Sawfly larvae, often mistaken for caterpillars, can rapidly defoliate ornamental trees, shrubs, and fruit plants. These pests are the immature stage of sawflies, which are non-stinging insects related to wasps and bees. Their feeding habits quickly diminish the health of susceptible plants like roses, pines, and fruit trees. Addressing an infestation requires a multi-step strategy, beginning with correct identification and progressing through targeted removal and long-term prevention.
Confirming the Pest: Sawfly Larvae Identification
Accurate identification is essential because sawfly larvae respond differently to common treatments than true caterpillars. Sawfly larvae are typically smooth-bodied, lack hair, and often appear greasy or slug-like. They are generally less than one inch long when mature.
The key distinction is their abdominal prolegs, the fleshy, stubby legs on the abdomen. Sawfly larvae possess six or more pairs, while true caterpillars have five or fewer. This difference is significant because the common organic insecticide Bacillus thuringiensis (Bt), which targets caterpillars, is completely ineffective against sawfly larvae.
Sawfly feeding damage often presents as “skeletonized” leaves, where the larvae consume the soft tissue between the veins, leaving a lacy network behind. Some species create a “windowpane” effect by chewing tissue from only one side of the leaf. Since they often feed in groups, look closely at the undersides of leaves for clusters of larvae, which cause rapid damage.
Starting Gentle: Non-Chemical Removal Methods
Initial control efforts should focus on immediate, low-impact, non-chemical methods. For small-scale infestations, manually removing the larvae is highly effective. Larvae can be picked off by hand and dropped into a container of soapy water, preferably in the early morning or evening.
A strong jet of water from a hose can physically dislodge the larvae from the foliage. Larvae knocked off the plant typically cannot climb back up to resume feeding. This method is useful for treating large clusters, and repeating the process every few days helps manage the infestation.
If the infestation is concentrated on a few branches, pruning off the affected section quickly removes a large portion of the population. This is useful for young plants or shrubs where localized damage is easier to isolate. For species that overwinter in leaf litter or soil, clearing debris and lightly raking the soil exposes the pupae to natural predators and harsh weather.
Escalating Treatment: Targeted Organic and Chemical Solutions
When physical removal methods are insufficient for controlling a widespread infestation, targeted sprays offer the next line of defense. Organic solutions like insecticidal soap and horticultural oil are effective because they work by contact, coating the soft-bodied larvae to cause dehydration or suffocation. Thorough coverage is important, particularly on the undersides of leaves where the larvae prefer to feed.
Neem oil is another widely used organic option that contains the compound azadirachtin, which disrupts the larvae’s growth and feeding behavior. It interferes with the molting process and functions as a deterrent. For best results, neem oil should be applied early against younger larvae, and application should be avoided during hot, sunny conditions to prevent leaf burn.
Another effective organic insecticide is spinosad, derived from a soil bacterium, which controls sawfly larvae upon ingestion or contact. Spinosad can be harmful to bees for several hours after application, so it should not be used on flowering plants. If organic methods fail to curb a severe outbreak, conventional chemical insecticides, such as synthetic pyrethroids, may be considered as a last resort. Use these products sparingly, following all label instructions, and avoiding application when pollinators are active.
Long-Term Strategy: Preventing Reinfestation
Preventing future sawfly outbreaks relies on diligent monitoring of susceptible plants. Regular inspection of host plants like roses, pine, and fruit trees, especially in the spring and early summer, allows for the detection and removal of eggs or newly hatched larvae before they cause significant damage. Early action is the most effective form of control.
Encouraging natural predators provides a long-term pest management solution. Birds readily feed on sawfly larvae, and attracting them with feeders or native plants helps keep populations in check. Parasitic wasps are also natural enemies, laying their eggs inside the larvae, which ultimately kills the pest.
Maintaining good garden hygiene is important for disrupting the sawfly life cycle, as many species pupate and overwinter in the soil or debris beneath the plants. Cultivating the soil in the fall or early spring exposes overwintering pupae to predators and the elements. Selecting plant varieties known to be resistant to local sawfly species also reduces the likelihood of severe, recurring infestations.