Pine Sawfly Larvae: Identification, Damage, and Control

Blackish-green "worms" covering a pine tree are almost certainly sawfly larvae—the only caterpillar-like insects that feed on pine needles. Despite their appearance, sawflies are primitive wasps (order Hymenoptera), not Lepidoptera. The adult female uses a saw-like ovipositor to lay eggs in needle tissue, giving the group its name.

Feeding Behavior and Damage

Larvae feed gregariously on previous year's needles, stripping entire branches before moving on. They avoid new growth (candles), which means pines rarely die from defoliation alone. However, repeated annual attacks weaken trees, reduce growth, and increase susceptibility to bark beetles and disease. Bark feeding on new shoots can cause twig dieback.

Life Cycle

Most species have one generation per year. Eggs overwinter in needle slits, hatching April–May. Larvae feed through June, then drop to soil to spin cocoons. Adults emerge late summer to mate and lay eggs.

Management Strategies

Mechanical: Clip infested branches, bag and destroy. Knock larvae into soapy water. Effective for small, accessible colonies.

Chemical: Foliar sprays with acephate, malathion, carbaryl, esfenvalerate, rotenone, or pyrethrin provide control. Target young larvae for best results. Systemic options like imidacloprid (soil drench) offer season-long protection but may affect pollinators.

Biological: Bacillus thuringiensis (Bt) is not effective against sawflies—it targets only Lepidoptera.

Healthy, well-watered pines tolerate moderate defoliation. Focus control on high-value specimens or severe, recurring infestations.