| Sawfly | |
|---|---|
 | |
| Tenthredo mesomela | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hymenoptera |
| Suborder: | Symphyta Gerstaecker, 1867[1] |
| Groups included | |
| Cladistically included but traditionally excluded taxa | |
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Sawflies arewasp-like insects that are in thesuborderSymphyta within the orderHymenoptera. Thecommon name comes from the saw-like appearance of theovipositor, which females use to cut into the plants where they lay their eggs. The name is associated especially with theTenthredinoidea, by far the largestsuperfamily in the suborder, with about 7,000 known species in the entire suborder. There are 8,000 described species in more than 800 genera.
Despite their given taxonomic rank of suborder, Symphyta is aparaphyletic group, consisting of severalbasal groups within the order Hymenoptera, with each one rooted inside the previous group; the various superfamilies of insects called "sawfly" do form a natural group orclade, but this clade also includes theApocrita – theants,bees, and wasps – which are not considered sawflies. The primary distinction between sawflies and the Apocrita is that the adult sawflies (theimago) lack a "wasp waist" orpetiole; the segments connecting theabdomen and thethorax smoothly transition between the two (leading to their scientific name). Sawflies first appeared 250 million years ago in theTriassic. The oldest superfamily, theXyeloidea, is still extant today. Over 200 million years ago, a lineage of sawflies evolved aparasitoid lifestyle, with carnivorouslarvae that ate the eggs or larvae of other insects. Sawflies aredistributed globally, though they are more diverse in the northernmost hemispheres.
Some sawflies areBatesian mimics of wasps and bees, and the ovipositor can be mistaken for astinger. Sawflies vary in length, most measuring 2.5 to 20 millimetres (3⁄32 to25⁄32 in); the largest known sawfly measured55 mm (2+1⁄4 in). The great majority of sawflies areherbivorous, though the members of the superfamilyOrussoidea are parasitoids. Their predators include birds, small mammals such asshrews, and other insects, such asDipterans and other Hymenopterans, which themselves may be parasitoids of sawflies. The larvae of some species haveanti-predator adaptations such as regurgitating irritating liquid and clustering together for safety in numbers.
The herbivorous larvae are often confused withcaterpillars, but can be distinguished from these by the number ofprolegs and the absence ofcrochets. Sawflies go through acomplete metamorphosis with four distinct life stages – egg, larva, pupa and adult. Adult sawflies are short-lived, with a life expectancy of 7–9 days, though the larval stage can last from months to years, depending on the species.Parthenogenetic females, which do not need to mate to produce fertilised eggs, are common in the suborder, though many species have males and practicesexual reproduction. The adults feed on pollen, nectar, honeydew, sap, other insects, includinghemolymph of the larvae hosts; they have mouth pieces adapted to these types of feeding.[3] The female uses her ovipositor to drill into plant material (or, in the case of Orussoidea, other insects) and then lays eggs in groups called rafts or pods. As the larvae approach adulthood, they seek a protected spot to pupate, typically inbark or the soil.
Large populations of species such as thepine sawfly can cause substantial damage to economicforestry, while others such as theiris sawfly are major pests inhorticulture. Outbreaks of sawfly larvae can defoliate trees and may causedieback, stunting or death. Sawflies can be controlled through the use of insecticides, natural predators and parasitoids, or mechanical methods.
