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.
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.
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.