What Are Galls?

Galls are unusual growths or deformities of plant tissue caused by an organism; be it insect, nematodes, fungi, bacteria or viruses. The presence of these organisms initiates an increased production of normal plant growth hormones and these plant hormones cause localized plant growth. The gall-making organism feeds inside the gall. The appearance of the gall is unique to the gall maker and host thus there is a lot of variation in shape, form, size, colour and texture. Galls can form on leaves, buds, petioles, flowers, twigs or branches.

Types of Galls:

• Leaf Galls; appear on leaf blades or petioles. These are the most common galls and may appear as leaf curls, blisters, nipples, or erineums (velvet-like growths) on the upper or lower leaf surface.
• Stem and Twig Galls; appear as deformed growth on stems and twigs that range from slight swellings to large knot-like growths.
• Bud or Flower Galls; appear as deformed bud or flower structures.

Gall Makers:

• Insects
  • Cynipid gall wasps
  • Eriophyid mites
  • Plant lice or psylids
  • Adelgids
  • Aphids
  • Midges
• Nematodes
• Fungi
• Bacteria
• Viruses

Eriophyid Mites:

Eriophyid mites are from the family Eriophyidae. They are microscopic mites so small that they require a 40x magnification to see them. They are yellow to pinkish white to purplish in color, worm like and only have 2 pairs of legs that extend from the front of their body. There are hundreds of species each feeding on a specific host. Some feed on leaves, others stems and twigs and still others feed on buds and flowers. They cause a wide variety of damage such as spindle-like growths protruding from the upper leaf surface (spindle gall), small wort like growths on leaves (bladder gall), crinkly leaf edges (leaf roll galls), velvety-like growth on the upper and/or lower leaf surfaces (Erineum or Velvet Galls), blisters or dimpling. When petioles are infested compound leaves can become twisted and grow out at odd angles. Flowers can become distorted even witches broom can form. Some Eriophyid mites are even used as biological agents to control weeds and invasive plant species.

Life cycle: Eriophyid mites typically overwinter on their host plant. They lay their eggs in the spring which hatch and begin feeding by sucking the juices out of the plant. Both the egg laying activity and the plants reaction to the insects saliva initiates gall formation. They continue to feed within the galls for the remainder of the season then as the trees go into dormancy they hibernate in buds and tree bark. Their primary method of population spread is by wind.

Eriophyid tiliae

Eriophyid tiliae is a species of Eriophyid mite that feeds exclusively on linden (Tilia sp.) aka. basswood or lime tree. The feeding and egg laying efforts of these mites cause slender, upward, oblique or curved growths to form on the upper leaf surfaces; with puckered pits forming on the lower leaf surface. The galls measure about 5-13mm (1/5 to 1/2 inch) long and may be greenish-yellow, pinkish, red, or brown in color (Washington State University (rev. 6/11/2014)). These spindle galls (aka. nail galls) typically appear around June and mature by July-August. The damage caused by this mite is considered more cosmetic and usually does not threaten the tree.

Another eriophyid mite to affect lindens is Eriophyes leiosoma. They cause velvet-like masses of tiny hairs to form on the back of the leaves instead of the protruding nail galls. These masses are known as erineum galls (aka. velvet galls) and may be yellow to brown in color, and the damaged areas may appear on the upper leaf surface as discolored patches (Washington State University (rev. 6/11/2014)

Other Common Galls Caused By Eriophyid Mites Include:

