Ash sawfly

The Ash sawfly (Tomostethus nigritus) is a member of the Hymenoptera order and the Tenthredinidae family. It is a specialized pest primarily affecting ash tree species (Fraxinus). This insect is commonly found in forests, urban parklands, and nurseries, where it can cause significant damage to the foliage of host trees during the spring and early summer periods.

The adult insect is a small black sawfly. The damage is exclusively caused by the larval stage, which feeds on the leaves of the ash tree. The life cycle consists of several larval instars, with the species having one generation per year. Development is closely synchronized with the phenology of the ash tree, ensuring that food is available immediately upon the larvae hatching.

The biology of Tomostethus nigritus involves overwintering as mature larvae in cocoons within the soil. Pupation occurs in the spring, and the emergence of adults coincides with the bud break of ash trees. Females deposit eggs into the leaf tissues, and the larvae emerge shortly thereafter to begin their feeding cycle on the developing canopy.

Larval feeding causes defoliation, which significantly weakens the tree's health. Early instars create small holes, while later stages consume entire leaves, leaving only the midribs intact. Severe infestations lead to premature leaf drop, reduced annual growth, and increased susceptibility to other biotic stresses, such as wood-boring insects or drought.

Management strategies for this pest include a combination of cultural and chemical controls:

• Soil cultivation around the base of the trees in late autumn to expose overwintering larvae to predators and frost.
• Application of biological insecticides such as Bacillus thuringiensis when larvae are in early developmental stages.
• Systemic insecticide treatments for highly valuable ornamental trees in nurseries or landscapes.
• Routine monitoring and inspection of ash canopy density during the spring months.