The mealybugs are being tended by Argentine ants (Linepithema humile), by far the most common here in Orange County. The "farm" is on an unripe lemon, on one of two citrus trees at the end of my drive. The other, a Satsuma mandarin, has been untouched by these critters.

More info on the scale is at http://www.gravena.com.br/planococus.htm and it appears to be identical to that on my lemon tree. Apparently, most of the research on this citrus pest has been done on Satsumas, chiefly in Japan.