Sawfly larvae can cause significant damage to ornamental plants and trees, often consuming large amounts of foliage almost overnight. These soft-bodied, cylindrical pests are frequently mistaken for true caterpillars, leading many gardeners to apply ineffective treatments. Sawfly larvae feed on a wide variety of plants, including roses, pine trees, and fruit trees, sometimes stripping entire branches bare. Prompt and accurate identification is the first step in saving your plants from rapid defoliation.
Confirming the Infestation
Distinguishing sawfly larvae from true caterpillars is paramount because the common treatment for caterpillars, Bacillus thuringiensis (Bt), is ineffective against sawflies. Sawfly larvae belong to the Order Hymenoptera (bees and wasps) and lack the necessary digestive system receptors for the Bt toxin to work. Correct identification prevents wasted effort and unnecessary pesticide use.
The easiest way to tell the two apart is by counting the abdominal prolegs, the fleshy, unjointed leg-like structures along the body. A true caterpillar will have five pairs of prolegs or fewer, typically on the middle abdominal segments. Sawfly larvae, conversely, will have six or more pairs of prolegs, often running down the entire length of the abdomen. True caterpillars also possess tiny hooks called crochets on their prolegs, which sawfly larvae lack.
Physical and Cultural Control
For small infestations, physical removal offers an immediate, non-chemical solution. You can hand-pick the larvae, which often feed in clusters, and drop them into a container of soapy water. This method is most practical for smaller shrubs and garden plants where the pests are easily accessible.
A strong, direct jet of water is an effective tool for dislodging sawfly larvae from larger or heavily infested plants. The water stream knocks the soft-bodied larvae off the foliage and onto the ground, where they are unlikely to return to the host plant. Using a tarp beneath the plant can help collect the dislodged larvae for easy disposal.
Pruning is effective when the infestation is confined to a few branches or needles, as sawflies often feed gregariously. Clipping off the affected sections and immediately disposing of them instantly removes a large portion of the pest population. Maintaining plant vigor through proper watering and fertilization allows healthy plants to better withstand minor damage. Cultivating the soil beneath the host plant in late fall or early spring can disrupt the sawfly’s life cycle by destroying overwintering pupae resting in the soil.
Applying Targeted Treatments
If the infestation is widespread, targeted spray treatments are necessary, but they must be applied when the larvae are small and actively feeding. Insecticidal soaps and horticultural oils are low-impact options that work by contact, suffocating the larvae or disrupting their cell membranes. These contact treatments require complete coverage, meaning you must spray both the tops and undersides of the leaves where the pests often hide.
Apply these sprays during the cooler parts of the day, such as early morning or late evening, to prevent leaf burn (phytotoxicity). This burn can occur when oils or soaps are applied in high heat or direct sun. Spinosad is another effective, low-toxicity option, derived from a naturally occurring soil bacterium. It works primarily through ingestion, affecting the sawfly’s nervous system, and retains effectiveness for several days.
For severe outbreaks, Pyrethrins, derived from the chrysanthemum flower, offer a fast-acting chemical option that causes immediate paralysis and knockdown. Pyrethrins are often combined with insecticidal soaps to create a powerful dual-action treatment for rapid pest reduction. Since these treatments can affect beneficial insects, always apply them with precision and avoid spraying flowering plants when pollinators are active.
Long-Term Management
Long-term control focuses on disrupting the sawfly’s life cycle before the larvae emerge to feed. The adult sawfly is a small, non-stinging, wasp-like insect that uses its saw-like ovipositor to insert eggs into plant tissue. Monitoring for these adult wasps in the spring and early summer signals that egg-laying is occurring and larvae will soon hatch.
Applying dormant oil in late winter or early spring targets overwintering eggs on woody plants and fruit trees. This application should be timed right before bud break, when temperatures are above freezing, to smother the eggs before they hatch. For species that overwinter as pupae in the soil, regularly turning the top layer of soil in fall and spring exposes the pupal cases to predation and harsh weather. Consistent monitoring for the first sign of damage, such as small skeletonized leaves, allows for immediate action against the vulnerable, newly hatched larvae.