Resources

The following resources have been assembled to assist with sawfly identification and understanding of sawfly ecology. Some of the resources may need updating to reflect modern taxonomic understanding. The internet is awash with incorrectly identified sawfly images so it is recommended to avoid identification by Google searches. The image galleries on this website and on the links below are sourced from known and trusted hymenopterists.

A glossary to common terminology used in identifying sawflies is available to download here.



An introduction to sawflies?

What is a sawfly?

Sawflies fall within a group of insects called the Hymenoptera, which includes the bees, wasps, ants and sawflies.

The hymenoptera have four membranous wings that are free from any dense scales or hairs. The fore and hind wings connect by a series of hooks called hamuli. The wings have fewer veins than in many insects and the longitudinal veins intersect with cross-veins so that the basal and apical veins do not appear continuous. The forewing has a stigma separated from the costal vein by a notch. A small sclerite called the tegula covers the base of the wing. The first abdominal segment (tergum or tergite) is fused to the thorax. The mouthparts are in the form of mandibles with a maxillar-labial complex.

The hymenoptera are for convenience split into the apocrita (bees, wasps and ants) and the symphyta (sawflies), though this simplistic split is not entirely supported when the phylogenetic relationships are studied. The apocrita have evolved a constriction on the second abdominal segment to create a waist between the gaster, or mesosoma, (the thorax and first tergite combined) and the abdomen. The sawflies do not have this constriction. In the females, the apocrita have a modified ovipositor creating a sting. In the sawflies the ovipositor is modified into a saw for cutting into plant material. The larvae of the apocrita generally feed on insect prey or pollen, whereas the sawfly larvae mostly feed on plant leaves or wood. One family of sawflies, the Orrusidae, is parasitic in nature.

 

The sawfly lifecycle

Stages

Sawflies undergo a complete metamorphosis in a similar way to butterflies and moths. In the typical lifecycle, the adult (imago) lays eggs into a notch cut into the membrane of a leaf. A caterpillar-like larva emerges and feeds on the leaf, undergoing several moults as it grows. The larva between moults is known as an instar. Once fully grown, the larva undergoes a final moult and becomes a non-feeding eonymph. The eonymph will burrow into soil and create a pupal cavity or it may spin a silken cocoon in leaf litter. Most species overwinter in this pre-pupal form before entering a short phase as a pupa in the spring, emerging as an adult after a few weeks.

Egg stage

Various egg laying strategies have evolved in sawflies. Most will lay eggs in a notch or pocket cut into the soft tissue of a leaf or stem. The Pamphilidae leaf-rollers, however, tend to glue the egg onto the surface of a leaf. The Siricidae wood wasps drill a tunnel into timber and lay the egg at the bottom of the tunnel. The Orussidae are parasitic and seek out tunnels made by wood-boring beetle larvae on which they feed.

Eggs may be laid singly or in clusters. The eggs swell as they mature and darken as the larva develops inside. Whilst most sawflies overwinter as a pre-pupal eonymph, a few species overwinter as an egg. These include some of the Diprionidae and the Apethymus species. The adults of these species are found much later in the year than other species.

Larval stage

The typical larvae are free feeding and caterpillar-like. They can be distinguished from the caterpillars of butterflies and moths by the number of prolegs and by the number of eye cells. Sawfly larvae have six or more pairs of prolegs (including the clasper at the tail). These are the fleshy false legs that follow behind the six true legs at the front of the larva. Butterfly and moth caterpillars on the other hand have five or fewer pairs of prolegs (including the clasper at the tail). Caterpillars also have Velcro-like hooks at the tips of the prolegs which are absent in sawfly larvae. On the head, sawfly larvae have a single eye (stemmata) on each side of the head whereas caterpillars typically have six stemmata on each side.

Many sawfly species feed only on specific plant species (monophagous), on certain plant families (oligophagous), or on a broad range of plants (polyphagous). The Siricidae wood wasps have a symbiotic relationship with certain fungi that they employ to break down timber to make it digestible. The Orrusidae feed on the larvae and pupae of timber-boring beetles. A small group within the Tenthredinidae live inside leaf mines or in galls. Some sawfly larvae have become modified to suit these various feeding strategies. The commonest modifications involve a reduction in the legs and a flattening of the body shape, as is the case in the leaf miners. The larvae of the Pamphilidae live either in leaf rolls, leaf tents or silken webs and have lost the abdominal prolegs. Larvae of the genus Caliroa have a slug-like appearance with the legs tucked under the body which is itself covered in a mucus.

