Spruce web-spinning sawfly

Cephalcia stigma is a species of web-spinning sawfly belonging to the family Pamphiliidae within the order Hymenoptera. As a serious conifer defoliator, this insect is known to cause significant damage to spruce forests, particularly impacting young plantations and mature stands under stress.

The primary host for this pest is Norway spruce (Picea abies). The larvae are the stage responsible for the damage; they feed voraciously on the needles, causing defoliation that weakens the tree's health and reduces its capacity for photosynthesis, often making the tree susceptible to secondary attacks by wood-boring insects.

The life cycle of the spruce web-spinning sawfly is typically univoltine, meaning it completes one generation per year, although larvae may enter prolonged diapause. Adult sawflies emerge in the spring. Females lay their eggs on spruce needles, and upon hatching, the larvae construct intricate silk web nests where they reside in groups, feeding on the needles within the safety of the web.

Damage is characterized by the presence of dense, silken nests attached to the spruce branches, filled with larval excrement and needle remnants. Affected foliage turns yellowish and eventually brown as it dies. Heavy infestations result in severe crown thinning and significant growth loss, which can eventually lead to the mortality of the host tree if the infestation persists over several seasons.

Effective management and protection strategies for this pest include:

• Implementation of pheromone traps to monitor adult population levels.
• Forest hygiene practices, such as the removal of heavily infested or weakened trees.
• Application of biological insecticides, including specific viruses or entomopathogenic fungi, to control larvae.
• Promotion of biodiversity within the forest to encourage the presence of natural predators and parasitoids.