What a resurrected and re-resurrected thread!
At this stage, I would climb and spray the egg masses with dormant oil, aka horticultural oil, to smother and kill the eggs. It's very labor intensive.
Alternatively, you could wait until just after they hatch and spray with a product like TalstarOne in the spring to kill them at that point. The caterpillars would've caused very little damage at that point, and Talstar (a.i. is bifenthrin, a synthetic pyrethroid) is very effective.
Alternatively, you could use Bt, bacillus thuringiensis, a biological agent that ravages their internal organs. That would also be a spring tool.
If it were in my yard, I would spray it all by hand with dormant oil. One patch at a time. For a customer, it would be too expensive, and I would recommend the latter two.