Question
There are some little tiny white insects on the bottom of the leaves on my plum tree that seem to be making the leaves turn white and curl up and die. Got any idea what they are.

Answer
Sounds like aphids. these are small insects that suck the plant juices from the leaf causing the leaf to curl. Usually there is a sticky liquid on the leaves form the secretion of the sap by the aphids.

There are several insecticides effective for aphid control when sprayed on plants. Perhaps most effective are those with systemic activity that allows them to move through the plant. Acephate (Isotox, Orthene) is the most widely available systemic insecticide.  Other insecticides used as sprays that have activity against aphids include insecticidal soaps .
Spray the foliage on both side of the leaves. I would make a second application in about 10 days to control the aphids you might have missed.