A mite that causes galls on ash trees

Ash flower gall develops when microscopic mites feed on the male flowers of ash trees. The galls appear as green or brown clusters hanging from branches on ash trees.

Infested ash trees require treatment for control
Armillaria takes advantage of injuries to the root system and attacks weakened or stressed trees. Trees suffering construction damage are a common target. Severed roots, compacted soil and changes to the level of soil over the roots all provide opportunities for Armillaria to invade a tree.

What to look for
• Ball-like irregular masses on the flowers.
• Current year galls are green early in the season.
• Older galls are reddish-brown.
• Galls are more noticeable when the leaves fall off the tree in autumn.
• Gall masses can persist for up to two years.

Life Cycle
• Female mites spend the winter under the bud scales or bark.
• Mites begin feeding on the male flower clusters of ash in early spring.
• Mite feeding initiates gall formation of flower tissue.
• Female mites then lay eggs in the developing galls.

Treatment and Prevention
Galls do not harm the health of infested trees. Once a gall starts growing it will continue to form even if the mite dies. Ash flower gall is difficult to control. Treatments must be applied in the early spring. Galls formed in previous seasons will remain on the tree, even if newly forming galls are prevented.

New galls early in the season.
Older galls later in the season.
Ash flower gall mite.