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Sawfly Larvae

Garden pest · Symphyta

Description​

Sawfly larvae are the immature stages of sawflies, a group of insects related to wasps and bees. Most garden species feed on leaves or conifer needles, although their host plants and feeding habits vary. Some look like smooth caterpillars; others resemble small, dark, slimy slugs. They may feed alone or in groups, chewing holes, eating inward from leaf edges, or stripping away soft tissue so that only the veins remain. Heavy feeding can defoliate a plant, but established plants often recover if their growing conditions are good. Outbreaks tend to be most noticeable when many larvae hatch at once. Adult sawflies generally do not cause the leaf damage. Because different sawfly species favor different plants, identifying the host can help narrow down which species is present. Repeated defoliation, especially of a young or already stressed plant, is more concerning than a single short-lived outbreak.

Symptoms​

  • Holes in leaves
  • Leaves dropping
  • Stunted growth
  • Insects visible on the plant

How to Identify​

Look for larvae on leaves, especially their undersides, and check for fresh chewing or skeletonized patches. Many sawfly larvae resemble caterpillars, but typical sawfly larvae have six or more pairs of fleshy abdominal prolegs; most butterfly and moth caterpillars have five pairs or fewer. Slug-like sawfly species are an exception to that easy visual rule. Feeding in clusters can also be a clue, though it is not definitive. Leaf damage without larvae present could have been caused by other chewing insects that have since moved on. Unlike slugs and snails, sawfly larvae do not leave slime trails across the plant or soil.

Prevention​

Inspect susceptible plants regularly during their growing season, paying particular attention to leaf undersides and new growth. Remove leaves bearing eggs or newly hatched larvae when practical, before feeding spreads. Fine-mesh insect netting can protect small plants if installed before adult sawflies lay eggs; secure its edges and remove it when pollinators need access to flowers. Keep plants adequately watered and avoid excessive nitrogen fertilizer, which can promote tender growth. Grow a variety of flowering plants to support predators and parasitoids, and avoid routine broad-spectrum insecticide sprays that kill them. If a particular plant suffers repeated outbreaks, learn the local sawfly species and its seasonal timing so inspections and protective netting coincide with egg laying.

Treatment​

Start by checking how much foliage has been lost. An established plant with light damage may need no treatment. Hand-pick or brush larvae into soapy water, or prune out heavily infested leaves or shoots; a firm water spray can dislodge larvae, but check whether they return. Treat young plants or large infestations promptly while larvae are small. Insecticidal soap or horticultural oil may work on exposed larvae when applied directly and thoroughly, including to leaf undersides; follow the label and avoid spraying heat-stressed plants. If damage continues, consider a product specifically labeled for sawfly larvae and the affected plant, such as spinosad where permitted. Follow all label directions and protect pollinators by avoiding sprays on flowering plants. Bacillus thuringiensis var. kurstaki (Bt), commonly sold for caterpillars, does not control sawfly larvae. Avoid broad-spectrum insecticides unless other measures have failed and the damage warrants them.

Plants Affected​

Alder, Cornus, Larch, Mahonia, Pine, Ribes, Rosa, Rose, Salix

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