Those are cocoons of sawflies.  Sawflies have caterpillar-like immature stages, but instead of growing up to be butterflies or moths, an adult sawfly is a wasp-like insect.  Adult sawflies are incapable of stinging, so you don't have to worry about that.  Some species of sawflies go through occasional outbreak cycles, with high populations.  Most of the ones that do this feed on pines.  Be on the lookout for caterpillars feeding heavily on pines, and consider treating trees if many caterpillars are present (you are easily able to count dozens on a branch).  Insecticidal soaps and several other pesticide products are available for sawfly control.  Be sure to read all product label information and follow directions.