Worms are devouring the needles on my mugo pine. What should I do?

The “worms” that are eating the needles on your mugo pine are the larvae of the European pine sawfly.  European pine sawfly larvae are grayish green.   Two light stripes and one dark stripe run down the sides of the body.  The legs and head are shiny black.  The larvae feed mainly on mugo, Scotch, and Austrian pines, though other pine species are occasionally damaged.  They do not feed on spruce or fir.  Larvae typically appear in mid to late May in Iowa and are usually gone within a few weeks. 

European pine sawfly larvae feed on needles produced in previous years.  (The needles on most  pines persist for 2 to 5 years.)  They do not harm the new needles developing on the branches.  As a result, the damage is mainly aesthetic.  Larval feeding does not destroy the affected branches. The branches simply have fewer needles than normal. 

To keep damage to a minimum, the larvae of the European pine sawfly can be controlled by knocking the larvae off affected branches into a bucket or other container and destroying them, pruning off and discarding infested branches, or spraying them with an insecticide, such as Sevin.