Left: The larval stage of the redheaded pine sawfly is the stage that does the most damage, feeding on foliage of conifers. Right: In cases of heavy infestation, entire trees can be defoliated.
There are more than 100 species of sawflies known in the U.S., and the most common feed on conifer foliage. Coincidentally, the sawfly is not technically a fly. It is in the order

Hymenoptera, which includes ants, wasps, and bees. The common name “sawfly” comes from the saw-like apparatus at the tip of the adult sawfly’s abdomen. It uses this “saw” to slit plant tissue which allows for easy insertion of eggs. Sawfly larvae resemble that of moth and butterfly caterpillars, but have six or more pairs of false legs (prolegs) on their abdomen. Caterpillars have five or fewer pairs of prolegs.

Source: The Morton Arboretum Photos: Gerald J. Lenhard, Louisiana State University, Bugwood.org; Steven Katovich, USDA Forest Service, Bugwood.org; USDA Forest Service - Region 8 - Southern , USDA Forest Service, Bugwood.org