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Insect and mite galls

Quick facts

  • 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

Yellowish bumpy spots on green leaves
Jumping oak gall caused by cynipid gall wasps

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

  • 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

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

Reviewed in 2019

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