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I borrowed this info from a poster at a tree site. I don't think he will mind. Sharing methods to combat this threat is the important thing.
Basically the product you should use is Imidacloprid (Merit). The delivery system can vary.
1) Mauget caps (J.J. Mauget) They come in 2 and 3 ml, use 3ml on trees over 16"DBH space every 4 to 5 inches at root flare. Timing in Michigan has been late May early June for best uptake (we are zone 5)
2) Arborjet- it is a high volume system that probably should only be bought if your doing multiple trees (cost $450 for the system and about the same for a quart of product). It works GREAT for the big trees putting in about 6 to 8 mls per inch of DBH. You need the volume to kill the bastard. He is one tough insect.

If the tree is very high value use trunk, limb and foliage spray w/ Bifenthrin (Onyx). In Michigan the timing is early June and early July. This is done to coincide w/the adult females feeding on the leaves for a week b4 egg deposition and then emergence. Also the use of soil applied Merit can be beneficial from the standpoint of 'building up' the parts per million of imdacloprid in the tree, This product has a VERY long residual 2 to 3 months under good conditions. The best results are gained from using a combination of these methods.