In California's Central Valley, when the winter holidays are over, butterfly season is just beginning. And that means it's time for Professor Arthur Shapiro's annual Butterfly-for-Beer contest. Now in its 38th year, the competition rewards the first finder of a live cabbage white butterfly.

Besides the distinction of beating the well-known UC Davis lepidopterist at his own game, the contest winner gets a pitcher of beer or its cash equivalent. Shapiro gets more data about the butterfly's biology.

The quarry, a common valley resident, is white or buff-colored and about 1 1/4 inches long. It may have a few black spots near its outer wing edges. The underside of its hindwing is yellow with a gray cast. Early specimens are most likely to be found in vacant lots and by roadsides where wild mustard grows, rather than in gardens.

Shapiro has found that the cabbage white, Pieris rapae, is now emerging about a week earlier than it did 30 years ago, a change that is most likely due to climate warming. Over the years, first sightings of the butterfly have ranged from Jan. 1 to Feb. 22. In 2008, the first specimen was caught on Jan. 19. Cool, wet, cloudy weather tends to delay the event; sunny, warm days hasten it.

Only adult butterflies captured outdoors in Sacramento, Solano or Yolo counties are eligible for the contest. Entries must be brought alive to the receptionist in the Evolution and Ecology office, 2320 Storer Hall, with full data about the time, date and place found. If a butterfly is captured on a weekend or holiday when the department office is closed, it can be stored (live) for a few days in the refrigerator.

For photos of white cabbage butterflies, go to http://www.wildutah.us/b_pieris_rapae_41006_12.jpg and http://butterfly.ucdavis.edu/butterfly/Pieris/rapae.