Swamp White Oak disease?

Asked June 13, 2018, 6:07 PM EDT

Howdy, we planted a swamp white oak in June 2017. A few weeks ago I noticed several of the green nodules, about 20, shown in the photo. Leaves are curled up too. What might this be? Wasps? Thanks so much!

Hennepin County Minnesota horticulture swamp white oak

1 Response

There are many gall wasps associated with oak (and will not affect the fruit trees in surrounding areas). Thanks for the picture of your tree; these appear to be a bulletgall wasp galls, found on bur and swamp white oak. Very heavy infestations can occur that largely cover twigs, reducing growth rate of the tree. They are very visible after leaf drop. The galls also exude a honeydew-like sweet material that is attractive to bees and wasps and fosters growth of sooty molds (this causes the blackened bark commonly seen on older oak trees).

Intensity of attacks by gall wasps is highly variable. Some individual trees appear to be resistant to attack, particularly those that have less fluted bark.

Female gall wasps emerge in late October and early November, after a hard frost. (A small, circular hole in the gall indicates emergence.) Eggs apparently are laid in the terminal growth (newest end of twig) during the fall. In late spring, the developing insect stimulates a pocket of stem tissue to produce a large rounded gall, in which the young wasp develops. Only a single wasp develops in each gall. There is one generation per season.

Effective controls have not been developed for gall wasps. Insecticide applications, if attempted, should be synchronized with periods when adult wasps emerge from galls and are laying eggs (regular monitoring – which you are doing – would be important). This may occur in late fall or winter (late October or early November), several months before symptoms of gall formation are first observed. Soil injections of Merit or Orthene and spring foliar (leaf) applications have not been effective in CSU trials

Timing gall removal: Removing galls on twigs before the wasps emerge (late Oct, early Nov) may be useful controls on smaller trees to prevent subsequent egg laying, however, take care not to injure the branches making them susceptible to other insects and/or disease. Gall wasps are heavily parasitized by other wasp species, and can be important in control. These wasps emerge in the spring, so removing galls during winter and spring (after the gall wasps have emerged) will have the adverse effect of destroying natural enemies, while not affecting the gall wasp.