Willow scale

Willow scale (lat. Chionaspis salicis) is a sap-sucking insect belonging to the order Hemiptera and the family Diaspididae. It is a common pest that infests the bark of deciduous trees, where it remains sedentary for most of its life cycle, protected by a hard, scale-like covering.

The primary hosts for this pest include willow, poplar, alder, birch, and ash trees. It is also occasionally found on fruit trees like apple and pear. The infestation is most common in landscapes with high humidity or near natural waterways where willow species grow.

The life cycle involves overwintering as eggs beneath the white, elongated scale of the female. In late spring, after the host plant has flowered, tiny crawlers emerge. These larvae move to new parts of the bark, settle down, and begin to feed, secreting a new waxy scale that serves as their permanent protective shield.

The damage caused by willow scale is severe because the insect drains essential sap, depriving the tree of nutrients. Heavy infestations result in chlorosis, reduced growth, twig dieback, and bark cracking. In extreme cases, the entire tree may weaken significantly, leading to premature death of branches.

Effective management strategies include the following:

• Physical removal of scales by scrubbing affected bark with a soft brush.
• Application of dormant oil sprays in early spring to smother overwintering eggs.
• Systemic insecticide treatments applied during the crawler emergence phase in spring.
• Pruning and destroying heavily infested branches to reduce the overall pest population.
• Promoting tree vigor through proper fertilization and watering to improve natural resistance.