County Ag News

Deciduous Tree Galls

Galls are abnormal growths on plants that can result from the feeding of living organisms such as bacteria, fungi, nematodes, insects and mites.  There are numerous galls that are caused by insects, the most common of which, in Wisconsin, are ash flower gall (University of Wisconsin Garden Facts XHT1048), hackberry leaf gall, hickory pouch gall, horned and gouty oak galls, and maple bladder gall.  

Gall formation often disfigures twigs and foliage leading to aesthetic damage, but rarely affects the health or vigor of host plants. However, some galls (e.g., horned and gouty oak galls) can cause serious injury to oak trees. For most of their lives, gall-making insects live inside gall tissue, sheltered from insecticide sprays. Consequently, timing of pesticide applications for their control is difficult. 

Furthermore, gall-making insects are difficult to treat with insecticides on taller trees. Yet, because galls are conspicuous, gall-making insects are easy targets of natural enemies such as predators and parasitoids.  Because most galls do not kill trees, the best management approach simply is to tolerate their presence.  

Hackberry Leaf Gall: Many of the galls on hackberry leaves are induced by psyllids or jumping plant lice.  Adult pysllids look like miniature cicadas.  In the fall, the adults leave the galls seeking places to hibernate, often invading homes. 

Control: Remove and destroy old galls before eggs hatch in the spring.  Dormant oil sprays may help reduce a hackberry gall problem.  However no insecticide treatment is necessary because the galls will not harm the tree.