There are a lot of galls on oak. One that can cause some problems at heavy densities is the oak bullet gall. These galls, sometimes called rough bullet galls, can quickly become unsightly. I usually see them on burr oak and occasionally on swamp white oak. They are caused by a gall wasp. The galls start out green-colored, eventually darkening to brown as the season progresses and the gall wasp larvae grow inside.After larvae complete development and adult wasps emerge, the dried galls can remain on the tree for more than a year, making the tree appear heavily galled although dieback is minimal. Over the past decade, I’ve been getting increasing numbers of reports of this pest which are usually associated with young plantations of severely galled trees. Additionally, in recent years I’ve seen increasing numbers of young trees showing decline and twig dieback due to this pest. For more information and a picture of an adult gall wasp, click on this link.
Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.