Before going into more expense and work, you might want to see if this tree even shows signs of life this spring? Is it worth saving? Borers in the trunk and branches are likely secondary pests, but what they have tunneled is already dead or was so weak that the beetle larvae overcame any resistance and kept on tunneling. Regardless of what you apply, that dead or tunneled tissue will not come back to life. That includes areas with peeling bark. 

While it will remove the unsightly dead limbs, pruning them will likely create some gaping wounds and the wounds will be attractive to even more borers.

I'm pretty sure I know what kind of product you used....most likely Bayer Advanced Tree & Shrub.....with imidacloprid. It's a systemic but not a silver bullet. Are you eating anything from this tree? The label states that it cannot be used on plants that produce something edible. The question is---with trunk and limbs already damaged, is there adequate live vascular tissue in the tree to move that product from the root zone to the ends of the branches? Will there be enough product to be effective when it reaches a grub? The larger grubs will be more tolerant of these products than the smaller ones.

You might think about putting your insecticide money toward a new crab apple tree. Cutting down the old one will give you an idea of how much damage has already occurred; tunnels winding through the wood or gouging the live bark were made by the larvae of a couple of common wood boring beetle families. They are everywhere in the state and are constantly in search of places to reproduce. You're not alone with this problem.

If you still want to treat and try to save what's left, be sure to read the label thoroughly and follow all instructions. The label is literally the law.

I hope this helps.