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.

Willow Leaf Gall Sawfly

The Willow Leaf Gall Sawfly, is as the name suggests, a sawfly (from the Euura spp.) that cause galls to form on the leaves of willow trees (Salix spp.). There are 867 described species of Euura worlwide (Taeger et al. 2018). With about 1/3 of those appearing in North America. Most species feed on willow trees, but not all species are gall makers. It is a difficult species to research as many changes have recently been made to the taxon, which now includes the genera Amauronematus, Pontania, Pachynematus, Pikonema, Phyllocolpa, and many species of Nematus (Baine Q., et al.,(updated Mar. 2022)).

Identification

Of the species of Euura that produce leaf galls on willows there is some variation in insect and gall appearance. Following is general description of them.

The galls, which are pea or bean shaped typically start off pale in colour becoming green and later red (if occupied). Their size can vary according to species but are about 10- 12 mm in length with some species appearing on the upper leaf surfaces and others on the underside or petiole. Some Euura species are host specific, only feeding on specific willow (Salix) species. The adults are rarely seen and it is usually the appearance of the galls that alerts you to their presence. The appearance of the galls are specific to the insect species and the host and can be used to help identify the specific species.

The adults are inconspicuous are rarely seen. They are stout, broad-waisted wasps that are quite small. Their colour and appearance can vary according to species. e.g.: Euura pacifica males are shiny black, and females are dull reddish, measuring about 10mm (2/5″) in length (U.C. IPM, (n.d.)); Euura proxima species are smaller with the females measuring about 2.9 – 4.6mm and the males 3.2mm (Sawflies.org.uk, (2021)).

The larvae can be light green, yellow, or whitish in colour with black eyes and a brownish head. At maturity they are about 10mm (2/5″) long.

The Pupae form a silken cocoon around themselves in the soil, near the base of the host tree. The cocoon is oblong in shape, measures about 6mm (1/4″) long, and reddish brown to creamy white in colour.

Life Cycle

The life cycle varies some what according to the specific species, but generally the insect over winters in the soil in the pupae stage. In the spring the adults, which are stout, broad-waisted wasps, begin emerging over a period of about two to three weeks. The males and females mate (although some species reproduce parthenogenetically (no fertilization of eggs)). The females oviposit their eggs, usually singly, into the young foliage of willows; as they do, they also inject a stimulant that causes a gall to begin forming around the egg. At this point both the male and female adults die. The larvae, which appear like tiny caterpillars, hatch within the gall and remain there through their 5-6 instar stages feeding (5 for males, 6 for females (U.C. IPM, (n.d.)). At this point the larva emerge from the gall and drop to the ground to pupate. Depending on the species and location there can be 1 to several generations per year.

Control and Management of Willow Gall Sawflies

Willow gall sawflies do not harm the over all health of willow trees, the damage is more cosmetic in nature. Thus no control is necessary. If you really feel you must control the insect, the least harmful way is to simply remove the galled leaves and destroy them. This will help to reduce the next generation. Spraying the adults with insecticidal soap is not very affective, as the adults only live for a couple of days but are emerging over a period of about two to three weeks. You would have to spray every day for 2-3 weeks to be affective, which would cause far more harm to your willow than the Sawflies ever would. Chemical control is not warranted. Willow Gall Sawflies have some natural predators and parasitoids that help to control their populations such as several wasp species, that feed on the larvae and galls and at least one weevil and moth. For a detailed account of the natural enemies of gall-forming sawflies on willows visit Research Gate.

