Insect and mite galls

Quick facts

  • Galls are abnormal plant growths caused insects, mites, nematodes, fungi, bacteria and viruses.
  • Galls can be caused by feeding or egg-laying of insects and mites.
  • Galls rarely affect plant health and their numbers vary from season to season.
  • Control is generally not suggested.

How are galls formed

Insects or mites cause feeding damage by chewing and their salivary secretions 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.

Gall formation usually happens during the accelerated growth period of new leaves, shoots and flowers in late spring.

  • 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 gall formation has started, it continues even after the insect dies.
  • Most galls remain on plants for more than one season since they become noticeable only after they are fully formed.

Types of galls

Leaf 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

Deform the 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/trees from galls

Control options are not required to protect plant health, as most galls do not cause any severe damage.

  • Chemical applications are often ineffective since the precise timing of sprays is critical.
  • Spray the product 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 insect or mite.
  • For insects or mites that spend the winter on the host plant, horticultural oil applications can be made before 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.

Browning leaves that look burnt
Erineum gall on viburnum caused by an eriophyid mite
Brown fuzzy balls on a branch
Bud gall on birch caused by an eriophyid mite
A cluster of dried up twigs that make the branch look like a witch's broom
Hackberry witches-broom gall caused by an eriophyid mite
Yellowish bumpy spots on green leaves
Jumping oak gall caused by a gall wasp
Brownish fruit like structures on branches
Oak bullet gall caused by a gall wasp
Round, green, fruit-like structures on leaves
Oak apple gall

Insects and mites that form galls

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Jeffrey Hahn, Extension entomologist and Mark Ascerno, former Extension entomologist

Reviewed in 2018

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