Symphyta: Sawflies and Woodwasps
 
 
 
Sawflies and woodwasps are several Families grouped in the Sub-order Symphyta and are relatively primitive members of the Hymenoptera. They lack the constriction or ‘wasp waist’ between the abdomen and thorax of the other bees, wasps and ants.  They have two pairs of wings that are transparent, but may have dark markings in some species.  The antennae are thread-like in most species but sawflies in the Cimbicidae family have clubbed antennae.  Sawflies vary in size, with some of the leaf-mining species having adults that are no more than 3mm long, while others are up to about 15mm long.  Woodwasps are much larger and can be up to 30mm long, including the sting-like ovipositor of the female.  Adult sawflies could sometimes be mistaken for true flies, but they have two pairs of wings while the true flies have only a single flying pair.
 
The larvae can strongly resemble the caterpillars  of moths and butterflies.  Both groups of larvae have three pairs of jointed legs on the thorax, but sawfly larvae have seven pairs of suckers or prolegs on the abdomen, whereas moth larvae have five or fewer pairs of prolegs.  
 
 
Wood wasps  - Family Siricidae and Xiphydriidae
 
The wood wasps are characterised by a backward projecting spike from the dorsal part of the last segment on the abdomen.  Females also have a very long ovipositer which is used to inject eggs deep into the wood the larvae feed on. We have 5 species of woodwasps in Britain and Ireland, and the best known is the hornet-sized Giant woodwasp Urocerus gigas, which is harmless but must enjoy some protection from its mimicry of dangerous wasps and hornets
 
 
 
 
Giant woodwasp or horntail Urocerus gigas Females can be up to 36mm in total length, and look scary because of the (harmless) long ovipositer.
 
Note the two projections at the end of the abdomen.  The upper is the spike from the last segment which characterises the wood wasps and is found on both sexes. 
 
The much longer lower organ is the ovipositer, which has "teeth" like a saw and can drill into wood with a racheting movement.
  Left: Siricid wood wasp Urocerus gigas larva 43mm long  Right: galleries excavated by the larvae of Sirex noctilio.
 
 
Sawflies 
 
The rest of the Symphyta are called sawflies, There are several families, of which much the biggest is the Tenthredinidae. The name "Saw"-fly comes from the big saw-toothed female ovipositer used to penetrate bark and wood for egg laying.  Some of the smallest sawflies have leaf-mining larvae.
  
 
Species in Britain and Ireland
There are 540 species in Britain and Ireland.   They are much more likely to be seen in gardens than wood wasps, especially those species with larvae that feed on garden plants.    Jennifer Owen recorded 7,814 specimens of sawflies, from 87 species in 3 families - nearly all tenthredinids.
 
Garden species on fruit trees and bushes include Common gooseberry sawfly, Nematus ribesii, and Small gooseberry sawfly, Pristiphora appendiculata, both on gooseberry and red currant foliage; Apple sawfly, Hoplocampa testudinea, in apple fruitlets; Plum sawfly, Hoplocampa flava, in plum fruitlets; Pear and cherry slugworm, Caliroa cerasi, on the foliage of pear, cherry, plum, hawthorn and other Rosaceae trees and shrubs and  Hazel sawfly, Craesus septentrionalis, on the foliage of hazel, birch and alder.
 
Sawfly larvae that eat the foliage of ornamental plants include those of
  • Aquilegia sawfly, Pristiphora rufipes, on aquilegia
  • Iris sawfly, Rhadinoceraea micans, on yellow flag iris and other waterside irises
  • Solomon’s seal sawfly, Phymatocera aterrima on Solomon’s seal
  • Geranium sawfly, Ametastegia carpini, on hardy geraniums
  • Geum leaf-mining sawfly, Metallus lanceolatus, on Geum species
  • Aruncus sawfly, Nematus spiraeae, on Goats beard
  • Robinia sawfly, Nematus tibialis, on false acacia
  • Rose slugworm, Endelomyia aethiops, Rose leaf-rolling sawfly, Blennocampa phyllocolpa, and large rose sawfly, Arge pagana, all on wild and cultivated roses
  • Berberis sawfly, Arge berberidis, on deciduous Berberis. 
 
