Mountain ash sawfly

The mountain ash sawfly (Pristiphora geniculata) is a significant defoliating pest belonging to the order Hymenoptera and the family Tenthredinidae. While the adult stage is relatively inconspicuous, the larvae of this species are notorious for their ability to cause severe damage to host trees during their intense feeding periods.

The primary host for this pest is the mountain ash (Sorbus species). They frequently attack both ornamental and native rowan trees in residential landscapes, parks, and nurseries. Young trees are particularly vulnerable to infestation, as repeated defoliation can lead to stunted growth and increased susceptibility to other stressors.

Regarding its biology, the sawfly typically completes one or two generations per year depending on local climatic conditions. The insect overwinters as a prepupa inside a silken cocoon located in the soil beneath the host plant. Adult emergence is synchronized with the spring leaf development of the host, allowing females to lay eggs directly into the leaf margins.

The damage caused by the larvae is distinct and characterized by leaf skeletonization. Larvae feed in groups, eating the leaf blade starting from the edge and working inward, leaving behind only the veins and midribs. A heavy infestation can lead to complete defoliation by mid-summer, which significantly reduces the tree's photosynthetic capacity and stored energy reserves.

Effective management strategies should be implemented to minimize damage:

• Monitoring the canopy for the presence of larval groups during late spring and early summer.
• Physical removal and destruction of infested leaves or larval clusters when infestations are limited.
• Cultivating the soil beneath the canopy in autumn to disrupt overwintering cocoons.
• Application of selective insecticides or microbial agents (such as Bt) when larvae are young and most susceptible.
• Promoting biodiversity to support the natural enemies of the sawfly, including parasitic wasps and predatory insects.