A rudimentary key to Gall Midges on Trembling Aspen (Populus tremuloides)
1. Galls on petioles and (or) leaves ..................................................................................... 2
Gall on apex of fresh twig, elongate or tapered: Cecidomyiidae sp. 6
2. Galls on petiole, purse- or flask-like ................................................................................ 3
Galls on leaves .................................................................................................................... 4
3. Galls roundish or pyramid-shaped, with relatively small, regular orifice: “Cecidomyiidae
petiole”.
Galls keel-like, longer than wide, opening along full length of gall (the gall itself formed by two
long “lips” extending from petiole, with single larva in between): Cecidomyiidae sp. 2.
4. Gall hard blister at leaf margin or formed by turned-in edge of leaf ................................ 5
Galls on leaf blade, closed .................................................................................................. 6
5. Gall swollen, hard and green; spindle-like blister at leaf margin; single larva inside:
Cecidomyiidae sp. 7.
Folded part of leaf not significantly thickened, often red, with larva inside separate ”envelope”:
Cecidomyiidae sp. 4.
6. Gall more or less globular in shape, sometimes with spine-like protrusion ................... 7
Gall spindle- or ridge-like hardening along underside of leaf; crosses leaf veins, wall swollen,
hard, green. Cecidomyiidae sp. 13.
7. Gall more or less spherical, no sharp protrusions ........................................................... 8
Gall with sharp, spine-like protrusion on lower part of the leaf, ca 1/3 of gall protruding above
upper leaf surface: “Cecidomyiidae pointed gall”.
8. Galls clearly visible / protruding on both sides of the leaf ............................................... 9
Galls clearly visible / protruding on only one side of the leaf (usually the upper surface) . 11
9. Galls always by junction between leaf blade and petiole; large as full-grown. Single bulb or
paired structure, protruding on both sides of leaf. Walls fleshy and juicy, full-grown gall woody;
often dark red or reddish-green in color. Gall opening downwards, orifice roundish but
irregular in shape, on full-grown gall surrounded by wart-like protrusions. Gall with no
pronounced central cavity; several larvae per gall: Cecidomyiidae sp. 3.
Gall not by junction with petiole; inside leaf blade proper. Pronounced central cavity; single
larva inside......................................................................................................................... 10
10. Gall thick-walled, rounded, green to reddish in color. Under-side half-spherical, surface
with distinctly marbled pattern. Opening with thick “lips”; these lips clearly protruding above
upper leaf surface even on young galls. Wall two-layered, with juicy outer layer and hard inner
layer. Cecidomyiidae sp. 10.
Gall roundish to oval, about equally protruding both above and below leaf blade. Located on
leaf vein, usually several galls on the same leaf. Orifice round, highly regular in shape, visible
even on young galls. Wall relatively thick, fleshy: Cecidomyiidae sp. 12.
11. Gall small (ca 2.5 mm), rounded, broadly attached to upper side of leaf. Wall thin but hard
and brittle, green to dark red in color. Opens with slit-like orifice on lower surface of leaf;
orifice not visible on young galls. Often many galls on one and the same leaf. Harmandia
globuli.
Gall larger (3-6 mm), rounded, very thinly attached to upper leaf surface. Wall thick, outer
layer juicy, inner surface of gall fibrous, hard. Color green to (usually) dark red. Often many
galls on one and the same leaf. Harmandia tremulae.