Pine trees  in gardens can be partly defoliated by larvae of the pine sawflies, Diprion pini and Neodiprion sertifer.  Willows, Salix species, are the food plant of many sawflies, including Nematus pavidus, N. oligospilus, N. bergmanni and Pontania species.  The last mentioned cause hard red or yellowish green galls, known as bean galls, in the leaves.
 
 
 
 
Woodwasps in the Siricidae family develop as larvae in the stumps, trunks and branches of recently dead or dying conifers; those in the Xiphydriidae family tunnel in the recently dead wood of various deciduous trees, including willow, alder and birch.  The European woodwasp Sirex noctilio has become a pest in many parts of the world where it is a significant cause of mortality in pine trees. When the female lays its eggs, it also adds a fungus inoculum  (Amylostereum areolatum) on which the young larvae feed, but which is pathogenic to the trees.
 
Woodwasp larvae can take two to four years to complete their feeding, and fully fed woodwasp larvae pupate inside the tunnels they have made while feeding.
 
 
 
The family Xiphydriidae have nearly spherical heads on long "necks", and rather shorter ovipositers. There are two species in Britain,  Xiphydria prolongata larvae feed on willow species, and could be found in southern English gardens. The Alder wood wasp Xiphydria camelus feeds on alder which is less common in gardens.
 
Left: Red-belted Necked wood wasp Xiphydria prolongata
Elm zig-zag sawfly.  Left: characteristic damage from young larvae. Centre: late instar larva. Older larvae still eat between major veins, but don't leave the zig-zag pattern.  Right: pupal stage in silk basket.
 
 
Sawflies complete their life cycle within one year, with some species having two or three generations during the summer.    The females deposit eggs on or in their host plants and these hatch a few days or weeks later.  Female sawflies have an ovipositer formed from a pair of blade-like structures, sometimes with a saw-tooth edge, that they use to make incisions in leaf blades, petioles or stems when they are inserting eggs in the larval food plants. 
 
Some species when laying eggs also secrete chemicals into the plant that induce abnormal growth in the plant.  Rose leaf-rolling sawfly females cause the leaf margins of expanded rose leaflets to roll downwards to form tubes in which the larvae will subsequently feed.  Similarly Pontania species secrete chemicals that induce the growth of galls, shaped like baked beans, in willow leaves within which the larvae will feed.
 
Those of sawflies generally go into the soil but some bore into rotten wood or spin cocoons in sheltered places above ground level.  Most sawflies overwinter as non-feeding prepupal larvae that will pupate a few weeks before the adult insects emerge in spring or early summer.  A few sawflies overwinter in the egg stage.
 
Role of sawflies and woodwasps in gardens
 
Woodwasps are generally not a problem in gardens, although adults sometimes lay eggs in or emerge from pine stems used to make rustic pergolas and bird feeders.  
 
The majority of sawflies are not garden pests.  Their larvae feed as individuals on grasses, sedges, rushes, ferns, horsetails, wild flowers, forest trees and shrubs.  Sawfly larvae provide food for some birds and other insect feeders; they are also attacked by some parasitic wasps and flies.
 
Some sawflies can be damaging to garden plants, particularly those with larvae that can cause extensive defoliation.  Fortunately, defoliated plants will recover, although they may be less vigorous in the following year.  Heavy infestations do not occur every year.  A sawfly that caused extensive damage one year may be completely absent in the following year, possibly because the larvae devoured all the leaves before they could complete their feeding. 
 
It would be fair to say that the biology of sawflies in gardens is poorly known, and this is an areas where keen amateurs could make a significant contribution to science.
 
