In California's Central Valley, when the winter holidays are over, butterfly season is just beginning. And that means it's time for Prof. Arthur Shapiro's annual Butterfly-for-Beer contest, which rewards the first finder of a 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. The cabbage white, Pieris rapae, typically emerges from its pupal state around Jan. 10. 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.