GALL CAUSING INSECTS
By
Louise Weyer
Program Assistant
Insects cause 95% of all galls with fungi, bacteria and viruses causing the remaining 5%. There are more than 2000 gall-producing insects in the United States. Specific aphids, wasps and midges (a minute fly) are the major vectors. All of these insects are very small, less than ¼ inch in length. Emergence is synchronized with the budding and blossoming development of the host plant, and peak emergence occurs as vegetative buds being to swell or during full flower bloom.
Aphids lay eggs in protected sites (bud scales, cracks and crevices in bark) on a plant toward the end of the season where they over winter. Aphids emerging in the Spring are known as “stem mothers”, and they give birth to daughters that give birth to more live aphids, and this cycle continues throughout the season. At the end of the growing season, males and egg laying females are produced. Mating occurs, the eggs are fertilized and laid in protected sites.
Female midges lay eggs on new plant tissue. The hatching larvae feed on the tissue causing the leaf to fold over and form a gall pod. Mature midges chew exit holes, emerge from the gall, and drop to the ground and over winter.
gall midge gall wasps
Wasps over winter in a gall that has fallen to a crevice on the ground. The female lays eggs on newly opening buds, and the larvae cause galls to develop. The insects mature and emerge as male and female. They mate, eggs are laid and the life cycle repeats.
A gall is an abnormal growth on woody ornamental shrubs and trees caused by parasitic insects. These growths are a plants response to insect egg laying and larvae feeding. Horticulturalists think that the organism introduces a growth-regulating chemical that causes enlargement and proliferation of cells. Since these cells are characteristic of a particular insect, a gall can be specifically identified by its structure and host plant.
Usually found on stems and leaves, galls also occur on trunks, flowers, leaf shoot terminals and petioles. These growths can vary in size from so tiny they are not noticeable to two inches in diameter. The shape can range from a sphere to a tube. They can be any color and the surface can be smooth, hairy or spiny. Galls caused by insects are commonly found on oaks, roses, willows and rhododendrons.
gall wasp gall midges witches broom
Insects’ eggs laid on new buds and leaves cause leaf curling and distortion. The terminal leaves become small and malformed. A growth known as witches broom (many twigs clustered together) may form at the site.
Generally, insects causing these deformities do no serious harm to healthy plants. A healthy plant is the best control. Treatment applied after galls are present is useless; the galls will not disappear. To control these abnormal growths with insecticides is impractical – timing of the application must be before the gall is formed and it is expensive. Galls and witches broom can be pruned out and destroyed. Galls on leaves will drop with the leaves and should be raked up.
These insects are not harmful to humans. Although midges look like mosquitoes, they do not bite people.
References: Insects That Feed on Trees & Shrubs, Warren T. Johnson & Howard H. Lyon, Cornell University Press
“Insect Galls”, Inst. Of Food and Agricultural Science, University of Florida
“Galls”, Virginia Cooperative Extension, Virginia Polytechnic Institute and State Univ.
Common Sense Pest Control, William Olkowski, Sheila Daar, Helga Olkowski, The Taunton Press