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Biology of two European Tenthredo species (Hymenoptera, Tenthredinidae) feeding on Gentiana
expand article infoAndrew Liston, Ewald Altenhofer§, Romana Netzberger|, Marko Prous
‡ Senckenberg Deutsches Entomologisches Institut, Müncheberg, Germany
§ Unaffiliated, Gross Gerungs, Austria
| Unaffiliated, Kollmitzberg, Austria
¶ University of Tartu, Tartu, Estonia
Open Access

Abstract

Very few sawflies using Gentianaceae as larval host plants have been recorded. We identified larvae collected in Austria on Gentiana asclepiadea L. as Tenthredo atra Linnaeus, 1758 and T. propinqua Klug, 1817. If its current taxonomic circumscription as a single species is accepted, T. atra is a highly polyphagous species, whereas T. propinqua may be more specialised: Gentiana asclepiadea is its first recorded host. We sequenced plant DNA from the head of one T. propinqua larva, which confirmed that it had been feeding on this plant. This is the first recorded use of G. asclepiadea by sawfly larvae. Larvae are illustrated, and identification characters are described.

Key Words

Gentianaceae, host plants, larvae, sawflies, Symphyta, Tenthredinoidea

Introduction

The study of the immature stages of sawflies, including the identification of their larval hosts, has a long tradition in Europe, reaching back to the pioneering studies of Réaumur (1740). Despite many advances since then, we still know little or nothing about the biology of some taxa. Here, we fill one of these gaps by documenting the host plant association of two Tenthredo species with a member of the Gentianaceae, a plant family which has hitherto seldom been mentioned as a host of sawflies.

Recently, DNA sequencing has proved itself as a potent tool for the identification of sawfly larvae (e.g. Shinohara et al. 2017; Prous et al. 2019). In this study we used DNA sequences to identify one of the sawfly species. Compared to the traditional method of rearing an adult from a larva and determining the adult using morphological characters, sequencing can provide an identification result much more quickly, and the risk is avoided of all individuals dying before they reach maturity, in which case no identification will be obtained. We also demonstrate that DNA sequencing can be used to identify or confirm the host of a larva, using DNA extracted from the larva. This is especially useful for larvae that were collected, for example by sweeping, without any clear indication of what they were feeding on, and might also help to identify the hosts of species of Tenthredininae which do not feed on the plant species in which eggs are laid, as reported by Chevin (2009) for some Macrophya species.

Methods

Nearly fully grown larvae of two Tenthredo species were beaten from, or detected visually on Gentiana asclepiadea L. in the Gesäuse National Park, Styria, Austria, by E. Altenhofer and R. Netzberger in 2016 and 2019. Some larvae were kept by EA for rearing, and others were preserved in 95% ethanol. Identification of Tenthredo atra Linnaeus, 1758 is based on the morphology of adults and larvae. Tenthredo propinqua Klug, 1817 was identified by genetic sequences obtained from a larva. Total DNA was extracted from the head of one T. propinqua larva (DEI-GISHym12639), and one mitochondrial (1087 bp of CO1) and two nuclear gene fragments (1654 bp of NaK and 2543 bp of POL2) were sequenced (methodology as in Prous et al. 2019). To test which host plant the larva had been feeding on, we used the same larval extract to amplify a plastid region between trnL 5′ exon and trnF using primers c and f (Taberlet et al. 1991). The region, which turned out to be 816 bp, contains two variable introns and the trnL 3′ exon and was sequenced with primers c, d, e, and f (Taberlet et al. 1991). The sequences have been deposited in the GenBank (NCBI) database (accession numbers MN856146MN856149).

Material examined

The abbreviation SDEI refers to the insect collection of the Senckenberg Deutsches Entomologisches Institut (SDEI), Müncheberg, Germany.

Tenthredo atra Linnaeus, 1758

Austria: Styria: Gesäuse, Kroisalm, 47.60N 14.63E, 900 m, 26.08.2016, 3 females reared from larvae on Gentiana asclepiadea (emerged May 2017), specimens were overlooked after emergence, and are in very poor condition, i.e. fragmented, with diverse parts gummed on one card (DEI-GISHym12664), and 1 larva, leg. E. Altenhofer (SDEI). Gesäuse, E Admont, 47.58N 14.62E, 11.09.2019, 10 larvae on Gentiana asclepiadea, leg. E. Altenhofer (SDEI). The last of these larvae entered the ground to overwinter on 21.09.2019. Gesäuse, Hartelsgraben, 47.59N 14.73E, 23.08.2019, larva on Gentiana asclepiadea, photographic record by R. Netzberger (Fig. 1).

Tenthredo propinqua Klug, 1817

Larvae:

Austria: Styria: Gesäuse (E Admont), between Gstatterboden and Hochscheibenalm, 47.58N 14.62E, 600–1150 m, 15.09.2019, 10 larvae on Gentiana asclepiadea, leg. E. Altenhofer (2 larvae in SDEI [DEI-GISHym12639, 12640], others retained by EA for rearing).

