Dichotomous table for gallers on Achillea

by Hans Roskam

1a On above-ground or underground shoots => 2

1b Roots with small, more or less spindle-shaped swellings, usually showing lateral roots. A. millefolium: Meloidogyne hapla

2a Malformations of inflorescences, flowers or fruits => 31

2b Malformations of vegetative parts => 3

3a Malformation by animal gall causers of whole plants or extensive parts, shoot tips or terminal buds => 25

3b Galls on stems, including root collar, or malformations of leaves or lateral buds => 4

4a Malformations with distinct sporulating areas on the surface => 22

4b Malformations without sporulating areas => 5

5a Galls on leaves => 12

5b Malformations on shoots, lateral buds or subterranean parts => 6

6a The gall causers are situated within the galls => 7

6b The gall causer is situated within a pit with ± thickened rim; galls usually clustered, causing thickening and distortion of green stems. A. millefolium, setacea: Planchonia arabidis

7a Galls on higher parts of the shoots => 10

7b Malformations on the root collar, subterranean parts of the stem, or on roots => 8

8a Galls oval or spindle-shaped => 9

8b Rotund, 5–8 mm long, thick succulent, one-chambered galls on the root collar or rhizomes; solitary or clustered. One larva per gall. A. millefolium, nobilis, crithmifolia: Oxyna flavipennis

= Larvae and pupae of Campiglossa argyrocephala have been recorded from A. ptarmica, inhabiting axillary buds, and dying stem tips below which rosettes of leaves are further developed, sometimes reminiscent of a gall-like malformation.

9a Closed spindle-shaped galls, with succulent wall; centrally with a chamber up to 12 mm long and 2 mm wide, later brownish-walled. Containing a single larva. A. millefolium, nobilis subsp. neilreichii: Dithryca guttularis

9b Midge gall, ovoid to jar-shaped, up to 6 mm long, initially depending on the location pale or green and soft, later brown, almost black, tough walled, unilocular. At the narrowed upper end with deflected haired lobes around opening when mature. One yellow larva: Rhopalomyia millefolii

10a Lateral swellings at different places on the stem => 11

10b Stem with spindle- or acuminate barrel-shaped swelling, up to 7 (9) mm long swelling, predominantly caused by a proliferation of the pith. Later on a larger area than the galled part is eaten by a caterpillar. Containing a single larva. A. millefolium: Unidentified lepidopteran

11a Stem with node-shaped compact closed proliferations, up to 6 mm long, or galls spindle-shaped, embracing the stem on all-sides over a length of 2–8 mm, lacking distinct chambers. A. millefolium, nobilis, etc.: Subanguina millefolii

11b Galls initially inconspicuous, egg-shaped, up to 2 mm high, unilocular, more noticeable after the emergence of the larvae by a straw-yellow coloration of the surroundings of the emergence hole. Containing a single larva per gall. A. millefolium: Rhopalomyia sp.

12a Leaves bent, curled, also rolled inwards or with little depressions => 18

12b Leaves with wart-, club-, spindle- or ± egg-shaped galls => 13

13a Galls larger than 1 mm => 15

13b Galls smaller than 1 mm, wart-shaped, compound; usually many on the lower leaves, also on stems; sometimes on the additionally deformed parts ± crust- or ridge-like converged => 14

14a Nutritive cells of the warts with golden yellowish content; hence galls yellow. A. ptarmica: Synchytrium aureum

14b Warts pearl-like, initially hyaline, later brownish; content of the nutritive cell colourless. A. millefolium: Synchytrium globosum

15a Galls usually several mm long => 16

15b Galls on both leaf surfaces; rotund oval, about 1–2 mm long, sometimes ± converging; initially whitish or yellowish green, later ± brown, sometimes clearly laterally curved over the margin. A. millefolium: Entyloma achilleae

16a Galls flat-spindle- or ± egg- to jar-shaped; with central larval chamber; caused by midge larvae => 17

16b Galls of different shape, about 1–8 mm long, on leaf blade ± club-shaped, on stem spindle-shaped, often curved, usually green, without larval chamber, often many on leaf blades as well as on stems. Achillea spp.: Subanguina millefolii

17a Galls laterally flattened, egg- jar-shaped, up to 6 (8) mm long, initially green and soft, later brown, finally glossy black and tough. Apically with a narrow, pubescent opening, the lobes enlarging and deflected when mature. One elongated chamber with yellow larva: Rhopalomyia millefolii

17b Galls protruding on upperside, about 2 mm long, oblong egg-shaped to jar-shaped, acuminate, about 4–8 mm long; wall soft paper-like to yellowish-green. Galls open at maturity with several deflected white felt-like pubescent lobes; top with narrow opening. One-chambered, solitary or gregarious. A. ptarmica: Rhopalomyia millefolii

