Sawflies are not true flies (Diptera). These wasps (Hymenoptera) are named for the adult female’s sawlike, abdominal appendage for inserting eggs into plant tissue.
Sawflies vary greatly in feeding habits. Willow gall sawflies feed inside tissues and cause the host to develop colorful swellings. Raspberry horntail bores in stems of caneberries, roses, salmonberry, and thimbleberry, causing shoot dieback. Species with larvae that chew openly on foliage include California pear sawfly, certain conifer sawflies, cypress and juniper sawflies, pear sawfly, and rose sawflies (roseslugs). Some species web foliage on which they chew and feed. Hosts of other sawfly pests include alder, birch, oak, and poplar.
Adults are usually 1/2 inch long or shorter and blackish or brownish overall with orange, red, or yellow. Their abdomen is broadly attached to the thorax; most other types of adult wasps have a narrow “waist” between the abdomen and thorax.
In addition to three pairs of legs on the thorax, larvae of sawflies that feed exposed on plants have six or more pairs of prolegs (fleshy stubs, or leglike appendages) on their abdomen. There are five or fewer pairs of prolegs on most other caterpillarlike larvae that occur openly on foliage.
Sawflies develop through four life stages: egg, larva, pupa, and adult. After hatching from an egg, larvae develop through several increasingly larger instars (growth stages), then pupate and emerge as an adult. Most sawflies in California overwinter as eggs in plant tissue or as mature larvae (prepupae) in litter, topsoil, or on hosts. Depending on the species and location, sawflies have from one to several generations per year.
Sawfly feeding varies greatly by species and includes larvae that chew openly on foliage, feed within leaves they roll or web with silk, or tunnel inside leaves or stems. Healthy trees and shrubs generally tolerate moderate defoliation, galling, or mining without significant loss of flowering, fruit yield, or plant growth. Species that sporadically become abundant and extensively defoliate hosts may cause decline and occasionally the death of conifers and plants that are young or already unhealthy from other causes.
Natural enemies keep many sawfly populations low and can cause outbreak populations to soon decline. Fungal and viral diseases, insectivorous birds, parasitic wasps, predaceous beetles, and small mammals commonly feed on and kill sawflies.
Clip off infested foliage or stems if larvae are on or inside a small portion of the plant. Most sawfly larvae that feed openly on foliage are relatively easy to control if thoroughly sprayed with almost any insecticide, including horticultural or narrow-range oil, insecticidal soap, neem oil, or spinosad.
Spinosad can adversely affect bees and certain natural enemies. Because it is toxic to bees for several hours after the spray has dried, do not apply spinosad to plants that are flowering. Avoid applying broad-spectrum, persistent insecticides including carbamates (carbaryl, or Sevin), organophosphates (e.g., malathion), and pyrethroids (e.g., bifenthrin, cyfluthrin, fluvalinate, and permethrin); these are harmful to bees and natural enemies and some products move to contaminate surface water and adversely affect aquatic organisms. Bacillus thuringiensis (Bt) is not effective against sawfly larvae because they are not caterpillars, the larvae of butterflies and moths (Lepidoptera).
See the sawflies named above for more information on those species. See Sawflies on Ornamentals from Oregon State University for information on sawfly pests of azalea, dogwood, mountain ash, oak, and certain other hosts.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Leaves holey from feeding by roseslug (sawfly) larvae.
Needles chewed by pine sawfly.
Larva of California pear sawfly.
Adult sawfly, bristly roseslug.
Distinguishing caterpillarlike insects.