 
Other sources of information
 
Websites
 
British Sawflies Website    See also their garden survey page
Mike Hackston's key to Superfamilies of hymenoptera without a "waist"
Mike Hackston's key to Family Cimbicidae
Checklist of British and Irish Hymenoptera: Symphyta
 
Books
Benson, R. B. (1951) Handbooks for the Identification of British Insects Vol 6 part 2a. Hymenoptera – Symphyta
Benson, R. B. (1952) Handbooks for the Identification of British Insects Vol 6 part 2b. Hymenoptera – Symphyta
Benson, R. B. (1958) Handbooks for the Identification of British Insects Vol 6 part 2c. Hymenoptera – Symphyta
All the above are out of print but can be downloaded from here.
 
Wright, A. (1990) British Sawflies (Hymenoptera: Symphyta) A key to adults of the genera occurring in Britain.  A Field Studies Council AIDGAP key
 
By Andrew Halstead  reviewed by Andrew Salisbury edited by Steve Head
 
Photo: By Slimguy - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=62262115
Three garden sawflies.  Top left: Robinia sawfly Nematus tibialis 6-7mm
 
Top right: Large rose sawfly Arge panana 10mm
 
Left: Common pine sawfly  Diprion pini 10mm. This species is an economic pest on several pine species
Biology and life cycle
 
Some sawflies, such as the Robinia sawfly and the Aruncus sawfly, occur as females only and are able to reproduce without mating (parthenogenesis).  For some sawflies, such as the Common gooseberry sawfly, mating is optional.  They normally mate and lay eggs that will produce both male and female adults.  However, if a female remains unmated, she can lay unfertilised eggs that will hatch and produce larvae that become male adults.
 
Most sawfly larvae eat the foliage of their host plants, often consuming entire leaves, especially in those species where the larvae feed gregariously. In severe attacks plants can be completely defoliated.  Some sawfly larvae feed inside leaves as leaf miners, while those of slugworm sawflies graze away the upper or lower surface of leaves.  Hoplocampa species have larvae that feed inside the developing fruitlets of various trees and shrubs in the Rosaceae plant family, usually causing the damaged fruitlets to drop off before they are ripe. 
 
Gregarious sawfly larvae feed together, often along the margins of leaves.  When disturbed or threatened, these larvae sometimes simultaneously lash the air with their abdomens, causing their bodies to form a characteristic "S" shape.
 
Left: Unidentified sawfly larva showing the 7 pairs of prolegs that distinguish sawfly larvae from caterpillars, and the "S" shape some adopt.
 
Centre: Oak sawfly larva Caliroa annulipes
 
Right:  Berberis sawfly larva Arge berberidis
 
Top left:  Pear slugworm sawfly larva  Caliroa cerasi covered in predator-repelling green slime.  Top right:  Social pear sawfly Neurotoma saltuum larvae in their communal web on flowering cherry
 
 
Bottom left:  Willow leaf mining sawfly, Heterarthrus microcephalus pupal cocoon in mine. Bottom right: "Baked bean" galls of Pontania vesicator on willow
Symphyta: Sawflies and Woodwasps
 
By Andrew Halstead  reviewed by Andrew Salisbury edited by Steve Head
 
 
Sawflies and woodwasps are several Families grouped in the Sub-order Symphyta and are relatively primitive members of the Hymenoptera. They lack the constriction or ‘wasp waist’ between the abdomen and thorax of the other bees, wasps and ants.  They have two pairs of wings that are transparent, but may have dark markings in some species.  The antennae are thread-like in most species but sawflies in the Cimbicidae family have clubbed antennae.  Sawflies vary in size, with some of the leaf-mining species having adults that are no more than 3mm long, while others are up to about 15mm long.  Woodwasps are much larger and can be up to 30mm long, including the sting-like ovipositor of the female.  Adult sawflies could sometimes be mistaken for true flies, but they have two pairs of wings while the true flies have only a single fling pair.
 