The suborder name "Symphyta" derives from the Greek wordsymphyton, meaning 'grown together', referring to the group's distinctive lack of a wasp waist between prostomium and peristomium.[4] Its common name, "sawfly", derives from thesaw-like ovipositor that is used for egg-laying, in which a female makes a slit in either a stem or plant leaf to deposit the eggs.[5] The first known use of this name was in 1773.[6] Sawflies are also known as "wood-wasps".[7]
In his original description ofHymenoptera in 1863, German zoologistCarl Gerstaecker divided them into three groups,Hymenoptera aculeata,Hymenoptera apocrita andHymenoptera phytophaga.[8] However, four years later in 1867, he described just two groups,H. apocritasyn.genuina andH. symphyta syn.phytophaga.[1] Consequently, the name Symphyta is given to Gerstaecker as thezoological authority. In his description, Gerstaecker distinguished the two groups by the transfer of the first abdominal segment to the thorax in theApocrita, compared to the Symphyta. Consequently, there are only eight dorsal half segments in the Apocrita, against nine in the Symphyta. The larvae are distinguished in a similar way.[9]
The Symphyta have therefore traditionally been considered, alongside the Apocrita, to form one of twosuborders of Hymenoptera.[10][11] Symphyta are the more primitive group, with comparatively completevenation, larvae that are largelyphytophagous, and without a "wasp-waist", asymplesiomorphic feature. Together, the Symphyta make up less than 10% of hymenopteran species.[12] While the terms sawfly and Symphyta have been used synonymously, the Symphyta have also been divided into three groups, true sawflies (phyllophaga),woodwasps or xylophaga (Siricidae), andOrussidae. The three groupings have been distinguished by the true sawflies' ventral serrated or saw-like ovipositor for sawing holes in vegetation to deposit eggs, while the woodwasp ovipositor penetrates wood and the Orussidae behave as externalparasitoids ofwood-boringbeetles. The woodwasps themselves are aparaphyletic ancestralgrade. Despite these limitations, the terms have utility and are common in the literature.[11]
While most hymenopteran superfamilies aremonophyletic, as is Hymenoptera, the Symphyta has long been seen to be paraphyletic.[13][14]Cladistic methods andmolecular phylogenetics are improving the understanding of relationships between the superfamilies, resulting in revisions at the level of superfamily and family.[15] The Symphyta are the most primitive (basal)taxa within the Hymenoptera (some going back 250 million years), and one of the taxa within the Symphyta gave rise to the monophyletic suborder Apocrita (wasps,bees, andants).[12][14] In cladistic analyses the Orussoidea are consistently thesister group to the Apocrita.[11][12]
The oldest unambiguous sawfly fossils date back to theMiddle orLate Triassic. These fossils, from the familyXyelidae, are the oldest of all Hymenoptera.[16] One fossil,Archexyela ipswichensis from Queensland is between 205.6 and 221.5 million years of age, making it among the oldest of all sawfly fossils.[17] More Xyelid fossils have been discovered from theMiddle Jurassic and theCretaceous, but the family was less diverse then than during theMesozoic andTertiary. The subfamilyXyelinae were plentiful during these time periods, in which Tertiary faunas were dominated by the tribe Xyelini; these are indicative of a humid and warm climate.[18][19][20]
The cladogram is based on Schulmeister 2003.[21][22]
| Symphyta within Hymenoptera | |||||||||||||||||||||||||||||||||||||||||
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| Symphyta (red bar) are paraphyletic as Apocrita are excluded. |
There are approximately 8,000 species of sawfly in more than 800 genera, although new species continue to be discovered.[23][24][25] However, earlier studies indicated that 10,000 species grouped into about 1,000 genera were known.[26] Earlyphylogenies such as that ofAlexandr Rasnitsyn, based on morphology and behaviour, identified nineclades which did not reflect the historical superfamilies.[27] Such classifications were replaced by those using molecular methods, starting with Dowton and Austin (1994).[28] As of 2013, the Symphyta are treated as ninesuperfamilies (one extinct) and 25 families. Most sawflies belong to the Tenthredinoidea superfamily, with about 7,000 species worldwide. Tenthredinoidea has six families, of whichTenthredinidae is by far the largest with some 5,500 species.[2][29]
Extinct taxa are indicated by adagger (†).
