north takes about 3 days to reach the Central Valley. Some butterflies have already arrived in Central Oregon. Their offspring will continue on to British Columbia.
Painted ladies occupy all the temperate regions in the US. They also migrate from the North African deserts to Europe, traveling as far north as Scandinavia. Around August, painted ladies will migrate south for the winter. And the cycle continues.
As for the butterflies whose fat reserve has been depleted, they will cease to migrate. Some will remain in our area to breed and lay eggs. As a result, the new batch of butterflies that emerge from their cocoons in May will migrate to the Pacific Northwest to breed.
Painted ladies are sometimes mistaken for Monarch butterflies, but a few field marks can help you tell them apart. The painted lady is orange-brown with white and black spots and has a wingspan of 2 to 2 7/8 inches, whereas a monarch butterfly is bright orange with black veins and borders with white spots and has a wingspan of 3 3/8 – 4 7/8 inches. A female monarch butterfly is orange-brown.
If you want to observe the painted ladies feed on plants, look for them in gardens and fields. They prefer plants 3-6 feet high, particularly thistles. You may also see them feeding on aster, cosmos, ironweed, blazing star and red clover, among others. They lay single eggs on top of host plants. The caterpillars feed on more than 100 host plants, particularly thistles and mallow.
For more information about painted lady butterflies, visit USGS Northern Prairie Wildlife Research Center, Butterflies of North America: Painted Lady (Vanessa cardui) at http://www.npwrc.usgs.gov/resource/distr/lepid/bflyusa/usa/225.htm.<p>
Article appeared in the <i>City Times and Gold River News, April 2005, Second Edition.
Arthur Shapiro, Evolution and Ecology
University of California Davis
Painted Lady Butterflies on the Wing