Question
I have very very small white insects that fly on my shrubs and going to some of my out door plants.  They are turning the leaves gray and brown and literally killing them the limbs are becoming bare.  they sit on the plants until i shake a limb and them they fly around.  There are thousands of them.

Answer
Sounds like Whiteflies. Whiteflies are tiny, soft bodied insects.  They are not true flies, but belong to the insect Order Homoptera which means "same wing", both the front and back wings are membranous and appear very similar.  Other homopterous insects includes: aphids, scale insects, leafhoppers, and mealybugs.  Whiteflies derive their name from white, waxy substances secreted by special glands on their abdomen.  The adults coat their bodies, particularly their wings, with this wax-hence the name "whiteflies."  The immature stages (nymphs) are also secrete wax in various forms, and the appearance and shape of these waxy strands, plates, and filaments is often useful in identification.
Whiteflies can seriously injure plants by sucking juices from them causing wilting, stunting, or even death.  Whiteflies also secrete a sticky substance called honeydew.  This material can coat any object, leaves, fruit, windshields, concrete walks, etc. on which it falls.  In addition, several molds, collectively called "sooty molds" because of their black appearance, can grow on this honeydew resulting in an unsightly, sticky mess.
There are a number of insecticides that will control whiteflies. I would suggest you check with your local nursery/garden type store and buy a systemic insecticide for whiteflies. The key is to spray all sides of the leavses and repeat the spray  every  week for at least three and then only as needed.