- Galls are abnormal plant growths caused by insects, mites, nematodes, fungi, bacteria and viruses.
- Galls can be caused by feeding or egg-laying of insects and mites.
- Insect galls rarely affect plant health and their numbers vary from season to season.
- Control is generally not suggested.
How galls are formed
Galls usually form during the accelerated growth period of new leaves, shoots and flowers in late spring.
- Insects or mites damage plants by chewing on them and their salivary secretions (spit) cause plants to increase production of normal plant growth hormones.
- Higher hormone production results in increased cell size or cell numbers. These abnormal cell growths are called galls.
- Mature plant tissues are usually not affected by gall-inducing organisms.
- The gall keeps growing as the gall-making insect feeds and grows inside the gall.
- If galls start to form, they continue to form even after the insects die.
Most galls remain on plants for more than one season since they become noticeable only after they are fully formed.
Types of galls
- Formed on leaf blades or petioles.
- Most common galls.
- Appear as leaf curls, blisters, nipples or erineums (hairy, felt-like growths).
- On the upper or lower leaf surface.
Stem and twig galls
- Deformed growth on stems and twigs.
- Range from slight swelling to large knot-like growth.
Bud or flower galls
- Deformed size and shape of buds and flowers.
Damage caused by galls
- Galls are growing plant parts and require nutrients just like other plant parts.
- Galls can steal vital nutrients from the plant and affect plant growth.
- Can be a problem when galls are numerous on very young plants.
- Damage may occur if there are many galls on branches or present for several years in a row.
- In most cases galls are not numerous enough to harm the plant.
How to protect your plants from galls
Most galls do not cause any severe damage to plant health. Control options usually are not needed. Gall numbers vary from year to year.
- Chemical applications often are not effective because the precise timing of sprays is critical.
- To be effective, apply pesticide before gall formation begins, but when insects and mites are active.
- Once galls start to form, it is too late for treatment, as the galls protect the insects or mites.
- For insects or mites that spend the winter on the host plant, apply horticultural oil before insect activity begins in the spring.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Insects and mites that form galls
These tiny wasps are the most common gall-producing insects with various species affecting leaves, stems and twigs.
Usually occur on oak and rose.
Some examples are: jumping oak gall, oak apple gall, oak bullet gall and stem galls on rose.
These tiny mites live through the winter on their host plant. They begin feeding and form galls in spring as leaf or flower buds open.
Ash flower gall
- Mites begin feeding on male ash trees in the spring before the flower buds fully expand.
Galls are initially green but turn brown and more obvious in August to September.
Can remain on the tree for up to two years.
- The tiny mites enter the male flower structure in mid to late April before the flower seems open.
- Control is difficult since gall formation begins much before people start to think about pest management.
- Spraying is effective when done seven to ten days before the flower bud is expected to open.
Other common galls caused by eriophyid mites:
- Maple bladder gall
- Spindle galls on maple and linden
- Erineum or velvet galls on maple, viburnum, birch and linden
- Bud gall on birch
Eriophyid mites, in combination with a powdery mildew fungus, cause hackberry witches-broom gall.
Psyllids or jumping plant lice
Psyllids cause some of the most common types of galls in Minnesota:
Hackberry nipple gall
Hackberry blister gall
The small insects spend the winter on hackberry or in other sheltered locations such as inside homes.
In spring, when leaves appear, they begin feeding and produce galls.
Larger nipple galls appear on the lower leaf surface.
Smaller, more numerous blister galls appear on the upper leaf surface.
These insects are small enough to pass through most window screens and become a nuisance in fall, when they are looking for shelter.
These aphid-like insects are most common on conifers. The most common galls formed by adelgids in Minnesota are the eastern spruce gall and Cooley spruce gall. Although alike in many ways, there are important differences between the two galls and their causes.
Eastern spruce gall
Occurs mainly on Norway and white spruce.
This adelgid completes its entire life cycle on a single host.
1/2 - 1 inch long green, pineapple-shaped galls are formed at the base of the new shoots.
- Such galls can weaken stems, with higher chances of breakage under heavy snow loads or high winds.
- Tree health is affected only if galls are numerous each year.
Cooley spruce gall
Occurs primarily on Colorado blue spruce and white spruce in Minnesota.
This adelgid usually requires two hosts to complete its life cycle: spruce and Douglas fir.
Galls are 2 to 4 inches in length, cone-shaped and are initially green-purple changing to brown as they dry in mid-summer.
Cooley spruce galls occur at the tips of the new growth.
- Damage is very rare and only when young trees have numerous galls repeatedly.
After forming the gall on spruce:
Winged adelgids fly to Douglas fir in mid-late summer and lay eggs.
Resulting adelgids feed and can produce yellow, twisted needles on the Douglas fir.
A winged generation of adelgids is then produced on Douglas fir.
They fly back to spruce in the fall, lay eggs and produce adelgids that live through the winter.
The females from this population lay eggs in spring to give adelgids that produce gall.
No galls are produced on Douglas fir.
Many species of aphids occur on plants and some can produce galls. The most common galls are the stem and petiole galls on poplar (poplar petiole gall) or cottonwood.
These tiny mosquito-like insects are responsible for causing irregularly shaped structures on leaves and buds of a variety of plant species.
Some of the common midge-produced galls are willow pine cone gall, gouty vein gall on maple and grape filbert gall.
Reviewed in 2019