Conifer sawflies—Acantholyda and Neodiprion spp.
More than one dozen species of Neodiprion sawflies (Diprionidae) feed openly on conifer needles. The species are named for a preferred host, most commonly a particular pine species.
More than 20 species of Acantholyda sawflies (Pamphiliidae) feed within silk nests on pines and other conifers. These are called webspinning sawflies, or false webworms.
Arborvitae, cypress, Douglas-fir, fir, hemlock, juniper, larch, and spruce are also hosts of Acantholyda or Neodiprion species or both. Cypress sawfly and other Susana species (Tenthredinidae) feed on arborvitae, cypress, and juniper.
Sawflies are wasps (Hymenoptera) named for the adult female’s sawlike abdominal appendage for laying eggs in plant tissue. Most conifer sawfly adults are 1/3 inch long or less and brownish to black overall. They have two pairs of wings with prominent veins and a broad waist, unlike most wasps that have a narrow constriction between the abdomen and thorax.
Larvae are commonly brownish, green, or yellowish green overall and resemble caterpillars (larvae of butterflies and moths, Lepidoptera). Larvae commonly have a black or brownish head and increasingly develop contrasting stripes or spots on the body as they mature.
Neodiprion larvae, especially when young, will feed several to a needle, often with their heads pointed away from the twig or their bodies wrapped around the needle. Larvae tend to disperse and feed singly as they mature.
Larvae of Neodiprion have pairs of prolegs (fleshy stubs, or leglike appendages) on abdominal segments two through eight (seven pairs) and their rear end is clasperlike. Five or fewer abdominal segments have pairs of prolegs on most other caterpillarlike larvae that occur openly on foliage.
It may not be possible to distinguish Neodiprion larvae to species; what appear to be the same species may be a complex of multiple species. Look-alike species can be given varying names, such as that named both balsam fir sawfly and Douglas-fir sawfly.
Acantholyda larvae spin nests or silken webs on conifer foliage. They feed inside the silk in groups or singly, varying by the species. Unlike openly feeding sawflies, these webspinning species have no abdominal appendages except for the clasperlike rear end.
Sawflies develop through four life stages. They hatch from an egg, grow through several increasingly larger instars, pupate, and emerge as an adult. Adult females lay eggs in niches (notches) in needles. The sawflies overwinter as eggs in plant tissue or as mature larvae (prepupae) or pupae.
Neodiprion species pupate inside a thin, papery cocoon in litter or topsoil. Acantholyda species generally pupate in an earthen cell in topsoil.
These sawflies require one or more years to complete one generation. Some individuals may remain in the cocoon or soil cell as resting-stage (diapausing) prepupae (mature larvae) or pupae for more than one year; these individuals may be active and feeding on foliage during only one several-week period over a time span of 2 to 3 years.
Neodiprion larvae chew foliage during either spring to summer, or fall to winter, varying by the species. Young larvae feed in a group on the outer tissue of mostly older needles, leaving the central ribs that later turn yellow brown and may break off. Later instars consume most of the needle. Infested trees have sparse foliage and only shoot terminals may remain green.
Acantholyda form and feed within "nests" on foliage during late spring and early summer. The nests are composed of cast skins, chewed needles, and frass (excrement) webbed together with silk. Nests often persist after larvae are no longer present.
Some species occasionally become very abundant and extensively defoliate hosts, most commonly small trees in forested areas and tree plantations. Otherwise-healthy conifers can be killed by a single complete or extensive defoliation. Widespread outbreaks sporadically occur in forests and can kill numerous trees.
In California, sawfly numbers are low during most years and their feeding commonly is innocuous to moderate and not threatening to plant health. Although their defoliation is occasionally very noticeable, sawfly feeding alone rarely kills or seriously harms conifers in landscapes.
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 a small portion of the plant. Species that feed openly are relatively easy to control if sprayed with almost any insecticide applied to achieve good coverage, including horticultural or narrow-range oil, insecticidal soap, neem oil, or spinosad.
Avoid applying broad-spectrum, persistent insecticides, such as 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.
For more information, see Insects Affecting Ornamental Conifers in Southern California and Western Forest Insects (PDF).
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).