''Nematus oligospilus'', commonly known as the willow sawfly, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

sawfly Sawflies are the insects of the suborder Symphyta within the order Hymenoptera, alongside ants, bees, and wasps. The common name comes from the saw-like appearance of the ovipositor, which the females use to cut into the plants where they lay ...

in the family

Tenthredinidae Tenthredinidae is the largest family of sawflies, with well over 7,500 species worldwide, divided into 430 genera. Larvae are herbivores and typically feed on the foliage of trees and shrubs, with occasional exceptions that are leaf miners, stem ...

. Native to central and northern Europe and Asia, it was first recorded in South America in the 1980s and New Zealand in 1997, and has also been introduced to Australia, South Africa and Lesotho. Its

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e feed on the leaves of various species of

willow Willows, also called sallows and osiers, from the genus ''Salix'', comprise around 400 speciesMabberley, D.J. 1997. The Plant Book, Cambridge University Press #2: Cambridge. of typically deciduous trees and shrubs, found primarily on moist s ...

Description

An adult is between long. It is a slender insect with two pairs of membranous wings with black veins, the fore-wings having a yellow leading edge. The head bears a pair of long antennae, a pair of black compound eyes and three

ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

. The sides of the thorax are pale yellowish-brown while the dorsal surface is mid-brown. The abdomen is greenish and the legs are yellowish-brown at the base and darker brown near the clawed tips. The larva resembles a

lepidoptera Lepidoptera ( ) is an order (biology), order of insects that includes butterfly, butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 Family (biology), families and 46 Taxonomic r ...

n caterpillar; it has a white head with brown markings, a green body, three pairs of true legs near the front and seven pairs of pro-legs behind.

Distribution

The willow sawfly is widespread in the

Holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical region ...

region and has spread to other parts of the world. It was recorded in South America for the first time in the 1980s, in Argentina and Chile where it caused severe defoliation of various species of willow ''(Salix)''. In 2017 it was recorded in Colombia on the ornamental willow ''

Salix humboldtiana ''Salix humboldtiana'', called Humboldt's willow, is a tree species of willow native to North and South America, growing along watercourses. Some authorities consider it a synonym of '' Salix chilensis'', which Molina described in 1782. Willdeno ...

''. It had reached New Zealand by 1997 and was also present around this time in Australia. In both of these countries it spread rapidly, with the larvae feeding on species of willow, which are not native plants in Australia and New Zealand. It has also been found in South Africa and Lesotho.

Life cycle

Eggs are laid on the leaves of the host tree, and the developing larvae feed on the leaf tissue. Larvae have a variable number of

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...

stages and fall to the ground when fully developed. They make their way underground and overwinter as non-feeding prepupae in the cocoons they form. However, in Colombia it was found that larvae remain on the tree and form their cocoons there; these are green and easily overlooked among the foliage. The prepupae pupate in the spring, the adults emerging when new foliage is available. Adults each lay about thirty eggs soon after emerging. In South America, all adults are female and reproduction is by

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and development ...

. The same is true in other parts of the Southern Hemisphere where it has been introduced, but in its native range in the Northern Hemisphere, both males and females occur. Shifting from sexual to parthenogenetic reproduction is fairly common in

invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...

and offers several advantages. The small number of individuals initially introduced into a new habitat would make it difficult for sexual partners to find each other. Also, a population that is all female can potentially expand at twice the maximum rate of a population with two sexes, allowing an invasive species to establish itself more rapidly. Sexual reproduction with its usual advantage of

genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...

is not as favored in invasive populations, which lack predators.

References

{{Taxonbar, from=Q14582407 Tenthredinidae Insects described in 1854 Unisexual animals