The larvae can strongly resemble the caterpillars  of moths and butterflies.  Both groups of larvae have three pairs of jointed legs on the thorax, but sawfly larvae have seven pairs of suckers or prolegs on the abdomen, whereas moth larvae have five or fewer pairs of prolegs.  
 
 
Wood wasps  - Family Siricidae and Xiphydriidae
 
The wood wasps are characterised by a backward projecting spike from the dorsal part of the last segment on the abdomen.  Females also have a very long ovipositer which is used to inject eggs deep into the wood the larvae feed on. We have 5 species of woodwasps in Britain and Ireland, and the best known is the hornet-sized Giant woodwasp Urocerus gigas, which is harmless but must enjoy some protection from its mimicry of dangerous wasps and hornets
 
 
 
 
Symphyta: Sawflies and Woodwasps
 
Sawflies and woodwasps are several Families grouped in the Sub-order Symphyta and are relatively primitive members of the Hymenoptera. They lack the constriction or ‘wasp waist’ between the abdomen and thorax of the other bees, wasps and ants.  They have two pairs of wings that are transparent, but may have dark markings in some species.  The antennae are thread-like in most species but sawflies in the Cimbicidae family have clubbed antennae.  Sawflies vary in size, with some of the leaf-mining species having adults that are no more than 3mm long, while others are up to about 15mm long.  Woodwasps are much larger and can be up to 30mm long, including the sting-like ovipositor of the female.  Adult sawflies could sometimes be mistaken for true flies, but they have two pairs of wings while the true flies have only a single flying pair.
 
The larvae can strongly resemble the caterpillars  of moths and butterflies.  Both groups of larvae have three pairs of jointed legs on the thorax, but sawfly larvae have seven pairs of suckers or prolegs on the abdomen, whereas moth larvae have five or fewer pairs of prolegs.  
 
Wood wasps  - Family Siricidae and Xiphydriidae
 
The wood wasps are characterised by a backward projecting spike from the dorsal part of the last segment on the abdomen.  Females also have a very long ovipositer which is used to inject eggs deep into the wood the larvae feed on. We have 5 species of woodwasps in Britain and Ireland, and the best known is the hornet-sized Giant woodwasp Urocerus gigas, which is harmless but must enjoy some protection from its mimicry of dangerous wasps and hornets
 
 
 
 
The family Xiphydriidae have nearly spherical heads on long "necks", and rather shorter ovipositers. There are two species in Britain,  Xiphydria prolongata larvae feed on willow species, and could be found in southern English gardens. The Alder wood wasp Xiphydria camelus feeds on alder which is less common in gardens.
 
 
Giant woodwasp or horntail Urocerus gigas Females can be up to 36mm in total length, and look scary because of the (harmless) long ovipositer.
 
Note the two projections at the end of the abdomen.  The upper is the spike from the last segment which characterises the wood wasps and is found on both sexes. 
 
The much longer lower organ is the ovipositer, which has "teeth" like a saw and can drill into wood with a racheting movement.
Above: Red-belted Necked wood wasp Xiphydria prolongata
 
Woodwasps in the Siricidae family develop as larvae in the stumps, trunks and branches of recently dead or dying conifers; those in the Xiphydriidae family tunnel in the recently dead wood of various deciduous trees, including willow, alder and birch.  The European woodwasp Sirex noctilio has become a pest in many parts of the world where it is a significant cause of mortality in pine trees. When the female lays its eggs, it also adds a fungus inoculum  (Amylostereum areolatum) on which the young larvae feed, but which is pathogenic to the trees.
 
Woodwasp larvae can take two to four years to complete their feeding, and fully fed woodwasp larvae pupate inside the tunnels they have made while feeding.
 
 
 
Top: Siricid wood wasp Urocerus gigas larva 43mm long  Below: galleries excavated by the larvae of Sirex noctilio.
 
