The rose sawfly, often called the “rose slug,” is a common garden pest whose larvae rapidly defoliate rose bushes. This insect is the larva of a non-stinging wasp, not a true caterpillar or slug. Addressing an infestation requires quick identification and targeted control measures. Effective strategies combine manual removal, cultural practices, and, when necessary, the careful application of low-impact products.
Identifying the Pest and Damage
The sawfly larva, often called a “rose slug” due to its slimy, soft-bodied appearance, is the primary culprit. Larvae are typically pale green or yellowish-green with a light brown head, growing up to three-quarters of an inch long. They feed on the underside of rose leaves, often remaining unnoticed until damage becomes obvious.
Sawfly larvae are distinct from true caterpillars, which is important for control methods. Sawfly larvae possess six or more pairs of fleshy, unjointed abdominal legs (prolegs). True caterpillars have five or fewer pairs. This anatomical difference explains why many caterpillar-specific treatments are ineffective against the sawfly.
The most characteristic sign of an infestation is leaf skeletonization, typically appearing in late spring or early summer. Young larvae chew the soft tissue from the underside of the leaves, leaving the tougher veins intact. This feeding pattern creates a translucent, window-pane effect, and the damaged areas eventually turn brown and dry. Control is easier when the larvae are small, making early detection important.
Manual and Cultural Control Methods
Non-chemical control methods are highly effective, especially when the infestation is caught early. For small numbers of pests, simple hand-picking is a direct solution. Regularly inspecting the undersides of leaves and physically removing the larvae prevents further damage.
A forceful jet of water from a garden hose can dislodge the soft-bodied larvae from the foliage. Larvae cannot easily climb back onto the rose bush once knocked to the ground. Directing the stream of water to the lower surface of the leaves ensures maximum contact with the pests.
Cultural practices focus on sanitation and removal to interrupt the sawfly life cycle. Clipping off and destroying heavily infested leaves or canes reduces the overall population. Since sawflies overwinter as pupae in the soil or debris near the plant base, remove leaf litter in late fall or early spring. Lightly cultivating the soil around the roses can also expose the pupae to the elements and predators.
Applying Targeted Insecticides
When manual controls are impractical due to a widespread or persistent infestation, targeted insecticides offer a solution. Recommended options are contact-based products like insecticidal soaps and horticultural oils, such as neem oil. These products work by suffocating or disrupting the cell membranes of the soft-bodied larvae and have a minimal residual effect once dried.
Thorough application is necessary, as these products must directly contact the pest to be effective. Since the larvae feed almost exclusively on the underside of the leaves, sprays must be directed there to ensure complete coverage. Reapplication may be necessary after rainfall, and should only occur when live larvae are actively present.
Avoid broad-spectrum synthetic insecticides, which can harm beneficial insects and pollinators. Also, the biological control product Bacillus thuringiensis (Bt), commonly used for true caterpillars, is ineffective against rose sawflies. Sawflies belong to a different insect order, meaning the toxins in Bt do not affect their digestive systems.