Aleiodes sp. (Mummy-wasp).
Size: approx. 5 mm. Photo: 04/26/12, 9 pm.

Over 200 species in this genus. Mummy wasps attack leaf-feeding caterpillars and mummify the victim. This unique method of parasitism leaves the caterpillar intact and identifiable while the wasp larva feeds and eventually pupates inside the caterpillar. 
bug guide (this photo):