An illustrated key to the galls of the trembling aspen (Populus
tremuloides) in central Alberta.
Tomas Roslin (tomas.roslin@helsinki.fi)
Key to gall-makers on Populus tremuloides. This key is only intended as a rough
guide-line to help you assign species to sensible morphospecies categories. Not all
taxa included have yet been discovered in North America. Species followed by a star
have been observed in 2002 around Drayton Valley / Ministik. Note that many entries
come with 3 alternatives.
1. Leaf mine: tunnel or pocket excavated by larva; fully inside leaf, between upper and lower
epidermis ............................................................................................................................. 2
Gall: insect affecting leaf morphology: leaf or petiole thickened, leaf-edge folded or generally
distorted; blob, knob or bump on leaf .................................................................................. 4
2. Long, silvery tunnel right under epidermis with black frass-string in the middle: moth
Phyllocnistis populiella (Gracillaridae), Fig. A
Blotch-like “pocket” inside leaf, equally conspicuous on both upper and lower leaf surface;
frass spread-out as small corns: Eriocraniidae sp. 1 (moth) , Fig. B
Pale blotch mine on lower surface of leaf; leaf usually bent into small tent along longitudinal
axis of mine, Fig.C ............................................................................................................... 3
3. Larva with front-half of body (including head) creamy-white, back-half orange-yellow, Fig.
D. Pupa orange-yellow with blackish head, Fig. E. Moth Phyllonorycter species group 1
(Gracillaridae)
Larva with black dorsal spots on abdominal segments, head brown, Fig. F. Pupa totally black:
moth Phyllonorycter species group 2 (Gracillaridae)
Any other type of leaf-mine: please describe
4. Galls attached to stem or twig ......................................................................................... 5
Galls on twigs, petioles or leaves ........................................................................................ 7
5. Galls with extensive convolutions and ridges at apices of stem, young galls light green, turn
reddish brown; poplar vagabond aphid Mordowilkoja vagabunda, Figs. G-J.
Galls without extensive convolutions and ridges ................................................................. 6
6. Galls woody in appearance along stem, poplar bud gall mite Aceria parapopuli*, Figs. K-N.
Galls of deformed leaves with rolled edges forming ball-like clusters of apices of stem or
twigs, mite Vasates dispar*, Figs. O-R.
7. Galls on apex of twig ....................................................................................................... 8
Galls on petioles and (or) leaves ....................................................................................... 10
8. Galls round or globular, poplar twig aphid Pemphigus populiramulorum, Fig. S.
Galls elongate or appear tapered, at base of fresh twig , midge Cecidomyiidae sp. 6*, Fig. T,
U.
Gall formed by enlarged winter bud, red larvae visible among scales: midge Dasineura sp.,
Fig. V.
10. Galls formed at junction of leaf blade and petiole, aphid ............................................ 11
Whart-like, paired gall at junction of leaf blade and petiole, 1-4 mm, irregular in size, redbrown mite Eriophyes diversipunctatus, Fig. X.
Galls on leaf or petiole ....................................................................................................... 14
11. Galls formed by swelling and twisting of petiole and leaf blade ................................. 12
Galls formed by swelling of base of leaf blade or apex of petiole ..................................... 13
12. Galls with a slit for exit hole, poplar bullet gall aphid Pemhigus populiglobuli, Fig. Y.
Galls with small round exit hole, poplar leaf-petiole gall aphid Pemphigus populicaulis, Figs. Z,
AA.
13. Galls formed mostly by swelling on upper side of leaf base with petiole, exit hole a slit,
aphid Pemphigus monophagus, Fig. AB.
Galls formed mostly of swollen petiole twisted on broadened leaf base, exit hole a long
crevice, poplar petiole-leaf gall aphid Pemphigus junctisensorius, Pemphigus nortonii, Fig.
AC.
14. Galls on petiole ............................................................................................................ 15
Galls on leaves .................................................................................................................. 18
15. Galls spherical ............................................................................................................. 16
Galls purse- or flasklike ..................................................................................................... 17
16. Galls ribbed, ribbed petiole gall caterpillar Ectoedemia populella, Figs. AE, AF.
Galls not ribbed with transverse slit, poplar leaf-stem gall aphid Pemphigus populitransversus,
Figs. AG-AI.
17. Galls large and tapered, wider than long (cone- to sickle-shaped), aphid Pemphigus
bursarius, Figs. AJ-AN.
