It sounds as if you are describing damage inflicted by the European pine sawfly. This insect generally begins feeding early to mid-May and the best time to treat is when they are present and small. They look similar to a caterpillar, have a black head, dark grey-green bodies with an off-white stripe down the middle of their back and slightly lighter stripes on either side. They feed in groups and when disturbed will sometimes rear up on their hind legs in unison. Once the larvae is about 3/4 inch long, they are full grown and it is not effective to treat them. These insects have only one generation per year, so they will not feed again until the following spring. Start scouting early next spring for the larva, and if you find it, spray them off the tree with a strong stream of water from the garden hose, or spray them with insecticidal soap. You can also shake them off or hand pick them off. If the tree is large and there are lots of larvae, you may want to chemically treat. Make sure the chemical is labeled for sawfly. When using chemicals, remember the good insects may be killed along with the undesirable ones. If you do not wish to do this yourself you could call an ISA certified arborist to monitor for and treat the insects.