Willow ermine moth

Systematic position: The willow ermine moth (Yponomeuta rorella) belongs to the order Lepidoptera and the family Yponomeutidae. It is a gregarious insect, well-known for its dramatic impact on tree health due to the large, visible silk webs produced by the larvae during their growth phase.

Host plants: The primary host plants for this species are various willow species (Salix), though it can occasionally affect poplars. In urban landscapes, parks, and riparian zones, the infestation can be so severe that entire rows of willow trees appear completely defoliated and enveloped in white silk.

Biology and lifecycle: This species has one generation per year. Larvae overwinter under protective shield-like scales on the tree bark. In early spring, they emerge and feed on expanding buds and young leaves. The larvae are colonial and work together to weave extensive silk webbing where they feed and eventually pupate in late June within communal cocoons.

Damage and severity: The economic and aesthetic damage is caused by the caterpillars feeding on leaves, resulting in skeletonization. During mass outbreaks, the trees lose almost all their foliage. This causes extreme stress to the tree, resulting in reduced annual growth, weakened immune response to secondary infections, and in severe cases, dieback of branches.

Control measures: Management strategies focus on both mechanical and chemical interventions. Key practices include:

• Pruning and destroying webbed branches during the early stage of larval development.
• Applying biological insecticides, such as those based on Bacillus thuringiensis, which are effective against young larvae.
• Targeted use of systemic insecticides during the early spring when larvae are active outside their hibernacula.