Many species of sawfly have retained their ancestral attributes throughout time, specifically their plant-eating habits, wing veins and the unmodified abdomen, where the first two segments appear like the succeeding segments.[30] The absence of the narrow wasp waist distinguishes sawflies from other members of hymenoptera, although some areBatesian mimics withcoloration similar to wasps and bees, and the ovipositor can be mistaken for a stinger.[31] Most sawflies are stubby and soft-bodied, and fly weakly.[32] Sawflies vary in length:Urocerus gigas, which can be mistaken as a wasp due to its black-and-yellow striped body, can grow up to20 mm (3⁄4 in) in length, but among the largest sawflies ever discovered wasHoplitolyda duolunica from the Mesozoic, with a body length of55 mm (2+1⁄4 in) and a wingspan of92 mm (3+1⁄2 in).[31][33] The smaller species only reach lengths of2.5 mm (3⁄32 in).[34]
Heads of sawflies vary in size, shape and sturdiness, as well as the positions of the eyes and antennae. They are characterised in four head types: open head, maxapontal head, closed head and genapontal head. The open head is simplistic, whereas all the other heads are derived.[35] The head is also hypognathous, meaning that the lower mouthparts are directed downwards. When in use, the mouthparts may be directed forwards, but this is only caused when the sawfly swings its entire head forward in a pendulum motion.[36] Unlike most primitive insects, thesutures (rigid joints between two or more hard elements on an organism) andsclerites (hardened body parts) are obsolescent or absent. Theclypeus (a sclerite that makes up an insects "face") is not divided into a pre- and postclypeus, but rather separated from the front.[37] The antennal sclerites are fused with the surrounding head capsule, but these are sometimes separated by a suture. The number of segments in theantennae vary from six in the Accorduleceridae to 30 or more in the Pamphiliidae.[38] Thecompound eyes are large with a number of facets, and there are threeocelli between the dorsal portions of the compound eyes.[37] Thetentorium comprises the whole inner skeleton of the head.[39]
Three segments make up the thorax: themesothorax,metathorax andprothorax, as well as the exoskeletal plates that connect with these segments.[40] The legs have spurs on their fourth segments, the tibiae.[41] Sawflies have two pairs oftranslucent wings. The fore and hind wings are locked together with hooks.[42] Parallel development in sawfly wings is most frequent in the anal veins. In all sawflies, 2A and 3A tend to fuse with the first anal vein. This occurs in several families including Argidae, Diprionidae and Cimbicidae.[43]
The larvae of sawflies are easily mistaken forlepidopteran larvae (caterpillars). However, several morphological differences can distinguish the two: while both larvae share three pairs of thoracic legs and an apical pair of abdominalprolegs, lepidopteran caterpillars have four pairs of prolegs on abdominal segments 3–6 while sawfly larvae have five pairs of prolegs located on abdominal segments 2–6;crochets are present on lepidopteran larvae, whereas on sawfly larvae they are not; the prolegs of both larvae gradually disappear by the time they burrow into the ground, therefore making it difficult to distinguish the two; and sawfly larvae only have a single pair of minute eyes, whereas lepidopteran larvae have four to six eyes on each side of the head.[10][31] Sawfly larvae behave like lepidopteran larvae, walking about and eating foliage. Some groups have larvae that are eyeless and almost legless; these larvae make tunnels in plant tissues including wood.[32] Many species of sawfly larvae are strikingly coloured, exhibiting colour combinations such as black and white while others are black and yellow. This is awarning colouration because some larvae can secrete irritating fluids from glands located on their undersides.[31]
Sawflies are widely distributed throughout the world.[44] The largest family, the Tenthredinidae, with some 5,000 species, are found on all continents except Antarctica, though they are most abundant and diverse in the temperate regions of the northern hemisphere; they are absent from New Zealand and there are few of them in Australia. The next largest family, theArgidae, with some 800 species, is also worldwide, but is most common in the tropics, especially in Africa, where they feed on woody and herbaceousangiosperms. Of the other families, theBlasticotomidae andMegalodontesidae arePalearctic; the Xyelidae,Pamphilidae,Diprionidae,Cimbicidae, andCephidae areHolarctic, while theSiricidae are mainly Holarctic with some tropical species. The parasitic Orussidae are found worldwide, mostly in tropical and subtropical regions. The wood-boringXiphydriidae are worldwide, but most species live in the subtropical parts of Asia.[23]
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Sawflies are mostlyherbivores, feeding on plants that have a high concentration of chemical defences. These insects are either resistant to the chemical substances, or they avoid areas of the plant that have high concentrations of chemicals.[45] The larvae primarily feed in groups; they arefolivores, eating plants and fruits on native trees and shrubs, though some are parasitic.[5][46][47] However, this is not always the case; Monterey pine sawfly (Itycorsia) larvae are solitary web-spinners that feed onMonterey pine trees inside a silken web.[48] The adults feed on pollen and nectar.[46]
