Common Galls of Southern Ontario
I wanted to compile a list of some of the common galls one might encounter here in Guelph, Ontario. I have been spotting a few lately and wanted to build a little database for myself and for others who may encounter them and want to know a little more. The galls are named by the inducer, and what I mean by that is the insect as all galls below induced by insects. I hope to make a series of posts over time.
Gall Inducer : Goldenrod Gall Ball Fly (Eurosta solidaginis)
Family: Tephritidae
Plant host : Tall Goldenrod complex (Which consists of three Solidago species, S. altissima, S. canadensis, S. gigantea).
Size of gall : Roughly 2.5 cm (1 in) diameter though can be found larger or smaller.
General description of gall : Spherical gall at middle of stem or higher. Sometimes two or three on single stalk.
Notes : Likely the most conspicuous and well known gall in Southern Ontario. Larvae commonly consumed as naturalist “party trick”. Tastes like butter.
Gall Inducer : Smooth Capitulum Gall Midge (Schizomyia racemicola)
Family: Cecidomyiidae
Plant host : Tall Goldenrod complex (Which consists of three Solidago species, S. altissima, S. canadensis, S. gigantea).
Size of gall : 3 mm (⅛ in) long x 2.5 mm in diameter.
General description of gall : Onion shaped striped tiny gall developing within the capitulum (the complete flower head made up of a bunch of flowers, as all in the floral family Asteraceae). Inside the gall is hollow and contains a single larva.
Notes : Larvae are bright, deep red-orange. Emerge as larvae (exit holes small and circular) and pupate on the ground.
Gall Inducer : Canada Thistle Gall Fly (Urophora cardui)
Family: Tephritidae
Plant host : Canada Thistle (Cirsium arvense)
Size of gall : Roughly 2 cm (¾ in) wide x 2.5 - 5.6 cm (1 - 2¼ in) long
General description of gall : Hard woody structure. Many possible shapes but seem to be consistently tapered, narrower towards the terminal growth. Inside of gall consists of several chambers, some hosting more than one larvae.
Notes : Non-native insect introduced as biocontrol to manage non-native host plant. Fly lays eggs on the stalk of Canada Thistle. Hatched larvae chew their way into stalk, gall forms around multiple larvae who overwinter. Adults chew a tunnel and emerge in Spring.
Gall Inducer : Mossy Rose Gall Wasp, or Pincushion-Gall Wasp (Diplolepis rosae)
Family: Cynipidae
Plant host : Various Rosa spp. including native R. blanda and non-native R. rugosa.
Size of gall : I did not get a measurement for this one, but I am assuming 6 or 7 cm in diameter.
General description of gall : Multi-chambered hard gall with branched filamentous mossy or hairlike projections . Starts out green, and then a pink-red, and then on to a rusty brown. Note, similar D. bassetti are single-chambered. Next time I’ll have to check.
Notes : Another parthenogenic species, of 60 or so eggs laid and hatched, less than 1% are male. D. rosae is the emblem of the British Plant Gall Society. Another common name, Robin’s Pincushion may refer to Robin Goodfellow, a common elfish sprite in British folklore.
Gall Inducer : Red Raspberry Knot Gall Wasp (Diastrophus turgidus)
Family: Cynipidae
Plant host : Red Raspberry (Rubus strigosus).
Size of gall : Roughly 6 cm (2⅜ in) long x ~ 3 cm (1⅛ in) wide.
General description of gall : Long abrupt swollen section of stem of the Red Raspberry, covered with spines or can also be found smooth. Multi-chambered with many larvae on the inside. Red-green and fleshy when young, ages to brown and dry.
Notes : Not much known about this one, though has a supposedly widespread distribution across Turtle Island/North America between 39th - 53rd parallel according to observations on Inaturalist. The book “Cynipid galls of the eastern United States” (Weld, 1959) says that a entymologist named “Brodie” reared the wasp who made this gall on or around May 21-27 in Toronto, so this could be a good time to observe these galls.. if you find one.
To learn more :
Emily Damstra’s research on Goldenrod galls shared through workshops
Cynipid galls of the Eastern United States by Lewis Hart Weld. Ann Arbor, Mich, 1959.
Inaturalist guide to Goldenrod Galls
Gallformers.org