Willow gall sawfly
‘‘WHAT are the red lumps on these willow leaves?’’ asked a Dunedin man at the museum recently.
His willow leaves bore galls of the willow gall sawfly, Pontania
proxima (Lepeletier, 1823) (Hymenoptera: Tenthredinidae), common throughout New Zealand.
The female is 3.5mm to 5mm long, shiny, black and wasplike. She lays eggs singly into leaves, at the same time inserting a secretion that causes plant cell multiplication. This produces a gall around each egg. The beanshaped galls are pinkish green or reddish. They measure 8mm long by 4mm wide and protrude 2.5mm above both the upper and lower leaf surfaces. Eggs hatch in 9 to 12 days and the five larval stages are completed in a little over two weeks. The larva is a fully legged caterpillar. It feeds internally on the gall and grows to 6mm long. This caterpillar has three pairs of thoracic legs and six pairs of abdominal prolegs, unlike caterpillars of moths and butterflies, which never have more than five pairs of prolegs.
The halfgrown larva makes a hole in the gall through which it periodically ejects faeces. When fully grown, the larva exits from the gall and spins a tough brown cocoon for its pupa. The cocoon remains on the underside of the leaf, in a crevice on the trunk or on the ground.
There are two or more generations a year. The winter generation overwinters in the cocoon. Reproduction is by parthenogenesis: females lay eggs that produce female offspring. Males are very rare and are presumed to be nonfunctional.
Galls are made on leaves of the weeping willow, Salix
babylonica, and the crack willow, S. fragilis. The willow gall sawfly is native to Europe and western Asia and was first found in New Zealand in 1929.
Two other European willow sawflies occur in New Zealand. The willow sawfly Nematus
oligospilus (Forster, 1854), came in 1997. This species does not make galls, its freeliving larva biting holes in leaves.
Amauronematus viduatus
(Zetterstedt, 1838) arrived in 2009. Its young larvae make leaffold galls in shoot tips, but older larvae are free living and chew holes in leaves.