Spiraea sawfly

The Spiraea sawfly (lat. Nematus spiraeae) is a specialized insect pest belonging to the order Hymenoptera and the family Tenthredinidae. It is a significant threat to decorative Spiraea plantings, as its larvae are voracious defoliators capable of causing severe aesthetic and physiological damage to ornamental shrubs.

The pest exclusively targets plants within the genus Spiraea. Due to their high reproductive rate and rapid larval development, they can cause extensive foliage loss within a short period. This impact is particularly noticeable in dense hedge plantings or individual bushes, where damaged plants lose their ornamental value and show reduced vigor in the following growing season.

The biological cycle of Nematus spiraeae involves several stages: egg, larva (often called a false caterpillar), pupa, and adult. They typically overwinter in the soil within protective cocoons. In the spring, coinciding with the flushing of new leaves on Spiraea, the adults emerge to mate and lay eggs in the leaf tissue, marking the beginning of the first annual generation.

Larval feeding causes characteristic damage to the foliage. Young larvae start by creating small holes or partial skeletonization of the leaf blades. As they grow through subsequent larval stages, they consume entire leaves, leaving only the primary veins. This process severely reduces the plant's photosynthetic capacity and can lead to stunted growth if the infestation is not managed in time.

Effective management requires a combination of monitoring and intervention strategies. Growers should perform regular inspections to identify egg clusters or young larvae.

• Manual removal of larvae for minor infestations on small shrubs.
• Application of selective insecticides if thresholds for control are exceeded.
• Cultural practices such as fall cleanup and soil cultivation to disrupt the overwintering pupae.
• Encouraging natural predators such as parasitic wasps and predatory beetles.
• Using biological control agents like Bacillus thuringiensis sprays during early larval development.