Heptamelus dahlbomi ( Thomson, 1870 )

Vikberg, Veli & Liston, Andrew D., 2009, Taxonomy and biology of European Heptamelini (Hymenoptera, Tenthredinidae, Selandriinae), Zootaxa 2112, pp. 1-24 : 4-7

Heptamelus dahlbomi ( Thomson, 1870) sp. rev.

Caenoneura Dahlbomi Thomson, 1870: 270 –271. Female, syntype series, type locality: Sweden, Scania, “Sällsynt; funnen af Prof. Dahlbom vid Sandhammaren i Skåne” [Rare; found by Prof. Dahlbom near Sandhammaren in Scania].

Coenoneura dahlbomi : Cameron 1878: 22, misspelling.

Coenoneura dahlbomi : Cameron 1882a: 272, synonymy with Heptamelus ochroleucus .

Heptamelus ochroleucus: Shaw & Bailey 1991 , misidentification: biology and parasitoids.

Notes on original description and type material. The original description of Caenoneura dahlbomi mentions the body colour as almost entirely black, with the scape and pedicel pale ( Thomson, 1870). These characters are considered diagnostic for H. dahlbomi (see key, above).

Thomson (1871) repeats his 1870 description of Caenoneura dahlbomi nearly unaltered, but adds a short description of what is here considered to be the female H. ochroleucus as a “variety”, characterized by its entirely black antennae, and more extensively pale marked thorax and abdomen. Cameron (1882b) adopted a similar approach and mentions two “varieties” of female, but referred to the species as Heptamelus ochroleucus and placed C. dahlbomi as its junior synonym.

None of the specimens in MZLU can be regarded as a syntype of Caenoneura dahlbomi , because the label data (locality, collector) do not agree with those published by Thomson (1870). A single female from Ringsjön, Sweden, apparently determined as C. dahlbomi by Thomson, belongs to H. ochroleucus . Possibly this is the specimen on which Thomson (1871) based his description of the “variety”. J. Ferrer (Naturhistoriska Riksmuseet, Stockholm) informs us (pers. comm.) that neither of the two specimens under the name H. ochroleucus in the Stockholm collection can be regarded as types, because these are from localities other than that published by Thomson (1870).

We refrain for the present from designating a neotype for H. dahlbomi , because the original description clearly characterizes this species and syntypes may still be located in other European museums where material collected by Dahlbom is deposited (see Horn et al. 1990).

Re-description [ Figs 1–7 ] (see key above and the following supplementary notes)

Body length: 3.6–5.5 mm (n = 54 Finnish females).

Clypeus flat (i.e. with lateral edge in same plane as central part; view from side!) or even slightly concave, with centre depressed; front margin with shallow, triangular medial emargination ( Fig. 7 ); black. Labrum dark brown or infuscate (but often hidden under clypeus). Mesopleuron entirely black; upper half of mesepisternum with fine punctures of regular size and distribution. Lancet ( Figs 5, 6 ): 13 annuli; denticles occupying only about half width of basal or medial annulus.

Variability. 7 or 8 antennomeres. Extent of pale (whitish) areas on scape and pedicel variable; both usually paler than flagellum, but occasionally only scape paler. Forewing stigma in two Canadian specimens largely pale, but pronounced black stripe present on anterior edge and posterior of disc slightly infuscate. This difference may however only be related to age or treatment of specimens.

Comments. The nominal taxa described by various authors from the Eastern Palaearctic require revision. Many are known only from single specimens or small series of only one sex. The association of sexes may in some cases be wrong. Two of these species seem to resemble H. dahlbomi more closely than H. ochroleucus , because of their dark colouration and weakly or finely punctured mesopleuron ( Malaise 1961): H. magnocularis Malaise, 1931 (described from Kamchatka, both sexes: Malaise 1931a) and H. ussuriensis Malaise, 1931 (described from Vladivostok Area, female only: Malaise 1931b). Should H. dahlbomi also occur in the East Palaearctic, it must be borne in mind that populations may occur which contain males. Even if one of the East Asian taxa is later found to be conspecific with H. dahlbomi , the European population will retain the name H. dahlbomi , because it is the second oldest species name in the genus.

Sex ratio. Male unknown. Shaw & Bailey (1991) reared 32 females, 27 of which were deposited in RSME and are now determined as H. dahlbomi . Observations by VV in Finland (below) and D. R. Smith (pers. comm.) in North America also indicate that this species is parthenogenetic. Furthermore, no European males of Heptamelus have been seen which exhibit characters diagnostic for the female of H. ochroleucus (e.g. shape of clypeus, puncturation of mesepisternum, darkened stigma of forewing) or which can be separated in any other way from those of H. ochroleucus .

