Question
QUESTION: I have several Live oaks with white sticky substance killing leaves. (3 years old)  I live in Houston TX,  I don't know if it is disease or insect related.  Worried I could lose the trees.  I have already lost 5 natural pecan trees to Ambrosia beetles and dont' want to lose the replacements.  Do you have any idea

ANSWER: Sounds like aphids. These insects secrete a substance called honeydew which is sticky form the high content of sugar. The foliage  will need to be sprayed with an insecticide. this is difficult on large trees since all the foliage will need to be sprayed. Small trees can be sprayed with insecticides.
Numerous contact insecticides are registered for aphid control. Since aphids are often placed under considerable pesticide pressure in field crops and greenhouses, they may be resistant to certain categories of insecticides. Therefore, if you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene),  bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon, malathion, nicotine sulfate, pyrethrum, , rotenone, resmethrin, and tetramethrin + sumithrin.

A systemic insecticide can be used that the tree takes the insecticide into the root system and to the leaves and twigs and kills the aphid when they suck the plant juices.
Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.


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QUESTION: First, thanks for answering.  
But if by aphids you mean the whiteflies I always get on my crete mertles, then it is not the same thing.  I actully have not seen any insect, yet.  The white substance is also fairly large and almost cottony.  It will rain down from the leaves if I spray then with water, unfortunately the leaves come down the next day.  I have tried the insecticide I use on the crete mertyles, but the stuff came back within a week.  Also the infecttion starts at the lowest limbs and works it way up over time.  Any more ideas?
Oh and thanks again.

Answer
No not white flies. I still think it is aphids. These insects have sucking mouth parts and can secrete a waxy material that is white. One of these is the Wooly oak aphid. You should be able to feel the sticky honeydew on the foliage. And it maybe raining down under the tree.

I would spray the foliage with one of the following insecticides.
Numerous contact insecticides are registered for aphid control. If you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene), bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon, malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.

Or use the soil applied insecticide.
These aphids will not kill the tree but may cause the tree to drop leaves. Aphids have a short life cycle and can reproduce fast so I would spray every 7-10 days for several times. This will kill the newly hatching eggs. This should stop the population.