Mealy Bug Destroyers
Tiny (1/8") black ladybugs, mealybug destroyers (Cryptolaemus ontmuzieri) are originally from Australia; and are some of the oldest, most successful biological controls. Back in the early 1900's they were credited with saving the Southern California citrus crop from mealybugs. They're still used today for the same purpose. As you can imagine, they work well under greenhouses conditions, too.

Mealybugs are their main diet, but they eat scales and aphids, too, when other food gets scarce. Mealybug destroyers have large appetites for such a small insect, so a few go a long way - we suggest 2-5 per infested plant, or 1 for every 2 feet of planted area. They'll set up breeding populations from there. Repeat as necessary.