Pine Sawflies

The two most common species of pine sawflies in Rhode Island are the redheaded pine sawfly and the European pine sawfly.

Redheaded Pine Sawfly

The redheaded pine sawfly is found from Southeastern Canada through the eastern United States. It is the most widespread and destructive of the pine sawflies, feeding is primarily on the two- and three-needled pines, including Jack, Scotch, red, shortleaf, loblolly, slash, longleaf, mugho, and pitch pines. White pine and Norway spruce may also [rarely] be defoliated by the redheaded sawfly.

Adult females are 1/4 to 3/8 inch long while males are 1/5 to 1/4 inch long.Males are completely black except for their legs and wings; females are predominantly black with a whitish underside of the abdomen.

This pest overwinters as a fully mature larva in a cocoon under host trees. When spring arrives the pupa is formed and the wasp-like adults emerge in a few weeks. Females find preferred pines and deposit eggs in rows of slits along the edges of needles. Eggs hatch in about a month into caterpillar-like larvae which take about one month to  mature into 1-inch long, yellow bodied insects with black spots and orange-red heads. Mature larvae drop to the soil and pupate. From New York north this insect has only onegeneration per year.

European Pine Sawfly

The European pine sawfly, which was accidentally introduced from Europe, is the most common sawfly infesting pines in landscapes, ornamental nurseries, and Christmas  tree plantations from southwestern Ontario through New England and west to Iowa. In landscape situations, this sawfly prefers mugho and tabletop pines. However, it readily attacks Scotch, red, Jack, and Japanese pines as well. It will rarely feed on white, Austrian, Ponderosa, shortleaf, and pitch pines,  especially if these are intermixed with the preferred hosts.

The caterpillar-like larvae are grayish green and have a light stripe down the back and a light stripe along each side followed by a dark green stripe. Fully grown larvae are about 1 inch long. Usually only one generation occurs and the winter is spent as an egg inserted into slits along the edge of needles. The eggs hatch in April through mid-May and the larvae may feed until mid-June. Mature larvae drop to the ground and spin tough, brown cocoons. Adults emerge in late August through September to mate and lay eggs.

Damage

Pine sawflies usually feed on young trees, preferably 1 – 14 feet tall. Trees growing under stress in shallow soils, very wet or dry sites, or under stress from competing vegetation are especially susceptible to infestation and heavy defoliation. Outbreaks occur periodically and tend to subside after a few years of heavy defoliation. Severe outbreaks have resulted in death or deformity of young pines.

The youngest sawfly larvae – those hatching from the egg – can only eat the needle surface, causing needles to turn brown and wilt and giving them a straw-like appearance. As the larvae grow, they feed in groups from the tip of a needle to the base. The larvae feed on older foliage and move from branch to branch as they strip the needles. Trees that are entirely defoliated are severely stunted but since new growth is rarely attacked the trees will survive. Larvae often migrate to new trees if the needles on their current host have been devoured. Heavily infested trees end up with a "bottle brush" effect – all the old needles are missing and only the current year's needles are present.

Control

Numerous natural control agents play important roles in keeping redheaded pine sawfly populations down. Outbreaks  often collapse from rodents feeding on the pupae and diseases killing the larvae. Best controls are obtained when the larvae are still small, so look for the straw-like needles left behind by the young larvae. Inspections should be made in late April and early May. The egg-laying scars can also be seen by inspecting the needles in late winter.

• Remove competing vegetation before planting pines.

• Avoid planting pines in high hazard areas, such as frost pockets or on excessively dry, wet, or nutrientdeficientsoils.

• If the needles containing overwintered eggs can be found before they hatch they can be pulled off the plants and destroyed. Do not simply throw them on the ground as the eggs can still hatch.

• Colonies of larvae can be easily removed by clipping off the infested branch. Place these branches in a plastic bag and destroy. Colonies can also be knocked off by sharply striking the infested branch. Crush the larvae or knock them into a pail of soapy water.

If few colonies are present, they can be controlled using the methods noted above, but large infestations are better controlled by general spraying. When necessary, chemical treatments can be used as an effective control. Several  horticultural oils and insecticidal soaps are labeled for control of sawflies on ornamentals. These usually work well when the sawfly larvae are small and thorough coverage of the colony can be achieved.

Adapted from: Virginia Cooperative Extension; Ohio State University Extension, 1999