INTERESTING INSECTS

By N A Martin (2015)

Willow sawfly

This adventive sawfly from the Northern Hemisphere was first found in New Zealand, in Auckland in February 1997. It is now present in the North and South islands. It feeds on most species of willow. Some trees may be defoliated. It is one of three species of willow feeding sawflies in New Zealand.

Conservation status: Widespread, a minor pest of some species of willow. It can weaken willow trees used to protect river banks.

The sawfly breeds from late spring to the autumn. It overwinters in its cocoons either as a larva or a pupa. There are up to 7 generations per year in North Island and up to 3 generations per year in the south of South Island. The willow sawfly overwinters in cocoons on fallen leaves or on the tree.

Adult wasps

The adults are about 8-10 mm long. They are pale yellow-brown with medium brown coloured areas on the dorsal (top) of the thorax (middle part of the body). The abdomen is greenish coloured. The head has black compound eyes and three black ocelli (single eyes). The underside of the head has a pair of sharp looking dark mandibles. The two pairs of wings and transparent with black veins and a yellow leading edge to the forewing. The two long antennae are brown while the three pairs of legs grade from yellowish basally to medium brown tarsi (feet). In New Zealand there are only female wasps, though in their homeland males are present.

Eggs and larvae

Female wasps lay single egg under the epidermis (skin) a leaf. The newly hatched larva chews a hole in the leaf away from the edge of the leaf and near the transparent egg shell. In the hole, the small larva feeds on the edge of the leaf. It is barely wider than the leaf thickness and can be easily missed. The larva gradually enlarges the hole. The hole may eventually encompass the edge of the leaf, or the larva may start chewing the leaf at a new place starting at the edge of the leaf. The larva is green. It has three pairs of true legs and seven pairs of abdominal prolegs (false legs) like caterpillars (larvae of moths and butterflies). Also like caterpillars, the sawfly larva has a head capsule. The head of the willow sawfly is white with brown markings, a triangular area above the jaws and a vertical stripe above each black eye. The sawfly larva has prominent jaws on underside of the head which are used for cutting through the willow leaf. The larva moults, or changes skin, as it gets larger. There are 5-7 larval stages (instars). The fully grown larva is about 12 mm long.

Differences between sawfly larvae and caterpillars (Lepidoptera larvae)

Caterpillars have fewer prolegs and caterpillar prolegs have crochets (tiny hooks), which are absent in sawfly larvae.

Cocoon and pupa

When the larva is fully grown it spins a pale brown oval cocoon on the underside of a leaf, on the tree or in the soil or litter. Inside the cocoon the larva becomes more compact before it pupates. The pupa is green and the form of the adult body including the legs and antennae are visible. The wings are represented by short wing buds. The adult emerges from the pupa inside the cocoon and at one end of the cocoon chews a hole through which it emerges.

In the summer it takes about 30-40 days from newly laid egg to adult emergence. Adults live for about 5 days.

Top view of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZR]

Underside of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZS]

Side view of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1605]

Recently hatched larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae), note the eggshell by the hole in the leaf. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZX]

Small larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZW]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1600]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1601]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1602]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1603]

Head of larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZY]

Head of larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1604]

Cocoon of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) on underside of leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZC]

Prepupal larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) exposed in cocoon. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZG]

Prepupal larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) exposed in cocoon. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZH]

Pupa of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) exposed in cocoon, note the brown moulted larval skin. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZI]

Pupa of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) exposed in cocoon, note the brown moulted larval skin. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZJ]

Pupa of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) removed from cocoon. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZN]

Cocoon of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) with exit hole made by adult wasp. Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZF]

There are three species of willow sawflies in New Zealand. In addition to the willow sawfly, Nematus oligospilus, there is the willow gall fly, Pontania proxima (Lepeletier, 1823), that causes oval leaf galls and the willow shoot sawfly, Amauronematus viduatus (Zetterstedt, 1838), whose larvae start life in a shoot tip leaf fold gall. Adults are similar in appearance to the willow sawfly, but darker.

