Cypress and juniper sawflies—Susana spp.
At least six species of Susana sawflies (Tenthredinidae) in California feed as larvae on cypress, juniper, or both. Arborvitae and western red cedar are less-commonly fed upon hosts. Cypress sawfly, S. cupressi, is the most common pest and primarily occurs along the coast and in southern California.
Sawflies are wasps (Hymenoptera) named for the adult females' sawlike, abdominal appendage for inserting eggs into plant tissue. Adults are about 1/3 inch long and have a broad waist, unlike most wasps that have a narrow constriction between the abdomen and thorax. Adults are stout and mostly black with some orange and yellow that varies by gender and species.
Larvae are grayish green with a wrinkled surface and rows of whitish dots and at maturity are 2/3 to 4/5 inch long. Susana larvae have pairs of prolegs (fleshy stubs, or leglike appendages) on abdominal segments two through eight (seven pairs) and a clasperlike rear end. Five or fewer abdominal segments have pairs of prolegs on most other caterpillarlike larvae that occur openly on foliage.
Acantholyda and Neodiprion sawflies also feed on conifers and more than one species can occur together. Neodiprion species chew and feed openly on foliage, mostly on pines, and have relatively smooth skin in comparison with the wrinkled surface of Susana sawflies. Species of Acantholyda feed within silk nests and are called webspinning sawflies, or false webworms.
Cypress and juniper sawflies feed as larvae during late winter to early summer. Larvae develop through five or six, increasingly larger instars (growth stages) as they feed and chew foliage. Larvae mature by early summer, then drop to the ground. Each mature larva (prepupa) forms a cell in topsoil where it overwinters in a brownish, oval, papery cocoon.
Pupation and adult emergence occur during late winter and spring. Adults emerge and mate, and females insert eggs in foliage mostly during February and March in southern California and about one month later in the San Francisco Bay Area. Susana species have one generation per year.
The larvae chew foliage, often in groups. When abundant, they cause hosts to have a sparse canopy of clipped foliage and patches of bare stems. Although abundant larvae make plants unsightly, otherwise-healthy hosts tolerate the loss of some foliage. However, if conifers lose most of their foliage they may die, especially small or young hosts and those already unhealthy from other causes.
Cypress and juniper sawfly numbers are low during most years, in part because they have many natural enemies. Sawfly feeding commonly is innocuous to moderate and this generally does not threaten the long-term health of plants.
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. The larvae are relatively easy to control if you thoroughly spray them directly with almost any insecticide, 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 true caterpillars, which are the larvae of butterflies and moths (Lepidoptera).
For more information see Insects Affecting Ornamental Conifers in Southern California and Review of the Cypress and Juniper Sawflies of the Genus Susana.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).