Imagines:

Ukraine: 1 male (DEIGISHym20102), Jablunitsa, Berkut, 48.72N 24.37E, 840 m, 08.07.2004, leg. E. Heibo (SDEI). 1 male (DEIGISHym20103), Jablunitsa, Berkut, 48.72N 24.37E, 840 m, 06.07.2004, leg. E. Heibo (SDEI). 1 male (DEIGISHym20104), Jasinja, Tatariv, 48.37N 24.56E, 710 m, 03.07.2004, leg. E. Heibo (SDEI). 1 female (DEIGISHym20105), Jablunitsa, Berkut, 48.72N 24.37E, 840 m, 06.07.2004, leg. E. Heibo (SDEI).

Austria: 1 female (DEIGISHym17738), Carinthia, Eisenkappel 10km E, St Margarethen, 46.46N 14.66E, 28.06.1993, leg. L. Behne (SDEI).

Tenthredo scrophulariae Linnaeus, 1758

The larvae illustrated in Figs 6, 7 were photographed by Henri Savina in France, Ariège, Aulus-les-Bains, 42.80N 1.33E, respectively on 08.09.2007 and 30.09.2007. Host: Scrophularia sp.

Results

Tenthredo atra

Figs 1, 2

Notes

Tenthredo atra has already been associated by various authors with larval hosts in many higher plant taxa. Taeger et al. (1998), in a summary of these records, mentioned the families Brassicaceae, Caprifoliaceae, Lamiaceae, Plantaginaceae, and Solanaceae. There are also records of larvae of T. atra feeding on Asteraceae (Pschorn-Walcher and Altenhofer 2006), Betulaceae and Salicaceae (Loth 1913), Ranunculaceae (Conde 1934), Rosaceae (Kangas 1985), and Urticaceae (Pschorn-Walcher and Altenhofer 2000). It is not clear whether records from Menyanthes trifoliata (Menyanthaceae) and Sedum telephium (Crassulaceae), which are sometimes named as hosts of T. atra (e.g. Taeger et al. 1998), really refer to this species, or respectively to the closely related T. moniliata Klug, 1817 and T. ignobilis Klug, 1817. Taeger et al. (1998) mentioned some additional plant taxa on which oviposition by T. atra has been observed but which have not been proved to be hosts of the larvae. Our identification of the larvae from Gentiana as T. atra accepts the premise that the name refers to only one, highly polyphagous species. However, a wide morphological variability, most obvious in the colour pattern of T. atra imagines, might indicate that more than one species are currently grouped under this name.

The larvae from Gentiana asclepiadea (Figs 1, 2) are in general appearance not distinguishable from larvae of T. atra from other hosts, nor from the larvae of the related T. moniliata on Menyanthes trifoliata (Conde 1934; Liston personal observations). Lorenz and Kraus (1957) did not examine larvae of T. atra, and their description is based on those of Cameron (1882) and Carpentier (1888). Lorenz and Kraus (1957) did not mention the faint, oblique, darker dorso-lateral stripes shared by the larvae from Gentiana, the larva described by Cameron (1882; as T. dispar Klug, 1817 from Succisa pratensis), and larvae of T. moniliata examined by Liston. Note also that Carpentier’s (1888) description, as T. dispar, is of larvae from Menyanthes trifoliata, and may therefore refer to T. moniliata, but he did not mention any body markings. On the other hand, the larva of Tenthredo ignobilis, another species in the complex with T. atra, possibly differs from T. atra in lacking the oblique body markings (Liston 2015: fig. 9).

Larvae of the later instars feed mainly on the leaves; they feed from the edge, leaving irregular holes. Inflorescences are also sometimes consumed, at least under rearing conditions (Fig. 2). The largest full-grown larvae are similar in size to those of T. propinqua, i.e. somewhat over 20 mm long.

Figures 1, 2. 

Tenthredo atra larvae, nearly full-grown, on Gentiana asclepiadea (photographed respectively on 23.08.2019, 11.09.2019). Photos: R. Netzberger (1), E. Altenhofer (2).

Tenthredo propinqua

Figs 3–5

Notes

A mitochondrial CO1 sequence from one larva (DEI-GISHym12639) corresponded closely (maximum divergence 0.5%) with sequences from T. propinqua imagines (DEI-GISHym20102, DEI-GISHym20103, DEI-GISHym20104, DEI-GISHym20105, DEI-GISHym17738). Nuclear sequences (NaK and POL2) are available only for the specimen sequenced here (DEI-GISHym12639). The sequenced plastid trnL-trnF region (816 bp) from the larval DNA extract confirmed Gentiana as the host. The closest (99–100% similarity) according to NCBI BLAST (https://blast.ncbi.nlm.nih.gov/Blast.cgi) were four species of Gentiana, among them G. asclepiadea. A shorter G. asclepiadea sequence in GenBank (accession AB453085, 387 bp) was identical to our sequence, while a longer one (AJ580515) differed by three substitutions and one deletion over the length of 781 bp (because of apparently numerous sequencing errors at the 3′ end of AJ580515, 21 bp of that sequence were excluded from the comparison).

