While doing some yard work last weekend, I saw the first leaf that had dropped due to an excess of Maple Bladder Galls growing on it. The leaf dropped not because of any direct injury to the leaf by these wart-like structures, but because there were so many of them that formed on the leaf that it could no longer stay attached to the branch.
Every year I receive numerous pictures from our Field Service Professionals asking about these weirdly shaped growths or galls form on the leaves of Maples, Ash, Oaks and Cottonwoods. The growths take on many shapes such as: the wart-like growths on maples, nipple galls on the underside of hackberry leaves, spindle galls on lindens or maples, velvet galls on maples and birches and the list of shapes and trees attacked goes on and on.
Source of the Gall Formations
There are several insects and insect relatives that are the source of the gall formations. These insects often secrete a substance on the leaf, which reacts by increasing its normal plant growth hormones. This results in an increase in the size or number of cells, which is what causes the gall.
Many of the galls are the result of the feeding by a tiny eriophyid mite that has overwintered on the plant and begins feeding in the spring as the leaves begin to develop. These mites are the source of most of the bladder, spindle and velvet galls. Psyllids, or jumping plant lice, are the source of nipple galls that form on the underside of leaves. Adelgids are an aphid-like insect that feeds on many types of conifers which results in the development of Cooley spruce galls.
Sometimes the insects will live inside the gall as it develops around them. One of the more interesting is the Jumping Oak Gall, which forms on White Oaks. A stingless female wasp will lay a single egg on a developing leaf bud. After hatching, the larva feeds on the gall tissue that forms around it.
In the early summer, the galls fall to the ground and the larva will jump in an effort to escape the gall, similar to the jumping of a Mexican Jumping Bean. It is an interesting site to see dozens of tiny little balls jumping on the ground underneath an oak tree.
Preventative Treatment for Leaf Bladder Galls
Trying to treat for these gall-producing insects or insect relatives is very difficult since most of the activity begins very early in the spring as buds are opening and before most people even think about treating for insect problems. The damage that is caused is not detrimental to the tree and is purely cosmetic. It definitely can be disconcerting to see leaves fall in the summertime, covered with bumps or spindles or other oddly shaded growths. A mature, healthy tree can have more than 200,000 leaves, so losing a few is not critical to the tree’s survival.
The exception to the cosmetic damage is the Cooley Spruce Gall Adelgid. Small infestations are not a major concern, but large outbreaks can disfigure the tree and cause entire branches to die back. There are control options to treat for the adelgids. If you think you may have a problem with this insect pest or have any other concerns on your home landscape, contact your neighborhood lawn and landscape professional at Spring-Green.