Tree Galls, Part 2

Kathleen Cue, Nebraska Extension Horticulture Educator

As mentioned in the previous garden update (Tree Galls-Part 1), galls that form on tree leaves rarely cause much in the way of tree stress. But there are also galls that form on other parts of trees. In most cases, gall formation on leaves and flowers are of little concern, while those affecting the twigs, branches and stems merit closer monitoring.

Ash Flower Gall

The male flowers of white ash are prone to invasion by the ash flower gall mite. Flowers will develop into bright green broccoli-like growths.  These growths eventually age to dry brown clusters that remain in trees until they break away.  The formation of galls prevents the male flowers from producing pollen, which is a good thing if you’re an allergy sufferer or don’t want the female ash flowers being pollinated and producing lots of seed. Even though tree owners find ash flower galls offensive, trees remain healthy and treatment is not necessary to control the ash flower gall mite.

Hackberry Nipple Gall

That hackberry is one tough native tree is undisputed.  Every year, the undersides of leaves display miniature barrel-shaped galls. These galls are formed of plant tissue in response to feeding by psyllids, otherwise known as jumping plant lice. While the number of galls on leaves can be alarming, no harm is caused and treatment is not necessary.

Oak Bullet Gall

Any time galls form on twigs and branches, the potential for long term damage increases. In the case of oak bullet gall, the formation of galls is on the perennial parts of the tree, in this case the twigs and small branches. Bullet galls, the result of feeding by the cynipid wasp, grow primarily on bur and swamp white oak. Initially galls are green, gradually darkening to brown and remaining on the tree long after the adult has emerged.  Older, well-established trees really aren’t harmed by oak bullet galls, but young and newly-planted trees can have branch dieback if there are a large number of galls. Trees that are thriving will have fewer detrimental effects from oak bullet galls.  Hanging feeders near young trees will attract birds ready to eat the non-stinging cynipid wasps. Pruning out heavily infested branches and then burning, burying or chipping them will decrease cynipid wasp numbers.

Weather is a huge factor impacting the number and variety of galls found in our landscapes, mainly because weather affects insect populations. Arborists and horticulturists see galls every year but the number of galls vary from one year to the next. Quite simply, galls and the insects that cause them are a thing of the past with autumn leaf drop.

Go to Dodge County Horticulture Web Page for more gardening information.

Photo: Oak Bullet Gall
Oak Bullet Gall