• Ash Flower Gall: irregular gall clusters, of about 12 mm in diameter are formed on male flowers in early spring before the flowers open. The galls are initially green but turn brown late in the season. The gall is caused by the mite Eriophyes fraxinifolia.
• Bladder Galls:
  • maple bladder gall: small, reddish, round wart-like structures about 1/16 to 1/8 inch in diameter. They tend to appear by mid-May. The galls start off green and as they mature become a deep red. Caused by the mite Vasates quadripedes. They occur most commonly on silver maple (Acer saccharinum) and red maple (Acer rubrum).
  • black tupelo bladder galls: look like pimples or worts. caused by eriophyid E. nyssae.
  • willow bladder galls: tiny balloon-like structures produced by the eriophyid mite Aculops tetanothrix.
  • bladder galls on poison ivy: by the eriophyid mite Aculops rhois (= A. toxicophagus)
  • bladder galls on fragrant sumac (Rhus aromatic) by the eriophyid mite Aculops rhois (= A. toxicophagus)
  • boxelder bladder galls: small, green bladdergalls that mature to large wart-like protrusions with velvet-like patches on the corresponding lower leaf surface. Caused by the mite E. negundi.
  • walnut bladder galls/walnut pouch galls: Upward bulging galls on the leaf surface. The upper surface of the galled areas are yellowish-purple, at maturity the galls break open to reveal tufts of creamy-white hairs. Caused by the eriophyid mite E. brachytarsus.
• Spindle Galls: erect, oblique or curved growths rising up from the upper leaf surfaces. Generally about 1/5 to 1/2 inch long. They may be greenish-yellow, pinkish, red, or brown in color. Puckered pits appear on the lower leaf surface where the mite exits the gall.
  • Maple Spindle Galls: Caused by the mite Vasates aceriscrumena. Species affected: silver, sugar, and red maples as well as Norway.
  • Linden Spindle Galls: Caused by the mite Eriophyid tiliae
• Erineum or Velvet Galls:
  • Maple: neon pink patches appear on the upper leaf surface of sugar maples (Acer saccharum). Caused by the mite Eriophyes elongates. The more subtle silvery-green erineum patches produced by E. modestus may be found on the lower leaf surface of sugar maples.  The patches become more evident later in the season when they turn rusty-red. 
  • Viburnum: commonly affects cranberry viburnums. Cranberry Erineum Mite Galls: These mites can be very destructive as their feeding strongly distorts the leaf. The distorted leaves can turn shades of pink and red by the mites. Heavy infestations will kill the leaves and the twigs on which they occur. 
  • Birch erineum galls: affecting (Betula nigra): pink, fuzzy, felt-like growths on the upper or lower leaf surfaces. Caused by the mite Acalitus longisetosus
  • Linden (Tilia): White fuzzy growth on the underside of linden leaves caused by the mite Eriophyes leiosoma.
  • Beech (Fagus grandifolia)  light green to a yellow patches on upper leaf surfaces with dimpling on the lower leaf surface beneath the patch. They will eventually become golden then rusty red to reddish brown later in the season. Caused by the eriophyid mite Acalitus fagerinea.
• Birch Budgall Mite: rosette-like galls, which consist of clusters of aborted buds caused by the mite Acalitus rudis (Canestrini).
• Hackberry Witches-broom Gall:  a dense bunch of twigs growing on a branch. The numerous thin, short, stubby twigs arise close together, often at a conspicuous swelling or knot on a branch. Caused by the mite Eriophyes celtis, synonym Aceria snetsingeri) in combination with a powdery mildew fungus (Sphaerotheca phytoptophila).
• Oak Leaf Blister Mite: Aceria triplacis produces raise leaf-blisters on the upper leaf surface of oaks (Quercus spp.) and hair-filled pockets on the lower leaf surface.
• Walnut Petiole Galls: eriophyid Aceria caulis The galls are specific to black walnut (Juglans nigra) and may occur on the petiole, rachis, and petiolules of the compound leaf. the compound leaves to become twisted and malformed, they do not disrupt the vascular flow.  
• Buttonbush Eriophyid Galls: fuzzy, cauliflower-like galls caused by the eriophyid mite Aceria cephalanthi
• Black Tupelo Leaf Roll Galls:  crinkled leaf edges caused by the eriophyid mite Eriophyes dinus.
• Grape Phylloxera: is a small aphid like insect that causes wort like galls on the underside of leaves or knot-like swelling on the rootlets of wild or cultivated grapes.

Control:

Typically the damaged caused by Eriophyid mite galls is more cosmetic and does not seriously threaten the tree. They can however mar the appearance and shape of trees and may kill leaves and branches, but they rarely kill the tree. Applying dormant oil spray on a warm day during dormancy will suffocate the over wintering mites and help to reduce populations.

There is a naturally occurring predatory mite that feeds on Eriophyid tiliae. If your population of this Phytoseiid mite is low you may be able to attract them to the area by soaking a couple of cotton balls in wintergreen essential oil, placing them in a small container then hanging that container in the affected tree.

Photo credits: all photos taken by the author

References

Boggs J. 2020, An Abundance of Eriophyid Mite Galls, Retrieved on Feb. 26, 2021 from: An Abundance of Eriophyid Mite Galls | BYGL (osu.edu)

Hahn J. and Ascerno M. rev. 2019, Extension entomologist and former Extension entomologist, University of Minnesota Extension, Insect and mite galls, Retrieved on Feb. 26, 2021 from: Insect and mite galls | UMN Extension

OMAF 2014, Guide to Nursery and Landscape Plant Production and IPM pg. 35, Retrieved from: Publication 841, Guide to Nursery and Landscape Plant Production and IPM (gov.on.ca)

Patterson S., Master Gardener, rev. Feb. 2021, Eriophyid Mite Control – Spotting And Treating Eriophyid Mite Damage, Retrieved on Feb. 26, 2021 from: Eriophyid Mite Control – Spotting And Treating Eriophyid Mite Damage (gardeningknowhow.com)

Plants Galore n.d., Maple Bladder and Spindle Galls, Retrieved on Feb. 26, 2021 from: Maple Bladder and Spindle Galls – Plant Insect, Disease and Environmental Problem Information from PlantsGalore.Com

University of Manitoba 2020, Insect and Mite Galls, Retrieved on Feb. 26, 2021 from: University of Manitoba – Faculty of Agricultural & Food Sciences – Insect and Mite Galls (umanitoba.ca)

Wawrzynski R.P., Hahn J.D., and Ascerno M.E. 2005, Department of Entomology, University of Minnesota Extension Service Insect and Mite Galls, Retrieved on Feb. 26, 2021 from: http://cues.cfans.umn.edu/old/extpubs/1009galls/DG1009.pdf

Wikipedia rev. Dec. 2020, Eriophyidae, Retrieved on Feb.25, 2021 from: Eriophyidae – Wikipedia

Wikipedia rev. May 2020, Eriophyes tiliae, Retrieved on Feb. 26, 2021 from: Eriophyes tiliae – Wikipedia

Washington State University (rev. 6/11/2014)., Urban IPM and Pesticide Safety Education, Linden(Tilia): Leaf galls, Retrieved on Feb. 27, 2021 from: Linden(Tilia)_Leaf_galls_Factsheets_2-27-2021.pdf

Updated on August 26th 2022

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