Identification

• Tree galls look like green or brown bumps, and may resemble a wart, blister or pouch.
  
  

What Causes Them

• Galls are created when insects, mites, nematodes or other organisms such as bacteria or fungi feed on a tree's leaves.
  
  

Effect

• Galls do not usually cause any serious damage to a healthy tree. Large numbers of galls, however, can affect the tree's appearance and cause premature leaf drop.
  
  

How to Treat

• According to R. Chris Williamson, turf and ornamental specialist with the University of Wisconsin, insects that lead to galls are easy targets of natural predators, and since the bugs do not kill trees it is best to simply let them exist.
  
  

How to Prevent

• The Iowa State University Extension does not recommend treating trees with miticides or insecticides to prevent future infestations as their application is not practical or warranted.
  
  

Types of Trees Affected

• Galls are quite common. Hickories, maples, oaks, ash, honey locusts, willows and hackberries are some trees that can be affected.