By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver
The willow blister gall is a grayish-white and red, somewhat round and irregular growth on the upper and lower surfaces of willow leaves, especially pussy willow.
The galls are caused by eriophyid mites. Eriophyid mites are tiny, carrot-shaped insects that can be seen only with a hand lens or microscope. What most people notice is the distortion or alteration of the host plant's tissues that results from the mite's feeding activities.
Each species of Eriophyid selectively affects a narrow range of host plants. They may cause damage ranging from harmless tissue deformities such as galls, to death of the plant. Overwintering on the willow's bark and branches, in spring they come out of hibernation and move to the developing leaves. Their feeding and presence on the tender leaves causes the leaf edges to swell, wrinkle and curl over to form a protective chamber. Additional bladder-like galls may occur on the entire leaf surface. The summer is spent feeding and reproducing. When large numbers of eriophyid mites develop, willow leaves may become completely curled and badly distorted.
Most galls do not adversely affect plant health, and control is generally not required. Chemical applications are often ineffective since the precise timing of sprays is critical. If desired, carbaryl or dicofol can be applied 7 to 10 days prior to bud break, before gall formation begins. Horticultural oils can be applied in late winter or early spring, when the plant is still dormant. Always read and follow directions carefully when using a pesticide.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010