Pieris rapae rapae
This Eurasian/N. African immigrant arrived in the United States in the 1860s and is now an established resident even in New Zealand. It can fly year-round, weather permitting, using mustard-family plants including garden varieties. The mustard oils make the adult butterfly distasteful to birds, while the larvae have a fluid on their setae (little hairs) that repels ants. They are actually quite an attractive, graceful butterfly with nicely-shaped wings.
Pieris rapae nectaring in the garden. April 18, 2014.
Females have two black dots on the forewing; males just one. La Jolla Canyon in the Santa Monica Mountains, January 12, 2006.
Male Pieris rapae in San Gabriel Canyon. March 8, 2006.
Some are more yellow than others on the underside; I believe this is due to uric acid. In my garden in Long Beach, October 25, 2005.
Pieris rapae eggs, the one on the left freshly laid. Both are on nasturtiums, which is a good larval host.
Larva of Pieris rapae on a nasturtium plant in my garden. March 23, 2009.
Up close, you can see the yellow mid-dorsal line, yellow dots along the side, small black spots, and the setae that have a repellent fluid. The white ocelli are ringed in black. Same larva as above.
Late in the year - or early - you can still find Pieris rapae while out for a stroll on a nice day. Another female, this one at the El Dorado Park Nature Center in Long Beach. October 20, 2005.
This butterfly was named by Carl Linnaeus himself. Here's what the reference looks like. It's from the landmark 10th Edition of the Systema Naturae of 1758. The species Rapae was the 59th to be named under the genus Papilio. Linnaeus included subgeneric groups under Papilio; rapae was with the "Danai candidi" group.
So when did Papilio rapae become Pieris rapae? In 1801, Franz von Paula Schrank's Fauna Boica was published in Ingolstadt, Germany. Here we find a new genus: Pieris. In 1836, Boisduval used Pieris for a subset of the species Schrank had used it for, including rapae, and his treatment was widely adopted.