Research (urgh~) *stressed*

Tomato Leaf Spray

Tomato plants, as members of the nightshade family, contain toxic compounds called alkaloids in their leaves. When the leaves of tomato plants are chopped, they release their alkaloids. When the alkaloids are suspended and diluted with water, they make an easy to use spray that is toxic to aphids, but still safe around plants and humans.

http://organicgardening.about.com/od/pestcontrol/a/spraysforaphids.htm