 
Sawflies 
 
The rest of the Symphyta are called sawflies, There are several families, of which much the biggest is the Tenthredinidae. The name "Saw"-fly comes from the big saw-toothed female ovipositer used to penetrate bark and wood for egg laying.
  
 
Species in Britain and Ireland
There are 540 species in Britain and Ireland.   They are much more likely to be seen in gardens than wood wasps, especially those species with larvae that feed on garden plants.    Jennifer Owen recorded 7,814 specimens of sawflies, from 87 species in 3 families - nearly all tenthredinids.
 
Garden species on fruit trees and bushes include Common gooseberry sawfly, Nematus ribesii, and Small gooseberry sawfly, Pristiphora appendiculata, both on gooseberry and red currant foliage; Apple sawfly, Hoplocampa testudinea, in apple fruitlets; Plum sawfly, Hoplocampa flava, in plum fruitlets; Pear and cherry slugworm, Caliroa cerasi, on the foliage of pear, cherry, plum, hawthorn and other Rosaceae trees and shrubs and  Hazel sawfly, Craesus septentrionalis, on the foliage of hazel, birch and alder.
 
Sawfly larvae that eat the foliage of ornamental plants include those of
  • Aquilegia sawfly, Pristiphora rufipes, on aquilegia
  • Iris sawfly, Rhadinoceraea micans, on yellow flag iris and other waterside irises
  • Solomon’s seal sawfly, Phymatocera aterrima on Solomon’s seal
  • Geranium sawfly, Ametastegia carpini, on hardy geraniums
  • Geum leaf-mining sawfly, Metallus lanceolatus, on Geum species
  • Aruncus sawfly, Nematus spiraeae, on Goats beard
  • Robinia sawfly, Nematus tibialis, on false acacia
  • Rose slugworm, Endelomyia aethiops, Rose leaf-rolling sawfly, Blennocampa phyllocolpa, and Large rose sawfly, Arge pagana, all on wild and cultivated roses
  • Berberis sawfly, Arge berberidis, on deciduous Berberis. 
 
Pine trees  in gardens can be partly defoliated by larvae of the pine sawflies, Diprion pini and Neodiprion sertifer.  Willows, Salix species, are the food plant of many sawflies, including Nematus pavidus, N. oligospilus, N. bergmanni and Pontania species.  The last mentioned cause hard red or yellowish green galls, known as bean galls, in the leaves.
 
 
 
 
Three garden sawflies.  Top left: Robinia sawfly Nematus tibialis 6-7mm
 
Top right: Large rose sawfly Arge panana 10mm
 
Left: Common pine sawfly  Diprion pini 10mm. This species is an economic pest on several pine species
Biology and life cycle
 
Some sawflies, such as the Robinia sawfly and the Aruncus sawfly, occur as females only and are able to reproduce without mating (parthenogenesis).  For some sawflies, such as the Common gooseberry sawfly, mating is optional.  They normally mate and lay eggs that will produce both male and female adults.  However, if a female remains unmated, she can lay unfertilised eggs that will hatch and produce larvae that become male adults.
 
Most sawfly larvae eat the foliage of their host plants, often consuming entire leaves, especially in those species where the larvae feed gregariously. In severe attacks plants can be completely defoliated.  Some sawfly larvae feed inside leaves as leaf miners, while those of slugworm sawflies graze away the upper or lower surface of leaves.  Hoplocampa species have larvae that feed inside the developing fruitlets of various trees and shrubs in the Rosaceae plant family, usually causing the damaged fruitlets to drop off before they are ripe. 
 
Gregarious sawfly larvae feed together, often along the margins of leaves.  When disturbed or threatened, these larvae sometimes simultaneously lash the air with their abdomens, causing their bodies to form a characteristic "S" shape.
 
Photo: By Slimguy - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=62262115
Top: Siricid wood wasp Urocerus gigas larva 43mm long  Below: galleries excavated by the larvae of Sirex noctilio.
 
