Sawflies are common garden and landscape pests that can rapidly defoliate trees and shrubs, causing alarm among gardeners. Despite their name, these insects are not true flies; they are non-stinging members of the Hymenoptera order, related to bees and wasps. Their name comes from the adult female’s saw-like ovipositor, which she uses to cut slits into plant tissue to deposit eggs. Effective control requires correctly identifying the pest and applying targeted removal strategies at the most vulnerable point in its life cycle.
Spotting Sawflies and Recognizing Damage
The most damaging stage of the sawfly is the larva, which looks remarkably similar to a moth or butterfly caterpillar. A defining characteristic is the number of fleshy prolegs on the abdomen; sawfly larvae have six or more pairs, while true caterpillars have five or fewer. These soft-bodied larvae often feed gregariously, which makes early detection easier.
Sawfly feeding damage varies by species and host plant. Larvae often skeletonize leaves by consuming the soft tissue between the veins, leaving behind a thin, lacy network that soon turns brown and brittle. For example, the common “roseslug” sawfly causes this skeletonization, while pine sawflies chew on last year’s needles, leaving new growth untouched. This concentrated feeding can quickly lead to severe defoliation, though established trees can tolerate moderate damage.
The Sawfly Life Cycle and Key Vulnerability Points
Sawflies undergo complete metamorphosis, progressing through four stages: egg, larva, pupa, and adult. Adults emerge in the spring or early summer to lay eggs, which hatch into the destructive larval stage within a few weeks. The larval stage is the longest, lasting several weeks as they feed voraciously on the host plant’s foliage.
Most species have one generation annually, but some produce two or more, extending their presence through the growing season. Once fully grown, larvae drop to the ground to prepare for the next stage. They typically overwinter as prepupae or pupae inside a cocoon, often burrowed beneath the soil surface or hidden in leaf litter. The larval stage is the primary vulnerability for control efforts, and treatments are most effective when applied to young, recently hatched larvae.
Physical and Organic Control Methods
Organic approaches are highly effective against sawflies, especially when the infestation is small. For accessible pests, physical removal is a direct solution. Larvae can be handpicked from the foliage and dropped into a container of soapy water, which quickly kills them.
A forceful jet of water from a garden hose can also dislodge groups of larvae. Once knocked down, the soft-bodied larvae are often unable to climb back up the host plant, leaving them vulnerable to predators or desiccation. For localized infestations, pruning and discarding affected branches or leaves eliminates eggs or young, clustered larvae.
To disrupt the pest’s overwintering stage, lightly disturbing the top two inches of soil around the host plant in late fall or early spring exposes the pupae. This mechanical action makes the cocoons vulnerable to freezing temperatures, desiccation, and foraging birds.
For contact-based organic treatment, insecticidal soaps made from potassium salts of fatty acids work by penetrating the insect’s outer shell, leading to dehydration. Neem oil, which contains azadirachtin, is another effective organic option that acts as both a contact killer and a feeding deterrent. It works best against younger larvae by disrupting their molting process. Both insecticidal soap and neem oil must be applied thoroughly, ensuring the spray directly contacts the larvae and coats the undersides of the leaves.
Chemical Treatment Options for Severe Infestations
When physical or organic methods are insufficient to manage a widespread infestation, targeted chemical treatment may be necessary. Bacillus thuringiensis (Bt), a common biological insecticide for caterpillars, is ineffective against sawfly larvae. Effective treatments must target the sawfly’s distinct biology.
Spinosad, a microbial insecticide derived from a naturally occurring soil bacterium, is an effective treatment against sawfly larvae. It acts on the insect’s nervous system through both contact and ingestion, leading to paralysis and death. Botanical pyrethrins, which are nerve agents derived from chrysanthemum flowers, offer a rapid contact kill.
Any chemical application must be precisely timed to coincide with the presence of young, actively feeding larvae for maximum effect. When using spinosad or pyrethrins, follow all label instructions and apply the product during the evening hours. This practice minimizes exposure to beneficial insects like honeybees, as these insecticides are toxic while wet, but their toxicity diminishes once they have dried.