Sawflies are eaten by a wide variety of predators. While many birds find the larvae distasteful, some such as thecurrawong (Strepera) andstonechats (Saxicola) eat both adults and larvae.[49][50] The larvae are an important food source for the chicks of several birds, includingpartridges.[51] Sawfly and moth larvae form one third of the diet of nestlingcorn buntings (Emberiza calandra), with sawfly larvae being eaten more frequently on cool days.[52]Black grouse (Tetrao tetrix) chicks show a strong preference for sawfly larvae.[53][54] Sawfly larvae formed 43% of the diet ofchestnut-backed chickadees (Poecile rufescens).[48] Small carnivorous mammals such as themasked shrew (Sorex cinereus), thenorthern short-tailed shrew (Blarina brevicauda) and thedeer mouse (Peromyscus maniculatus) predate heavily on sawfly cocoons.[55] Insects such as ants and certain species of predatory wasps (Vespula vulgaris) eat adult sawflies and the larvae, as do lizards and frogs.[56][57]Pardalotes,honeyeaters andfantails (Rhipidura) occasionally consume laid eggs, and several species of beetle larvae prey on the pupae.[50]
The larvae have severalanti-predator adaptations. While adults are unable to sting, the larvae of species such as thespitfire sawfly regurgitate a distasteful irritating liquid, which makes predators such as ants avoid the larvae.[5][58] In some species, the larvae cluster together, reducing their chances of being killed, and in some cases form together with their heads pointing outwards or tap their abdomens up and down.[50][59] Some adults bear black and yellow markings that mimic wasps.[31]
Sawflies are hosts to many parasitoids, most of which are parasitic Hymenoptera; more than 40 species are known to attack them. However, information regarding these species is minimal, and fewer than 10 of these species actually cause a significant impact on sawfly populations.[60] Many of these species attack their hosts in the grass or in other parasitoids.[clarification needed] Well known and important parasitoids includeBraconidae,Eulophidae andIchneumonidae. Braconid wasps attack sawflies in many regions throughout the world, in which they are ectoparasitoids, meaning that the larvae live and feed outside of the hosts body; braconids have more of an impact on sawfly populations in the New World than they do in the Old World, possibly because there are no ichneumonid parasitoids in North America. Some braconid wasps that attack sawflies includeBracon cephi,B. lisogaster,B. terabeila andHeteropilus cephi.[60][61][62] Female braconids locate sawfly larvae through the vibrations they produce when feeding, followed by inserting the ovipositor and paralysing the larva before laying eggs inside the host. These eggs hatch inside the larva within a few days, where they feed on the host. The entire host's body may be consumed by the braconid larvae, except for the head capsule andepidermis. The larvae complete their development within two or three weeks.[60]
Ten species of wasps in the family Ichneumonidae attack sawfly populations, although these species are usually rare. The most important parasitoids in this family are species in the genusCollyria. Unlike braconids, the larvae are endoparasitoids, meaning that the larvae live and feed inside the hosts body.[60] One well known ichneumonid isCollyria coxator, which is a dominant parasitoid ofC. pygmaeus. Recorded parasitism rates in Europe are between 20–76%, and as many as eight eggs can be found in a single larva, but only oneCollyria individual will emerge from its host. The larva may remain inside of their host until spring, where it emerges and pupates.[60]
Several species in the family Eulophidae attack sawflies, although their impact is low. Two species in the genusPediobius have been studied; the two species are internal larval parasitoids and have only been found in the northern hemisphere. Parasitism of sawflies by eulophids in grass exceeds 50%, but only 5% in wheat. It is unknown as to why the attack rate in wheat is low.[63] Furthermore, some fungal and bacterial diseases are known to infect eggs and pupa in warm wet weather.[50]
Outbreaks of certain sawfly species, such asDiprion polytomum, have led scientists to investigate and possibly collect their natural enemies to control them. Parasites ofD. polytomum have been extensively investigated, showing that 31 species of hymenopterous and dipterous parasites attack it. These parasites have been used in successful biological control against pest sawflies, includingCephus cinctus throughout the 1930s and 1950s andC. pygmaeus in the 1930s and 1940s.[64][65]
