Ambler Arboretum Spotlight: A Lot of Galls

Cedar Apple Ruse, a type of gall

By Kathy Salisbury
Director, Ambler Arboretum of Temple University

What are those things on the leaves? This time of year as you continue your nature walks around the neighborhood or start venturing into local parks, you may notice weird growths on the leaves of plants. They may look kind of alien or gross to you but they are really fascinating. These growths are called galls. 

Galls are abnormal cell growth. While we mostly see them on leaves or stems when we are exploring, they can also form on roots and even flowers. These lumps and bumps are caused by irritation to the plant's surfaces. While they can be caused by an infection, usually it is an insect causing the problem. 

Feeding insects can cause galls and some insects even lay their eggs in between the surfaces of the leaf causing the reaction. The insects that create the galls tend to be specific to a particular type of plant. Many of these gall forming insects have two host plants they use during various portions of their life cycle. Kathy Salisbury has noticed a few in her recent travels around her garden and neighborhood and presents them as examples:

  • Cockscomb Gall is named for its appearance, which is like a rooster's comb. Usually found on elm trees — those in the photo are on a slippery elm tree (Ulmus rubra) — these galls are created by the cockscomb aphid (Colopha ulmicola). In early summer, these aphids leave the gall for their summer home in the grasses and when it is time in fall, a winged form of the aphid returns to start the cycle again. 

  • Oak Apple Gall is created by a gall wasp. The leaf tissue grows to surround one larva located at the center of the gall. 

  • Cedar Apple Rust Gall are formed by a fungus, which also requires a co-host such as apples or crabapples. Throughout most of the year you will find these galls on Eastern red Cedar (Juniperus virginana). When the conditions are right — usually the warm, rainy weather of spring — slick orange telial horns emerge from the galls, releasing spores.  

  • Hickory Aphid Galls are formed by small aphid-like insects called Phylloxera. 

  • Maple Bladder Galls are created by mites on the upper surface of red and silver maples. 

  • Witch-Hazel Leaf Galls are conical galls on the upper leaf surface. 

Nature never ceases to amaze! How many of these can you find on your explorations?