Birch sawfly

Taxonomy: The birch sawfly (Craesus septentrionalis) is an insect species belonging to the order Hymenoptera and the family Tenthredinidae. It is a well-known defoliator that affects several species of broad-leaved trees across the Northern Hemisphere.

Host range: The pest primarily targets trees such as birch (Betula), alder (Alnus), hazel (Corylus), poplar (Populus), and willow (Salix). It frequently colonizes both mature trees in forest environments and ornamental specimens in residential gardens and public parks.

Biology and lifecycle: The species overwinters as prepupae within durable cocoons located in the soil or leaf litter. Adult wasps emerge in late spring to coincide with leaf flush. Females insert eggs into the leaf tissue along the margins. The larvae are social during their early instars, often feeding in groups before dispersing as they mature.

Damage and significance: The larvae are highly voracious and cause significant defoliation. Typically, they feed by gripping the edge of the leaf and consuming the tissue between the veins, often leaving only the main veins behind (skeletonization). Repeated or severe attacks can stunt tree growth, cause premature browning, and render trees more susceptible to secondary stressors like diseases and drought.

• Monitor trees regularly during late spring and early summer for signs of feeding or colonial clusters of larvae.
• Manually remove and destroy infested leaf clusters if the infestation is localized to small trees.
• Utilize microbial insecticides based on Bacillus thuringiensis for targeted control that spares beneficial pollinators.
• Apply contact or systemic insecticides if the population density threatens the overall health of the tree canopy.
• Manage the soil around the base of trees through light cultivation to disturb overwintering pupae and increase mortality rates.