Gall Making Insects
If you have trees, there is a good chance at least one of them has a gall on it. Galls are a swelling in the plant tissue caused by an injury from an insect or a disease. There are over 2,000 gall causing insects in the United States and thousands more throughout the world. Certain insects usually only produce galls on a specific plant, which makes it easier to diagnose the cause of the problem and treat it.
Galls usually appear on twigs and leaves, but can appear on any part of the plant: flowers, roots, fruits, twigs, branches, leaves, and trunks. Galls develop in three phases: initiation, growth, and maturity. The gall starts, or initiates, because of something the insect does to the plant tissue. Sometimes the very presence of the insect is enough to start gall formation, but more often it is caused by the laying of eggs or hatching of larvae, feeding on the plant, or by salvia or other excretions by the insect.
Once the gall begins production, it surrounds the eggs or immature larvae of the insect or mite. Both the gall and the insect live off of the plant material. Eventually, the gall stops growing and the insect feeds off of the gall material until it is mature. The insect then chews through the gall and escapes to reproduce.
Gall making insects are not generally considered a threat to the plant on which they reside. Some galls are very beautiful and are used in flower and plant arrangements. However, if the plant is to be sold in the nursery or plant industry, galls can lower the value of the plant.
One of the most common species of gall making insects is the gall making wasp. There are over 1,000 species of this type of insect alone. The mealy oak gall wasp is a good example of this type of wasp. It produces one of the most common galls on oak trees. These insects produce both sexually and asexually and the galls each type of generation makes are different. Galls of the asexual generation appear on branches and twigs of live oaks in late summer and early fall. They are generally 1/8 to 1 inch in diameter and are spherical. New galls are pink to pinkish brown outside and a soft yellow green inside. As the gall matures, it becomes brown and dry inside. The wasps chew their way out of the galls in December and January. They are all female and lay their eggs in swollen leaf buds.
The eggs that were laid on the swollen leaf buds hatch in early spring as the leaf buds start to open. The larvae develop in the leaf tissue and cause small, beige leaf galls that resemble kernels of wheat. Adults of both sexes emerge from these galls after a few weeks and begin to mate. The females then lay eggs in twigs and branches. After a dormancy of three to five months, the larvae hatch and cause the spherical galls of the asexual generation.
Control of gall making insects is generally not necessary. They rarely hurt their hosts. When the adult insects vacate the gall, beneficial insects such as spiders and insect predators move in. Removing the galls or spraying them hurts these beneficial insects and spiders without effecting the gall making insect.