Sawflies 
 
The rest of the Symphyta are called sawflies, There are several families, of which much the biggest is the Tenthredinidae. The name "Saw"-fly comes from the big saw-toothed female ovipositer used to penetrate bark and wood for egg laying. Some of the smallest sawflies have leaf-mining larvae.
  
Species in Britain and Ireland
There are 540 species in Britain and Ireland.   They are much more likely to be seen in gardens than wood wasps, especially those species with larvae that feed on garden plants.    Jennifer Owen recorded 7,814 specimens of sawflies, from 87 species in 3 families - nearly all tenthredinids.
 
Garden species on fruit trees and bushes include Common gooseberry sawfly, Nematus ribesii, and Small gooseberry sawfly, Pristiphora appendiculata, both on gooseberry and red currant foliage; Apple sawfly, Hoplocampa testudinea, in apple fruitlets; Plum sawfly, Hoplocampa flava, in plum fruitlets; Pear and cherry slugworm, Caliroa cerasi, on the foliage of pear, cherry, plum, hawthorn and other Rosaceae trees and shrubs and  Hazel sawfly, Craesus septentrionalis, on the foliage of hazel, birch and alder.
 
Sawfly larvae that eat the foliage of ornamental plants include those of
  • Aquilegia sawfly, Pristiphora rufipes, on aquilegia
  • Iris sawfly, Rhadinoceraea micans, on yellow flag iris and other waterside irises
  • Solomon’s seal sawfly, Phymatocera aterrima on Solomon’s seal
  • Geranium sawfly, Ametastegia carpini, on hardy geraniums
  • Geum leaf-mining sawfly, Metallus lanceolatus, on Geum species
  • Aruncus sawfly, Nematus spiraeae, on Goats beard
  • Robinia sawfly, Nematus tibialis, on false acacia
  • Rose slugworm, Endelomyia aethiops, Rose leaf-rolling sawfly, Blennocampa phyllocolpa, and Large rose sawfly, Arge pagana, all on wild and cultivated roses
  • Berberis sawfly, Arge berberidis, on deciduous Berberis. 
 
Pine trees  in gardens can be partly defoliated by larvae of the pine sawflies, Diprion pini and Neodiprion sertifer.  Willows, Salix species, are the food plant of many sawflies, including Nematus pavidus, N. oligospilus, N. bergmanni and Pontania species.  The last mentioned cause hard red or yellowish green galls, known as bean galls, in the leaves.
 
 
 
 
Top left: Unidentified sawfly larva showing the 7 pairs of prolegs that distinguish sawfly larvae from caterpillars, and the "S" shape some adopt.
 
Top right:  Oak sawfly larva Caliroa annulipes
 
Bottom right:  Berberis sawfly larva Arge berberidis
 
Elm zig-zag sawfly.  Left: characteristic damage from young larvae. Centre: late instar larva. Older larvae still eat between major veins, but don't leave the zig-zag pattern.  Right: pupal stage in silk basket.
 
 
Sawflies complete their life cycle within one year, with some species having two or three generations during the summer.    The females deposit eggs on or in their host plants and these hatch a few days or weeks later.  Female sawflies have an ovipositer formed from a pair of blade-like structures, sometimes with a saw-tooth edge, that they use to make incisions in leaf blades, petioles or stems when they are inserting eggs in the larval food plants. 
 
Some species when laying eggs also secrete chemicals into the plant that induce abnormal growth in the plant.  Rose leaf-rolling sawfly females cause the leaf margins of expanded rose leaflets to roll downwards to form tubes in which the larvae will subsequently feed.  Similarly Pontania species secrete chemicals that induce the growth of galls, shaped like baked beans, in willow leaves within which the larvae will feed.
 
Those of sawflies generally go into the soil but some bore into rotten wood or spin cocoons in sheltered places above ground level.  Most sawflies overwinter as non-feeding prepupal larvae that will pupate a few weeks before the adult insects emerge in spring or early summer.  A few sawflies overwinter in the egg stage.
 