Gentiana asclepiadea is the first recorded host plant of the hitherto unknown larva of T. propinqua, which is a close relative of T. scrophulariae Linnaeus, 1758. These species have long been known to strongly resemble each other in the morphology of their imagines, but they are distinguishable using some colour characters (Enslin 1912). Their larvae are also closely similar in general appearance (see the description of a T. scrophulariae larva by Lorenz and Kraus 1957 and below). However, T. scrophulariae has a very different host plant spectrum, which consists mainly of Scrophularia and Verbascum species, but sometimes Buddleja species (Muche 1962), all of which belong to the Scrophulariaceae.

Differences in the pattern of black markings may enable T. propinqua larvae to be distinguished from T. scrophulariae, but a larger number of T. propinqua larvae should be checked, to confirm that the differences are consistent. In T. propinqua, each of the medio-dorsal black spots on the abdominal segments occupies only the width of single annulet (Figs 3, 4), whereas in T. scrophulariae, these spots occupy parts or the whole width of two annulets (Figs 6, 7). The position of the corresponding spots on the thorax is, however, similar in both species. At least in the later instars of T. scrophulariae larvae, the position of these markings is thought to be stable: compare Fig. 6 (half-grown) with Fig. 7 (nearly full-grown). After each moult, the integument of the larvae of both species temporarily lacks the covering of white wax and has a greenish ground colour. The head colour pattern of T. propinqua (Fig. 5) is the same as described by Lorenz and Kraus (1957) for T. scrophulariae. In practice, the identity of their host plant should be sufficient to distinguish larvae of these species.

The feeding habits of Tenthredo propinqua larvae are similar to those of T. atra, i.e. irregularly shaped parts of the leaf-blade are consumed from the edge. But, unlike for T. atra, we did not observe feeding on the inflorescences by T. propinqua. The largest full-grown larvae are 22–25mm long, which is about the same as given by Lorenz and Kraus (1957) for T. scrophulariae.

Figures 3–7. 

Larvae of Tenthredo propinqua and scrophulariae. 3–5. Tenthredo propinqua, from Gentiana asclepiadea. Arrows indicate some of the black dorso-median markings, which on the abdomen occupy the width of a single annulet; 3. Preserved in ethanol (wax dissolved); 4. Alive, with wax coating (photographed on 11.09.2019); 5. Head, preserved in ethanol. 6, 7. Tenthredo scrophulariae larvae, on Scrophularia species. Arrows indicate some of the black dorso-median markings, which on the abdomen extend across two annulets at least in part; 6. Half-grown (photographed on 08.09.2007); 7. Nearly full-grown (photographed on 30.09.2007). Photos: A. Liston (3, 5), E. Altenhofer (4), H. Savina (6, 7).

Discussion

As far as we are aware, neither Gentiana nor any other member of the Gentianaceae has previously been recorded as a larval host of a sawfly, except by Wang et al. (2015), who studied in China the effect of florivory by larvae on Halenia elliptica D. Don; they referred the larvae to as an undescribed species of Tenthredinidae. Otherwise, the only reported interaction between a sawfly species and a species of Gentianaceae involves visits to the inflorescences of Frasera speciosa Douglas ex Griseb. by the Nearctic Tenthredo erythromera Provancher, 1885 (Norment 1988).

Tenthredo propinqua is a rather rarely collected species (Ritzau 1998), whose known distribution comprises south-eastern Europe, Turkey, and the Transcaucasus (Lacourt 1999). Although the eastern part of the range of Gentiana asclepiadea is more or less congruent with that of T. propinqua, the sawfly has not yet been recorded further west than Berchtesgaden (Bavaria, Germany), although the plant is widespread in Switzerland and occurs as far west as northern Spain (Zajac and Pindel 2011). Tenthredo propinqua has been considered to be to some extent endangered or even locally extinct, at least in the Alps of eastern Bavaria on the north-western edge of its range (Ritzau 1998; Liston et al. 2012). In the future, we should be able to more effectively assess its distribution and conservation status by searching for its larvae.

Approximately 400 species of Gentiana occur worldwide in Eurasia, North Africa, the Americas, and eastern Australia, but South-East Asia is a hotspot of diversity of this genus, with 248 species known from China alone, whereas only 27–29 species occur in Europe (Ho and Pringle 1995; Mel’nyk et al. 2014). Because China also possesses a very rich fauna of Tenthredo species (Wei et al. 2006), it is possible that Gentiana is more widely used there as a host plant by these sawflies than it is in Europe.

Acknowledgements

The Museum für Naturkunde, Berlin, generously paid for open access publication of this work. Henri Savina (Toulouse, France) kindly permitted us to reproduce his images of larvae of Tenthredo scrophulariae. Erik Heibo (Lierskogen, Norway) provided tissue samples of T. propinqua. Dustin Kulanek and Katja Kramp (SDEI, Müncheberg) did some of the lab work. Akihiko Shinohara and Spencer Monckton reviewed the manuscript and suggested many worthwhile improvements.

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