August and September are good months to check your trees for next year's population.  The recently deposited egg masses will be darker in color and not eroded and full of holes like the "old egg masses".  It is from the recent egg masses that the next generation of caterpillars will hatch in May of next year. 

It is best to leave the new egg masses alone for a few months.  A small egg parasite uses these eggs to develop it's population so it is beneficial to allow this tiny wasp to thrive.  Under good conditions, this wasp can kill about half the gypsy moth eggs.  The wasp, Ooencyrtus kuvanae, has several generations in the fall and can be active until late November.  Starting in December it is best to collect and destroy all the egg masses that you can.

By mid winter the distinction between old and new egg masses is less clear as even the new have bleached in the sun light.  Collect and destroy any that have a firmness to them.  They contain the eggs.