Role of sawflies and woodwasps in gardens
 
Woodwasps are generally not a problem in gardens, although adults sometimes lay eggs in or emerge from pine stems used to make rustic pergolas and bird feeders.  
 
The majority of sawflies are not garden pests.  Their larvae feed as individuals on grasses, sedges, rushes, ferns, horsetails, wild flowers, forest trees and shrubs.  Sawfly larvae provide food for some birds and other insect feeders; they are also attacked by some parasitic wasps and flies.
 
Some sawflies can be damaging to garden plants, particularly those with larvae that can cause extensive defoliation.  Fortunately, defoliated plants will recover, although they may be less vigorous in the following year.  Heavy infestations do not occur every year.  A sawfly that caused extensive damage one year may be completely absent in the following year, possibly because the larvae devoured all the leaves before they could complete their feeding. 
 
It would be fair to say that the biology of sawflies in gardens is poorly known, and this is an areas where keen amateurs could make a significant contribution to science.
 
 
Other sources of information
 
Websites
 
British Sawflies Website    See also their garden survey page
Mike Hackston's key to Superfamilies of hymenoptera without a "waist"
Mike Hackston's key to Family Cimbicidae
Checklist of British and Irish Hymenoptera: Symphyta
 
Books
Benson, R. B. (1951) Handbooks for the Identification of British Insects Vol 6 part 2a. Hymenoptera – Symphyta
Benson, R. B. (1952) Handbooks for the Identification of British Insects Vol 6 part 2b. Hymenoptera – Symphyta
Benson, R. B. (1958) Handbooks for the Identification of British Insects Vol 6 part 2c. Hymenoptera – Symphyta
All the above are out of print but can be downloaded from here.
Wright, A. (1990) British Sawflies (Hymenoptera: Symphyta) A key to adults of the genera occurring in Britain.  A Field Studies Council AIDGAP key
 
Page drafted by Andrew Halstead, reviewed by Andrew Salisbury edited by Steve Head
Elm zig-zag sawfly
Recently the Elm zig-zag sawfly Aproceros leucopoda has caused some concerns among foresters. It is a parthenogenetic species - males have never been found - and has been spreading from its native China and Japan, probably helped by importation of plants.  It was first found in Europe in 2003, and in England in autumn 2017.  The sawfly is restricted to our three species of elms (Ulmus procera, U. glabra and U. minor) and can cause severe defoliation, but doesn't normally kill the trees.
 
The younger larvae cut a very characteristic zig-zag pattern in the leaves they eat, so its presence is easy to spot. It is unclear how much damage they will do to our elms, already severely reduced by Dutch elm disease.  There is more concern that they will compete with other elm-feeders, such as the already rare white-letter hairstreak butterfly (Satyrium w-album)
Top left:  Pear slugworm sawfly larva  Caliroa cerasi covered in predator-repelling green slime.  Top right:  Social pear sawfly Neurotoma saltuum larvae in their communal web on flowering cherry
 
Bottom left:  Willow leaf mining sawfly, Heterarthrus microcephalus pupal cocoon in mine. Bottom right: "Baked bean" galls of Pontania vesicator on willow
Elm zig-zag sawfly
Recently the Elm zig-zag sawfly Aproceros leucopoda has caused some concerns among foresters. It is a parthenogenetic species - males have never been found - and has been spreading from its native China and Japan, probably helped by importation of plants.  It was first found in Europe in 2003, and in England in autumn 2017.  The sawfly is restricted to our three species of elms (Ulmus procera, U. glabra and U. minor) and can cause severe defoliation, but doesn't normally kill the trees.
 
The younger larvae cut a very characteristic zig-zag pattern in the leaves they eat, so its presence is easy to spot. It is unclear how much damage they will do to our elms, already severely reduced by Dutch elm disease.  There is more concern that they will compete with other elm-feeders, such as the already rare white-letter hairstreak butterfly (Satyrium w-album)
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