Dichotomous table for gallers on Corylus
by Hans Roskam
1a On above-ground parts => 2
1b Nodules on roots. Corylus spp.: Subanguina radicicola
2a On catkins or fruits => 13
2b On vegetative organs => 3
3a On leaves => 9
3b On buds, young shoots or older branches => 4
4a On buds and young shoots => 7
4b On woody parts => 5
5a Cancers or witches’ brooms => 6
5b Thicker branches or stems with ± distinct nodular to club-shaped swellings. Corylus cornuta: Viscum album
6a Bark of woody parts with cancer-like proliferations. C. avellana: Neonectria ditissima
6b Small witches’ broom-like malformations. C. avellana: Cause unknown – ? gall mite
7a On young shoots => 8
7b Buds swollen, rotund, up to 10 mm long. Scales enlarged, thickened like the outer leaf primordia and inside with tubercular proliferations. C. americana, avellana, colurna, maxima: Phytoptus avellanae
7c Enlarged globular bud. Contains caterpillar in a large central cavity. C. avellana: Unidentified lepidopteran
8a Young shoots stunted. Usually leaves of several buds per twig are disfigured; their leaf blades curled, folded, partially rolled and more densely pubescent. C. avellana: Cecidophyopsis vermiformis
8b Young shoot with local, pustule-shaped swellings; apical part of shoot ring-barked, withering and soon breaking off. C. avellana, colurna: Oberea linearis
9a Leaves folded or curled => 11
9b Leaf blades with flat, rotund archings => 10
10a Flat upward proliferations, surrounded by a 6 mm long, slightly discoloured and thickened area; the underside cavity contains a single vitreous to whitish larva. C. avellana, maxima: Mikomya coryli
10b Leaf blade with parenchyma galls about 3–5 mm wide, weakly protruding on both sides, initially hardly discoloured. Containing a single larva. C. avellana, colurna, maxima: ? Arnoldiola tympanifex
11a Leaf blade curled => 12
11b Leaf blade folded and crumpled; at base, with white, jumping larvae. C. avellana: Contarinia cybelae
12a Younger leaves, if heavily infected, folded and curled. With green, rarely reddish, ca. 2.5 mm long aphids on the underside. C. avellana, colurna, maxima: Corylobium avellanae
12b Younger leaves, if heavily infested, with discoloured bulges. With pale yellow to yellowish-white, ± 2 mm long aphids on underside. C. avellana, colurna, heterophylla, maxima: Myzocallis coryli
12c Leaf blades loosely contracted together, curled. C. avellana: Philaenus spumarius
13a Malformations of the catkins => 14
13b Gall-like malformations in the developing fruits, from the outside inconspicuously changed. C. avellana: Curculio nucum
14a Catkins completely or only partially swollen => 15
14b Catkins sprouting. C. avellana: Unidentified gall mite
15a Catkins locally or completely pear- or barrel-shaped, swollen. Scales enlarged, glabrous; stamens ± stunted, larvae white, glossy, jumping. C. avellana, maxima Contarinia coryli
15b In similar galls. Larvae white, dull, non-jumping. Inquilines of the previous species: Dasineura corylina
15c Catkins locally swollen, often at their base. Scales ± irregular, sometimes elongated, lacerate, ± pubescent, slightly thickened, spreading at top. Stamens enlarged. C. avellana: Phyllocoptruta coryli
15d Male catkins distorted, often twisted screw-like. Containing one to several caterpillars. C. avellana, ? colurna: Unidentified lepidopteran