Canadian Species of Willow Gall Sawflies

•  Euura proxima) (formerly Pontania proxima) feed on a range of narrow leafed willows such as: white willow (S. alba), weeping willow (S. babylonica), S. x blanda, S. excelsa, crack willow (S. fragilis) and bay willow (S. pentandra) (Wikipedia (updated 9 April 2022)). The coffee bean shaped galls are most intense on the upper leaf surface and measure about 12 mm x 6 mm (1/4″ – 1/2″). They are green when unoccupied or red if occupied. This species tends to reproduce parthenogenetically (no fertilization of eggs).
• Willow Apple Gall Sawfly (Euura californica) galls protrude on both sides of the leaf. They feed on Salix lasiolepis and Salix lemmonii.
• Euura salicisdesmodioides feed on white willow and crack willow.
• Euura salicispisum feed on pussy willow (Salix discolor). The pea-like galls are hollow, pale yellowish-green in colour, and appear on the underside of leaves.
• Euura salicispomum
• Euura petiolaridis
• Euura obducta
• Euura pacifica (formerly Pontania pacifica) are host specific and feed on Arroyo Willow (Salix Iasiolepis) causing green or reddish galls to form on the leaves. The galls are about 8mm (1/3″) and globular to oval in shape.
• Euura pedunculi (formerly Pontania pedunculi) feed on Goat willow (Salix capraea), Grey willow, (Salix cinerea), and Eared willow (Salix aurita) (Wikpedia, ( 28 February 2021)). The pea-shaped galls appear on the underside of leaves, at first appearing green but turn reddish and hairy as the gall matures. The galls measure about 5mm (1/5″).

Photo Credits: all photos by the author.

References:

Baine Q., Looney C., Smith D.R., Schiff N.M., Goulet H., Redford A.J. (updated March 2022). Euura. idtools.org. https://idtools.org/id/sawfly/factsheet.php?name=17505&msclkid=a9cd264db02711ecb02ebfc0b7e5afb4

Gov. of Canada, (updated 2020-01-24). Willow Redgall Sawfly. https://agriculture.canada.ca/en/agriculture-and-environment/agricultural-pest-management/diseases-and-pests/willow-redgall-sawfly?msclkid=a2f91fe8aecf11ecaad766d4d5862047

Hadley D., (2019). Gall Wasps. The Habits and Traits of Family Cynipidae. https://www.thoughtco.com/gall-wasps-family-cynipidae-1968088

Kopelke, J.-P. (2002). Natural enemies of gall-forming sawflies on willows (Salix spp): (Hymenoptera: Tenthredinidae: Euura, Phyllocolpa, Pontania). Entomologia Generalis. 26. 277-312. https://www.semanticscholar.org/paper/Natural-Enemies-of-Gall-forming-Sawflies-on-Willows-Kopelke/69d4c0c1fed88a93f506de88d2a2fbdff17c88d8

Marlatt C. M. (1896) Revision of the Nematina of North America, a Sub-family of Leaf-feeding Hymenoptera of the Family Tenthredinidae.  https://www.biodiversitylibrary.org/page/52142981#page/114/mode/1up

Nature Spot. Willow Gall Sawfly – Pontania pedunculi. https://www.naturespot.org.uk/species/willow-gall-sawfly

Sawflies.org.uk, (2021). The Sawflies (Smphyta) of Britain and Ireland. Euura proxima (Serville, 1823). https://www.sawflies.org.uk/euura-proxima/

Taeger, A., Liston, A.D., Prous, M., Groll, E.K., Gehroldt, T. & Blank S.M., (2018): ECatSym – Electronic World Catalog of Symphyta (Insecta, Hymenoptera). Program version 5.0 (19 Dec 2018), data version 40 (23 Sep 2018). – Senckenberg Deutsches Entomologisches Institut (SDEI), Müncheberg. https://sdei.de/ecatsym/ Access: 30 Mar 2022

U.C. IPM, (n.d.). Willow gall sawflies—Euura and Pontania spp. Pests in Gardens and Landscapes. http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/willowgallsaw.html

Utah State University, (n.d.). Cynipid Gall Wasps. Integrated Pest Management. .https://extension.usu.edu/pests/ipm/ornamental-pest-guide/arthopods/gall-wasps/cynipid-gall-wasps

Whitney Cranshaw, Colorado State University, (rev. 16 February 2010).Willow Gall Sawflies. https://wiki.bugwood.org/HPIPM:Euura

Wikipedia (updated 9 April 2022). Euura proxima. https://en.wikipedia.org/wiki/Euura_proxima

Wikpedia, ( 28 February 2021). Euura pedunculi. https://en.wikipedia.org/wiki/Euura_pedunculi

Wikipedia, (Updated on Dec. 2021). Gall wasp. https://en.wikipedia.org/wiki/Gall_wasp?msclkid=1ac029efaec611ec834b2e60877383d6

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