Further descriptions of Dolerus Larvae (Hymenoptera: Tenthredinidae), with notes on larval identification and feeding habits.

Abstract

In 1957, for his PhD study, Manfred Kraus co-produced with Herbert Lorenz 'Die Larvalsystematik der Blattwespen' (Lorenz & Kraus 1957). The work represented the first serious effort to provide a classification and identification key for European sawfly larvae, and to this date it has not been superseded. The task of larval identification remains neglected in relation to the progress with adult sawflies. Some progress has been made however with more detailed descriptions of individual species and genera (e.g. Benes 1973, Vikberg & Nuorteva 1997). This paper aims to continue the work of description and identification of larvae in one genus, Dolerus, which was begun by the present author in a key to eight graminivorous species (Barker 1998).
The genus Dolerus is unusual among sawfly genera in specialising on Poaceae, Cyperaceae, Juncaceae, and the primitive genus Equisetum (horsetails). Liston (1995) lists 70 currently recognised European species and Lacourt (1999) 82 species from the Western Palaearctic although revision of the genus is likely to change these estimates. They have some economic relevance as sporadic pests of cereal crops (e.g. Freier & Wetzel, 1984), and have also been studied for their role in providing food for higher trophic levels in farmland ecosystems (e.g. Green 1984; Potts 1986) and as possible biocontrol agents for horsetails, which can cause weed problems (Leblanc & Goulet 1992). They are almost all univoltine and fly early in the season (March-June) in lowland regions (flight periods are later in highland and subarctic regions). Dolerus overwinter as prepupae and pupae (Lorenz & Kraus 1957, Barker 1998) and are difficult to rear through, which has slowed the descriptive process. Lorenz & Kraus (1957) described 21 Dolerus species and 3 Loderus species, based on their own observations and the descriptions of Conde (1933, 1937); these two genera have subsequently been reunited under Dolerus (Goulet 1986). Leblanc & Goulet (1992) described larvae of eight Nearctic species.
Barker (1998) described the morphology of larvae of Dolerus species common in southern Britain in grasses and cereals, including the previously undescribed Dolerus puncticollis Thomson, 1871. These species were almost all very similar in overall morphology so that identification depended on subtle differences in shape, colour pattern and chaetotaxy. One European Equisetum-feeding species was also described for comparison. The current paper presents descriptions of Dolerus species from different host plant families, and updates the identification key. Their host spectra and feeding behaviour are examined in relation to mouthpart morphology to explore further the observation made in Barker (1998) that the labrum and mandibles vary in a way that might relate to diet.