Aphid predators
(Aphidoletes aphidimyza) lay their eggs near aphid colonies, which soon hatch and begin feeding on aphids. To reach full development takes at least 10 aphids, but when aphid populations are high, many more are destroyed - first paralyzed with a poison, then sucked dry. Killed aphids remain attached to plant leaves, and eventually dry up. Are these guys bad, or what?

Aphid predators have worked so well in many greenhouses, that by summer aphids may be hard to even find. And they usually set up breeding populations from a single release. 250 is enough to handle a small greenhouse, while 1000 covers a larger greenhouse. Commercial greenhouses have used 3000- 4000 per acre, repeated weekly during the aphid season until control. (Regular releases get them going faster.)

Aphid predators come as cocoons waiting to hatch, packaged in bottles. Spread the contents on the soil or leaves, and they'll do the rest.