Galls keel-like: small and not tapered, longer than wide, midge Cecidomyiidae sp. 2, Fig. AD.
18. Felt-like growth on leaf surface, more or less pronounced ”bump” on leaf (open, NOT
closed gall), mite ................................................................................................................ 19
Felt-like growths not present on leaf surface ..................................................................... 20
19. Felt first greenish-yellow, later blackish-brown, mite Phyllocopotes populi*, Fig. AO-AR.
Felt first reddish, later brown, bumps (supposedly) low, mite Aceria varia*, Fig. AR.
20. Galls near or at leaf margin or formed by turned-in edges of leaf ............................... 21
Galls not near or at leaf margin ......................................................................................... 25
Galls on leaf blade, by junction with petiole. Single bulb or paired structure, protruding on both
sides of leaf. Walls fleshy and juicy, often dark red or reddish green. Gall (often?) opening
downwards, roundish, slightly irregular orifice. Cecidomyiidae sp. 3*, Fig. AS.
21. Galls in a row, beadlike or pocketlike, beadlike cottonwood gall aphid Parathecabius
populimonilis, Fig. AT.
Gall hard, swollen and spindle-like blister at leaf margin; single larva inside: midge
Cecidomyiidae sp. 7*, Fig. AU.
Galls not in a row, leaf margin folded ................................................................................ 22
22. Leaf-fold simple ........................................................................................................... 23
Fold part with maggot inside separate ”envelope” within folded leaf: midge Cecidomyiidae sp.
4*, Figs. AV, AX.
23. Leaf may be completely folded along mid-vein; thickened, yellowish-reddish; containing
aphids: folded leaf poplar aphid Thecabius affinis or Pachypappa sacculi, Figs. BG-BI.
Folded edge narrow, 0.5-1 mm broad, tight, not hairy. No animal visible: mite Vasates dispar,
Fig. Q.
Folded edge broader (several mm), loose, with white maggot(s) ..................................... 24
24. Folded edge very hairy, containing several maggots: midges Dasineura populeti and
Prodilopsis morrisi, Fig. AY.
Folded edge not hairy, single jumping larva: midge Contarinia tremulae, Fig. AY.
25. Galls elongate, single gall beside midvein, aphid Pemphigus populivenae, Figs. BA-BC.
Galls globular, with closed spherical galls ......................................................................... 26
26. Galls clearly visible on both sides of the leaf ............................................................... 27
Galls clearly visible on only one side of the leaf (usually the upper surface) .................... 30
27. Gall opening with round orifice; with inner surface of gall much harder than outer .... 28
Gall opening with slit-like orifice; gall-wall homogeneous in structure ............................ 29
28. Thin-walled, spherical, low, blister-like, ca 3 mm broad, surrounded by thin, sharp edge.
Usually by somewhat thickened leaf-vein. With oval, hard inner part of gall; yellow-red larva.
Midge Harmandia pustulans
Thick-walled, round, 4-5 mm. Green-reddish. Under-side half-spherical, somewhat more
than ½ above upper leaf surface, low “spike-like” protrusion in the middle. With oval, hard
inner part of gall; orange larva. Midge Lasioptera populnea
29. Gall thick-walled, ca 1/3 above upper leaf surface, round, ca 5 mm, greenish-reddish.
Opening slit-like, parallel with leaf vein. Midge Harmandia cavernosa*, Fig. BD.
Thin-walled, mostly on under-side of leaf, round, 3-4 mm. Harmandia populi*
30. Small, ca 2.5 mm, round, thin-walled, green to dark red, broadly attached to upper side of
leaf. Slit on under-side of leaf. Often many galls on one and the same leaf. Midge Harmandia
globuli*, Fig. BE.
Larger, 3-6 mm, round, thick-walled, dark red, very thin attachment to upper leaf surface
Harmandia tremulae*, Fig. BF.
Rudimentary key to gall-making arthropods:
legless and “headless” maggot ...................................................gall midge, Cecidomyiidae
legless larva with distinct head, jaws ................................................ moth, Microlepidoptera
larva with distinct head capsule and row of legs evenly spaced along middle and rear-end
segments .......................................................................................... moth, Microlepidoptera
larva with distinct head capsule and distinct gap in row of legs between middle and rear-end
segments ............................................................................................................... saw wasp
soft-looking bug with six legs, greenish or blackish, long antennae; may or may not have
wings ............................................................................................................................. aphid
apparently nothing but hair- or felt-like stuff on the leaf ................................................. mite