17c Petiole, rachis or midrib of the leaflets with sometimes several, very weak, spindle-shaped, ± reddish galls; occasionally accompanied by slightly curved weak enlargements. Per gall one sulphur yellowish to bright brick red larva. A. millefolium: Lasioptera francoisi

18a Leaves rolled in different ways, twisted or curled => 19

18b Leaf blade on the underside with small depressions; bulges on corresponding upper side. A. millefolium: Craspedolepta nervosa

19a Malformations or abnormal pubescence => 20

19b One or several, stacked leaves on the stunted shoot stem, densely white-silky pubescent; leaf blades ± rolled or curled downwards. Rachis and veins slightly thickened, ± shortened, sometimes bent strongly downwards. A. millefolium: Unidentified gall mite

20a Malformations of usually many densely stacked leaves at the ± stunted shoot tip; caused by aphids or cercopids => 27

20b Leaf blade ± nest-like joined or leaf margin closely rolled downwards => 21

21a Leaflets with narrow, not abnormally pubescent downwards rolled leaf margin. A. erba-rotta subsp. moschata, nobilis: Unidentified gall mite

21b Leaves, depending on size, completely or partially strongly curved, sometimes almost nest-like bent downwards on the stunted shoots with several; usually strongly curled and dark green coloured near the froth-covered larva. Achillea spp.: Philaenus spumarius

22a Sporulating areas dark-coloured => 23

22b Globular to oblong, exceptionally slightly callus-like bulged aecia on the leaf underside or, more rarely, on stems. A. ptarmica: Puccinia vulpinae

23a Sporulating area tough, bulged on leaves; up to 5 mm long, globular to oblong, with brown telia. Strongly infected stems ± bent or twisted => 24

23b Whole plant stunted by shortened internodes, densely bushy, often non-flowering. Stem and leaves with small pustule-like depressions, which cause irregular bends. A. millefolium, ptarmica: Didymaria matricariae

24a On A. millefolium, setacea and closely related species: Puccinia cnici-oleracei

24b On A. salicifolia, ptarmica and closely related species: Puccinia ptarmicae

24c Shining black, less than a mm long elliptic blisters on the stems and undersides of the leaves. A. ptarmica: Schizothyrioma aterrimum

25a Galls of largely defined shape at the tips of sometimes conspicuously stunted shoots, on terminal- or lateral buds; caused by gall midges or flies => 30

25b Malformations of many leaves at the end of main- or ± stalked lateral shoots => 26

26a Malformations not abnormally pubescent; caused by aphids or cercopids => 27

26b Leaves stunted, on the often strongly compressed tips of main shoots or lateral buds ± in dense tuft-like clusters, twisted and at the margins downwards rolled, often ± thickened, sometimes abnormally pubescent. A. ptarmica: Unidentified gall mite

26ca Malformations of terminal twig-parts and leaves. Containing a fly maggot. A. ptarmica: ? Tephritis sp

27a Malformations of rosette leaves or more closely to the tip of usually distinctly stunted stems, or leaflets deflected, ± curled; occupied by aphids => 28

27b Leaflets crooked, irregularly twisted. Whole plant stunted or ± bent. A. erba-rotta subsp. moschata, ptarmica: cf. Psylla ptarmicae

28a Malformations at the tip of well-developed shoots => 29

28b Leaf rosettes dense bushy; leaves malformed in different ways; grey-green aphids between the lobes. A. millefolium: Coloradoa achilleae

28c For A. millefolium and subspecies a causer of similar malformations has been reported, with early dying rosette leaves standing rigidly brush-like upwards and having the leaf segments upwardly directed: Macrosiphoniella usquertensis

29a On A. salicifolia, ptarmica. At the top of vegetative shoots the leaves in large numbers more densely arranged, ± downwardly bent and rolled: Aphis fabae and/ or Aphis fabae subsp. solanella

29b Similar malformations on A. collina, millefolium. Aphid 2.5–3 mm long; body green, covered with grey wax powder: Macrosiphoniella millefolii

30a Tip of shoot, possibly including inflorescences or also lateral buds, changed into a spongy, whitish or reddish, ± globular-oval, smaller or up to 30 mm long, moderately frequently whitish pubescent, multilocular gall, often disc-like framed by some ± shortened and broadened leaves. One white larva per chamber. A. ptarmica, rarely millefolium, ochroleuca: Rhopalomyia ptarmicae

30b Galls laterally flattened, egg- to jar-shaped, about 4–8 mm long; very different in position; their wall soft and green in the beginning, later tough and brown to glossy black. At the rather acute top with a narrow, on the inner surface pubescent opening, the lobes of which are enlarged and deflected at maturity. Sometimes several galls together, also partially connate. One bright yellow to yellow larva per chamber: Rhopalomyia millefolii

30c Bud hypertrophied, transformed into a subglobular gall, woody, plurilocular, covered with a dense whitish pubescence and enveloped by some disfigured and haired leaves. Contains a single red larva: Rhopalomyia achillearum

30d Buds in leaf axils transformed into jar-shaped galls, about 4–8 mm long; their wall soft paper-like to yellowish-green, one-chambered, which open at maturity with several deflected whit felt-like pubescent lobes. A. ptarmica: Rhopalomyia sp.