Hostplants. Athyrium filix-femina (L.) Roth ( Woodsiaceae ) ( Shaw & Bailey 1991; observations below), but possibly also other ferns (see under H. ochroleucus ).

Observations on biology. In 2000 and 2001 a rather large population of Heptamelus dahlbomi occurred in the grove of Hangastenmäki; a moist, deciduous wood with numerous fern species, but dominated by the lady fern ( Athyrium filix-femina ). Females in flight were netted, or swept from lady fern, from end of May to mid June. Two females were used for oviposition experiments on potted A. filix-femina brought indoors. Plants were then searched for signs of larval feeding, leaf petioles opened and larvae and their feeding traces studied.

Oviposition experiments (OE):

OE 11/00: On 3 June, 2000 1Ψ swept from flowers of Sorbus aucuparia . Young lady fern offered, on which she oviposited during the evening of the same day. She first touched the apical, unopened pinnae and upper part of the rachis with her sawsheath tip (or saw tip?), walked down the rachis and then turned, with the head directed upwards and at the level of lower pinnae, and oviposited in sides of convex surface of rachis. Oblique oviposition scars were slightly visible on lateral parts of the rachis. The result of oviposition was not studied.

OE 9/01: One small female was caught on 1 June, 2001. At 17.00–18.00 local time, she laid an egg or eggs in the petiole below the lowest pinnae laterally and medially in the convex side of petiole. Before oviposition she lightly touched the unopened apical part of the leaf with apex of sawsheath or tip of saw. Oviposition lasted 10–15 min. On 10.6. signs of feeding were visible on upper part of petiole; there were 4–5 broader, translucent areas of feeding and between them narrow, darker areas of intact petiole. On 12.6. petiole 14 cm long, of which upper 11 cm mostly dried and brown. Feeding area had extended further downwards and only 2 cm of base remained entire. On 15.6. petiole was opened; one larva about 8 mm long situated rather basally, with brownish head and reddish-violet body. Indoors, development of the larva was rapid; taking 15 days from egg to full grown larva.

Observations on larvae under natural conditions:

24.6.2000: two larvae were found in lower part of the same leaf petiole. A whitish parasitoid larva was attached to underside of abdomen of upper larva. Lower larva 10.5 mm long, eating towards the petiole base. The leaf-blade was withered, brownish. Bodies of larvae grey, turning white in alcohol. Head capsules of two exuviae were found; they showed a distinctive pattern of pigmentation.

26.6.2000: about 20 larvae were taken from opened leaf petioles; the smallest were whitish, larger ones greyish and the largest violet-grey. Mostly one larva was found in one petiole but sometimes two fed in the same petiole. Feeding area usually begins below the lowest pinnae in uppermost part of petiole. Two larvae carried an external parasitoid wasp larva. In lower parts of leaf petioles a distinct, regular “zebra” pattern of feeding traces is found. If the petioles are opened and spread, within a length of 5 cm some 19 dark transverse areas (each less than 1 mm long) and some 18 pale areas (each about 2 mm long) are seen. The larvae were put in alcohol.

22.11.2000: lower parts of leaves of Athyrium filix-femina were collected and on 4.1.2001 a female of Bracon (Orthobracon) virgatus Marshall emerged from petioles of the fern. In late summer of 2000 at the locality where Heptamelus larvae occurred, some females of B. virgatus had previously been swept. This was the first record for North Europe of this idiobiont braconid ectoparasitoid, specific to Heptamelus . Shaw & Bailey (1991) reared and recorded it from the English Lake District as Bracon lineifer van Achterberg, which was synonymized with B. virgatus by Papp (1999).

28.6.2001: 7 petioles of lady fern containing feeding Heptamelus larvae were cut and placed in a glass of water. The feeding progressed downwards. On 1.7. the petioles were opened and 9 larvae were counted. Two petioles contained two larvae each. These were small and white. Larvae occurring singly in a petiole were blue-violet and many of them were full-grown. The length of larvae living singly was 6–12 mm, the width of head approximately 1.0 mm and width of frons approximately 0.60 mm. All larvae were put into alcohol.

The shed larval skins (mostly head capsules) were collected, as far as possible, and the width of the frons measured. The size-classes are as follows:

1st instar: frons width 0.20 mm (n = 1),

2nd instar: frons width 0.28–0.33 (n = 3),

3rd instar: frons width 0.35–0.40 (n = 6),

4th instar: frons width 0.46–0.53 (n = 6).

There are thus five larval instars (in female sex).

Figures

FIGURES 1 – 7. Heptamelus dahlbomi (Thomson), Ψ (Germany, Bavarian Forest National Park, leg. G. Merkel-Wallner). 1, whole insect, lateral; 2, head, dorsal; 3, sawsheath, lateral; 4, sawsheath, dorsal; 5, lancet, apex; 6, lancet; 7, clypeus. Scale bars = 0.5 mm.