Issue 2, April 30, 2010
Galls, Galls, Galls
Spring commonly brings large numbers of leaf galls, with oaks having the greatest share of leaf and other galls. Galls are plant growths formed due to stimuli provided by insects, mites, disease organisms, or mechanical injury. They can be thought of as benign plant tumors. Frequently, insects and mites feed on the plant tissue, releasing chemicals that cause the plant to form the gall. Sometimes, even the physical injury caused by feeding or egg-laying can apparently cause the gall to be formed.
Gall insects and mites typically attack areas with large amounts of meritstematic tissue such as leaf, flower, and stem buds as well as expanding leaves and elongating stems. Meristematic plant tissue contains embryonic or undifferentiated cells that will form various organs of the plant based on hormonal (chemical) stimulation by the plant. Gall insects and mites use their own chemical messengers to stimulate gall formation. The developing insects or mites then feed on the gall tissue and are protected from natural enemies by it. The galls that are formed by the plant are so specifically located and shaped as to allow identification of the gall maker to species.
Most galls produced by insects and mites cause little or no harm to the health of the plant. Of the common galls, only gouty and horned oak galls and Cooley and Eastern spruce galls require control if numerous to prevent damage to plant health. Plants having other types of galls are apparently as healthy as those that do not have them. They grow as fast, grow as tall, and live as long as nearby plants of the same species. It is common for individuals of the same species to host vastly different gall numbers, making it likely that host plant resistance is common with galls.
Over the next few weeks, you are likely to find many leaf galls on woody plants, many of bizarre appearance. These are very unlikely to cause any harm to the plant, which is convenient because there is no control for formed galls except physical removal. The biology and life cycle of most galls is unknown, making the timing of insecticides or miticides to prevent gall formation an educated guess at best. Because gall tissues are not normal in terms of cell makeup, including xylem and phloem conductive tissue, systemic insecticides and miticides have limited value.
In the last issue, we discussed galls associated with pin oak and silk tree or mimosa. This week, we received photos on our Distance Diagnosis System of Neuroterus minutus on white oak in southern Illinois. This is a leaf petiole gall wasp that forms large masses at leaf bases. We also saw specimens earlier in the month of apparently Dasineura porrecta, a gall midge that attacks developing seeds on American elm. The homeowner was seeing tens of thousands of tiny larvae about one-fiftieth of an inch long on the ground under the tree. In these cases, the outcome of gall attack are reduced leaf canopy or seed production, neither of which are likely to cause serious health effects to the tree. They are nuisances in that the client has less shade or has to sweep the driveway a couple extra times to remove dropping larvae.
All of the above galls are ones that we had not noticed before. There are enough gall makers that there is always something new. Thankfully, they are rarely damaging to their hosts.--Phil Nixon, Brenda Roedl