Typically, sawfly larvae undergo 5 or 6 instars. The number of instars in a species may differ according to the gender. The final instar is the pre-pupal eonymph. The eonymph lacks most of the morphological features of the prior instars and so is unlikely to be identifiable at this stage.

Pre-pupal stage

The eonymph will either make a silken cocoon, a cocoon incorporating soil or plant material, or may even simply excavate a cavity. Cocooning may take place above ground in a plant, within the leaf litter at ground level, at the interface with the soil or up to several inches below ground. Alternatively, cocooning may take place inside a convenient location such as the hollow stem of a pithy plant or even in old galls. Some eonymphs simply excavate a cavity in the soil with no structural support. Most sawfly species will overwinter as an eonymph within their chosen location. The exceptions are the Neodiprion and Apethymus genera which overwinter as eggs and the Dolerus genus which overwinter as a pupa within a mud cavity. In adverse conditions, eonymphs may enter diapause which can last for up to three years. The exact cause of diapause is unclear but may relate to humidity or temperature.

Pupal stage

In the spring the eonymph will metamorphose into a pronymph, a very short-lived stage where the pupal eyes become apparent. Within a day or two, the pronymph fully transforms into a pupa where all the adult structures can be seen. Depending on climatic conditions the pupal stage can be as little as a week or last for several weeks or more. When conditions are suitable the imago emerges from the pupa. If the pupa is within a silken cocoon, the imago will use its mandibles to cut a hatch into the end of the cocoon and push through.

Adult stage

Depending on the species, the imago may persist for a few days or live for several weeks. Males of Trichiosoma species have been known to defend a territory for four weeks. Due to their haplo-diploid reproductive system, some species produce both males and females whilst other species produce only females and reproduce parthenogenically. Males are normally haploid having half the full complement of chromosomes. Females are diploid and are produced either from a fusing of haploid gametes from a male and a female, or the fusing of two gametes from the female.


Recommended Reading

Books:

Benson, R.B., 1950. An introduction to the natural history of British sawflies (Hymenoptera Symphyta) Transactions of the Society for British Entomology, Vol.10 Part 2. Sydenham & Co., Bournemouth.
Benson, R.B., 1952. Handbooks for the identification of British insects. Hymenoptera, Symphyta, Vol 6, Section 2(a), Royal Entomological Society, London

Benson, R.B., 1952. Handbooks for the identification of British insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London

Benson, R.B., 1952. Handbooks for the identification of British insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London

Lacourt, J., 2020. Hymenoptera of Europe 2. Sawflies of Europe. NAP Editions, Verrières-le-Buisson.

Viitasaari, M. ed., 2002. Sawflies (Hymenoptera, Symphyta), I: a review of the suborder, the Western Palaearctic taxa of Xyeloidea and Pamphilioidea (Vol. 1). Tremex Press.

Wright, A., 2019. British sawflies: A key to the adults and genera occurring in Britain. 2nd edn. FSC Publications, Telford.

Articles:

Liston, A.D., Heibo, E., Prous, M., Vårdal, H., Nyman, T. and Vikberg, V., 2017. North European gall-inducing Euura sawflies (Hymenoptera, Tenthredinidae, Nematinae). Zootaxa, 4302(1), pp.1-115.

Liston, A., Prous, M. and Vårdal, H., 2019. A review of West Palaearctic Hoplocampa species, focussing on Sweden (Hymenoptera, Tenthredinidae). Zootaxa, 4615(1), pp.1-45

Liston, A., Mutanen, M. and Viitasaari, M., 2019. On the taxonomy of Heterarthrus (Hymenoptera, Tenthredinidae), with a review of the West Palaearctic species. Journal of Hymenoptera Research, 72, p.83.

Prous, M., Kramp, K., Vikberg, V. and Liston, A., 2017. north-Western Palaearctic species of Pristiphora (hymenoptera, tenthredinidae). Journal of Hymenoptera Research, Vol. 59

Prous, M., Kramp, K., Vikberg, V. and Liston, A., 2018. Corrigenda: North-Western Palaearctic species of Pristiphora (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research, 59: 1–190.

Prous, M., Liston, A., Kramp, K., Savina, H., Vårdal, H. and Taeger, A., 2019. The West Palaearctic genera of Nematinae (Hymenoptera, Tenthredinidae). ZooKeys, 875, p.63.

 

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