Overview of Surface Feeding Sawfly

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 resemble a small, dark olive-green slug. 

Different sawfly species prefer a variety of hosts. Conifer sawflies chew needles or buds; a few mine shoot 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, and 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.

Overview of Borer and Miner Sawfly

Borers and miners are the larvae of various insects, particularly beetles, flies, and moths. Many of the miners that cause damage to annuals, perennials and vegetables as well as woody plants are the larvae of sawflies.

Many woody plants are vulnerable to sawflies, of which there are two main types: gall sawflies and stem sawflies. Gall sawflies are mostly leaf feeders, although some burrow internally in buds, petioles, twigs, or stems, usually producing galls. Susceptible trees include maples and willows. If the leaves of your maple trees begin to wilt, yellow, or turn brown and drop beginning in mid-May, the petioles may be infested with sawflies; rake and burn or otherwise destroy the infested fallen petioles promptly and daily. Damage to willows is likely to occur west of the Rockies and is generally limited to the formation of unsightly galls which may also weaken stems. There are a number of natural predators that help to keep these pests in check, including several species of birds as well as ants and grasshoppers. Some commercially available parasites have also proven to be effective controls.

Stem sawflies bore into tender shoots of trees and shrubs, occasionally inflicting serious damage in localized infestations; however, injury is seldom widespread. Vulnerable woody plants include blackberry, currant, raspberry, and roses, as well as some oak species, poplar, viburnum, and willows. If the canes of your roses or berry plants wilt and die back, you may have a sawfly problem; prune suspect canes below the infested section and destroy them. The above-mentioned trees are most likely to manifest damage through wilting and drooping terminals and branch ends. In these instances, the most effective control entails pruning and destroying infested growth.

Integrated Pest Management Strategies

1. Live with the insect. 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 larvae for birds could be a good and beneficial solution. 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 them 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. 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 correctly identified your insect pest. 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.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against sawflies. Not following the pesticide label before usage is a violation of federal law.