31a Galls in single florets, on bracts or fruits => 42

31b Malformations of capitula or on the whole inflorescence => 32

32a Malformations of receptacle, or flower head => 39

32b Malformations usually extending over larger parts of the inflorescence => 33

33a Malformations caused by aphids => 38

33b Different gall causers => 34

34a Galls not connate to a consistent locular body => 35

34b Single or occasionally multiple capitula or even complete inflorescences reshaped to small or considerable extent, sometimes up to 30 mm long, more or less rotund to oval, multilocular galls. Wall spongy, whitish or reddish, sometimes pubescent. Galls occasionally framed by a cup of several, partially shortened and broadened leaves. Each chamber with one white larva. A. salicifolia, ptarmica, more rarely A. millefolium: Rhopalomyia ptarmicae

34c Gall midge larvae develop in flower heads of A. millefolium: Macrolabis achilleae

= From Greece on A. millefolium the gall midge Contarinia achilleae has been recorded.

35a Malformations more or less white woolly => 37

35b Malformations without conspicuous pubescence => 36

36a Capitula to a large extent deformed. All flower parts enlarged and more or less curled. Stamens occasionally reshaped to form a second corolla. Bracts more or less enlarged and misshapen. A. abrotanoides, millefolium: Aceria kiefferi

36b Capitula enlarged, more or less distorted, non-flowering. A. millefolium: Thrips nigropilosus

37a Inflorescence completely transformed into a dense white tomentose mass; capitula completely or partially greened. A. erba-rotta subsp. moschata, nana, nobilis, pectinata: Aceria achilleae

37b Similar malformations in A. millefolium: Unidentified gall mite

38a The whole inflorescence stunted in all developmental phases. Capitula staying small, not opening, stacked. Aphid 2.5–3 mm long, body green, greyish frosted: Macrosiphoniella millefolii

38b Similar malformations. Aphid 1–2 mm long, pale yellowish to yellowish-green. A. millefolium: Brachycaudus helichrysi

38a Malformations of receptacle by fly maggots => 40

39b Capitula differently deformed, more or less stunted. Marginal florets occasionally enlarged, distorted, slightly thickened, occasionally containing oospores. Mycelium in many cases later on fruits with dirty grey to violet, branched conidiophores. A. ptarmica: Peronospora radii

40a Receptacle swollen; malformations not affecting the stalk of the flower head => 41

40b Receptacle together with the stalk of the flower head thickened. Infected capitula clearly standing out from the normal ones. A. millefolium: Unidentified fly

41a Receptacle acute-conically swollen, sclerotic; at the upper end with a round opening after the fly has emerged. Containing a single larva. A. millefolium and subspecies, A. ptarmica: Tephritis nigricauda

41b Receptacle similarly but more strongly deformed, conspicuously standing out from the normal capitula. One white larva. A. millefolium: Eurasimona stigma

41c Flower head aborted, inconspicuously disfigured or hypertrophied. Containing a single larva. A. millefolium. Also on Artemisia: Tephritis dioscurea

41d Slight malformation of the flower head. A. maritima: Tephritis stictica

42a In florets and fruits => 43

42b Basal parts of bracts more or less egg-shaped, swollen. One initially white, later on orange larva. A. ptarmica: Rhopalomyia palearum

43a Malformation of single or many florets => 44

43b Ovary or fruit, sometimes also the basal part of corolla, lightly swollen. One yellow larva. A. millefolium, ptarmica (main hosts), biserrata, nobilis, nobilis subsp. neilreichii: Ozirhincus millefolii

44a Several florets deformed in a frequent thick-walled gall, about 3–5 mm long, initially more or less egg-, later jar-shaped, protruding distinctly from the flower head. One yellow larva. Achillea spp.: Rhopalomyia millefolii

44b Many capitula per inflorescence intensely dark red coloured. Flowers often distinctly stalked, hardly swollen, not opening. One reddish larva per floret. A. ptarmica: Unidentified gall midge

= Larvae of Contarinia sp. developing in flower heads of Achillea millefolium, ptarmica have been recorded from Denmark, and ditto on Achillea sp. from Greece

= Furthermore larvae of Jaapiella sp. developing in flower heads of Achillea millefolium have been recorded from Denmark

mod 19.xii.2019