Like all other hymenopteran insects, sawflies go through acomplete metamorphosis with four distinct life stages – egg, larva, pupa and adult.[66] Many species areparthenogenetic, meaning that females do not need fertilization to create viable eggs. Unfertilized eggs develop as male, while fertilized eggs develop into females (arrhenotoky). The lifespan of an individual sawfly is two months to two years, though the adult life stage is often very short (approximately 7 – 9 days), only long enough for the females to lay their eggs.[31][50][67] The female uses its ovipositor to drill into plant material to lay her eggs (though the familyOrussoidea lay their eggs in other insects). Plant-eating sawflies most commonly are associated with leafy material but some specialize on wood, and the ovipositors of these species (such as the familySiricidae) are specially adapted for the task of drilling through bark. Once the incision has been made, the female will lay as many as 30 to 90 eggs. Females avoid the shade when laying their eggs because the larvae develop much slower and may not even survive, and they may not also survive if they are laid on immature and glaucous leaves. Hence, female sawflies search for young adult leaves to lay their eggs on.[31][50]
These eggs hatch in two to eight weeks, but such duration varies by species and also by temperature. Until the eggs have hatched, some species such as the small brown sawfly will remain with them and protects the eggs by buzzing loudly and beating her wings to deter predators. There are six larval stages that sawflies go through, lasting 2 – 4 months, but this also depends on the species. When fully grown, the larvae emerge from the trees en masse and burrow themselves into the soil to pupate. During their time outside, the larvae may link up to form a large colony if many other individuals are present. They gather in large groups during the day which gives them protection from potential enemies, and during the night they disperse to feed. The emergence of adults takes awhile, with some emerging anywhere between a couple months to 2 years. Some will reach the ground to form pupal chambers, but others may spin a cocoon attached to a leaf. Larvae that feed on wood will pupate in the tunnels they have constructed. In one species, thejumping-disc sawfly (Phyllotoma aceris) forms a cocoon which can act like a parachute. The larvae live in sycamore trees and do not damage the upper or lower cuticles of leaves that they feed on. When fully developed, they cut smallperforations in the upper cuticle to form a circle. After this, they weave a silkhammocks within the circle; this silk hammock never touches the lower cuticle. Once inside, the upper-cuticle's disc separates and descends towards the surface with the larvae attaching themselves to the hammock. Once they reach the round, the larvae work their way into a sheltered area by jerking their discs along.[31][50]
The majority of sawfly species produce a single generation per year, but others may only have one generation every two years. Most sawflies are female, making males rare.[50]
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Sawflies are major economic pests offorestry. Species in the Diprionidae, such as the pine sawflies,Diprion pini andNeodiprion sertifer, cause serious damage topines in regions such as Scandinavia.D. pini larvae defoliated 500,000 hectares (1,200,000 acres) in the largest outbreak in Finland, between 1998 and 2001. Up to 75% of the trees may die after such outbreaks, asD. pini can remove all the leaves late in the growing season, leaving the trees too weak to survive the winter.[68] Little damage to trees only occurs when the tree is large or when there is minimal presence of larvae.Eucalyptus trees can regenerate quickly from damage inflicted by the larvae; however, they can be substantially damaged from outbreaks, especially if they are young. The trees can be defoliated completely and may cause "dieback", stunting or even death.[50]
Sawflies are serious pests inhorticulture. Different species prefer different host plants, often being specific to a family or genus of hosts. For example,Iris sawfly larvae, emerging in summer, can quickly defoliate species ofIris including theyellow flag and other freshwater species.[69] Similarly the rose sawflies,Arge pagana andA. ochropus, defoliate rose bushes.[70]
The giant woodwasp or horntail,Urocerus gigas, has a long ovipositor, which with its black and yellow colouration make it a good mimic of ahornet. Despite the alarming appearance, the insect cannot sting.[71] The eggs are laid in the wood ofconifers such asDouglas fir, pine,spruce, andlarch. The larvae eat tunnels in the wood, causing economic damage.[72]
Alternative measures to control sawflies can be taken. Small-scale, mechanical methods include visually confirming larval presence on a plant and subsequently removing them, either by pruning damaged leaves or removing the larvae from the leaves they are on. Larvae typically try to remain hidden on the underside of foliage. Upon removing larvae and/or the affected leaves from plants, they may be dispatched by squishing, or, alternatively, the cut leaves with larvae still attached may be fed to birds; if larger animals do not prey upon them, other insects will. However, this is not practical or useful for some, thus the larvae can be quickly dispatched by simply dropping foliage into a vessel of plain or saltwater, diluted hydrogen peroxide or isopropyl alcohol, insecticidal soap, or other garden chemical. In large-scale, industrial settings, where beneficial insect predators can also be used to eliminate larvae, as well as parasites, which have both been previously used in control programs.[50][64] Small trees can be sprayed with a number of chemicals, includingmaldison,dimethoate,carbaryl,imidacloprid, etc., if removing larvae from trees is not effective enough.[50]
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