The willow sawfly adults are medium sized pale brown adult wasps. Those of the willow shoot sawfly, Amauronematus viduatus, are similar in size, but darker colour. Adults of the willow gall sawfly, Pontania proxima, are smaller, black and have a distinct waist. In New Zealand there are two other species of sawfly that live on other plants.

The green caterpillar-like larvae of the willow sawfly are associated with holes in the centre of willow leaves and chewed edges of leaves. They can be difficult to see when their body is closely applied to the leaf edge, but are more obvious when they lift their abdomen up. The head capsule of the larva has a prominent dark strip above the eye. The larva of the willow shoot sawfly, Amauronematus viduatus lacks this stripe.

Top view of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 160A]

Underside of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1609]

Side view of adult female willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 160B]

Small larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZV]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1601]

Two cocoons of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) on underside of leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZB]

Cocoon of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) on underside of leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZD]

No pathogens of the willow sawfly are known in New Zealand.

Parasitoids

One parasitoid has been reared from cocoons of willow sawfly. Adult females of a wasp, 'Gelis' tenellus (Say, 1836) (Hymenoptera: Ichneumonidae) lay an egg on a larva in its cocoon. The wasp larva kills the sawfly and the fully grown wasp larva pupates in the sawfly cocoon. The adult wasp chews its way out of the cocoon.

Predators

Adult Chinese paper wasps, Polistes chinensis (Hymenoptera: Vespidae), were observed feeding on willow sawfly larvae. It is likely that adults and larvae are preyed upon by birds, spiders and other predatory insects. Sawfly larvae living on riverside willows are probably eaten by trout if they drop into the water.

The willow sawfly feeds on a selection of willow species including crack willow, a pest of waterways, and ornamental species such as weeping willow and tortured willow. The larvae make holes in leaves and can completely eat a leaf.

Small larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 15ZW]

Larva of willow sawfly, Nematus oligospilus (Hymenoptera: Tenthredinidae) feeding on leaf of weeping willow, Salix babylonica (Salicaceae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1601]

Willow sawflies in New Zealand

There are three species of willow sawflies in New Zealand. The willow gall sawfly, Pontania proxima (Lepeletier, 1823), causes oval galls on willow leaves in which the larvae live. This European species was first found in Canterbury in 1929. The next to arrive was the willow sawfly, Nematus oligospilus Forster, 1854, which was first found in New Zealand, in Auckland in February 1997. This species has a free living larva that makes holes in leaves. The last species to be discovered was Amauronematus viduatus (Zetterstedt, 1838), another northern hemisphere species that is also in Australia. It was first found in Auckland 2009 as part of MAF Biosecurity surveillance for unwanted organisms. The larvae first live in leaf fold galls at the shoot tip. Older larvae are free living and make holes in leaves. There is one generation per year.

Berry JA 1997. Nematus oligospilus (Hymenoptera: Tenthredinidae), a recently introduced sawfly defoliating willows. New Zealand Entomologist 20: 51-54.

Charles JG, Allan DJ 2000. Development of the willow sawfly, Nematus oligospilus, at different temperatures, and an estimation of voltinism throughout New Zealand. New Zealand Journal of Zoology 27: 197-200.

Charles JG, McIvor I, Hurst S 2004a. Willow Sawfly, Nematus oligospilus Part III. Willow sawfly research. Leaflet produced by HortResearch, Sustainable Farming Fund: 1-2.

Charles JG, McIvor I, Hurst S 2004b. Willow Sawfly, Nematus oligospilus Part II: How can we combat the willow sawfly threat. . Leaflet produced by HortResearch, Sustainable Farming Fund: 1-2.

Charles JG, McIvor I, Hurst S 2004c. Willow Sawfly, Nematus oligospilus Part I: A threat to our willows. Leaflet produced by HortResearch, Sustainable Farming Fund: 1-2.

Charles JG, Allan DJ, Froud KJ, Fung LE 1999. A Guide to Willow Sawfly (Nematus oligospilus) in New Zealand. hortnet.co.nz/publications/guides/willow_sawfly/wsawfly.htm

Plant-SyNZ: Invertebrate herbivore-host plant association database. plant-synz.landcareresearch.co.nz/.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.

John Charles for information about the sawfly.