Sawflies are not true flies, but rather are in the same order as ants, wasps, and bees. Their name derives from the adult female's abdominal appendage, which she uses to insert eggs in foliage. Adult sawflies have 2 pairs of wings and are dark, wasplike, somewhat flattened insects, usually 1/2" long or shorter. Most surface feeding larvae have six or more pairs of prolegs on the abdomen and one large "eye" on each side of the head. An exception is the pear sawfly, whose larvae resembles a small, dark olive green slug.
Different sawfly species prefer a variety of hosts. Conifer sawflies chew needles or buds; a few mine shoots and cause tip dieback. The larvae often feed several to a needle. They can be found throughout the United States. While over 2 dozen species are native, several foreign species have been introduced in the East, for example, the European pine sawfly and the European spruce sawfly. Overall, pine is the most common host of these species, but they can also feed on arborvitae, cypress, fir, hemlock, juniper, larch and spruce.
Broadleaf sawfly species cause more variable damage. Some skeletonize or chew holes in leaves; others mine tissue. A few examples are the pear sawfly and the bristly rose slug. The adult pear sawfly is shiny black with dark wings; the larva is described above. Also called the pear slug, this slimy little larva skeletonizes the leaf surface of most fruit trees, especially cherry and pear, and less often other trees such as ash or hawthorn. The bristly rose slug is, as the name implies, another sluglike sawfly larva, actually one of several that feeds on roses. It is shiny black to pale green; by maturity it develops the many bristlelike hairs on its body whence its name derives. Young larvae skeletonize the lower leaf surfaces of their favored host, while mature larvae chew large holes in leaves.
Fortunately, healthy trees and shrubs tolerate moderate defoliation without significant loss in growth, flowering or fruit yield. Furthermore, a number of natural enemies keep most sawfly populations low, including parasitic wasps, insectivorous birds, small mammals, predaceous beetles, as well as fungal and viral diseases. In eastern states, where introduced species are a menace to Christmas tree farms and forests, sawfly populations have been managed by introducing and/or augmenting sawfly parasites and insect specific viruses. Sawflies in western forests, which are primarily native species, rarely damage forest trees.
Control Strategies
1. Live with the insect. Just a few sawfly larvae on a large or mature plant may do little damage and can be tolerated. Also, when larvae are one-half full grown or larger they will do little more feeding. Leaving the larvae for the birds could be a good solution and beneficial. Sawflies feeding mid- to late summer also do less harm to a plant. This said, many sawflies feed in large groups and can quickly devour needles on many conifers and the foliage of several deciduous trees. Early detection and prompt action may be necessary to avoid extensive needle loss or defoliation of young plants.
2. Hand-pick or wash off the insects. On a small plant, brush the insects off with a gloved hand or wash off with a forceful stream of water. The insects will not climb back onto the plant. Quick action can be very effective.
3. Use insecticides. First it must be noted that even though sawfly larvae resemble caterpillars (the larvae of moths and butterflies) they are not controlled with Bacillus thuringiensis (Bt), a biological control that is very effective against moth and butterfly caterpillars. Be sure you have your insect pest correctly identified. An organic solution, insecticidal soap, is effective against young sawfly larvae but may have little effect on more mature larvae. As a last resort, chemical options include: acephate (Orthene), bifenthrin, carbaryl (Sevin), malathion and permethrin. Two organic pesticides are Neem and Spinosad.