Nematus

Taxonomy

Family: Tenthredinidae
Family common name: common sawflies
Subfamily: Nematinae
Tribe: Nematini
Genus: Nematus Panzer, 1801
Subgenera: none

Background

The Tenthredinidae are the most species-rich family and are found throughout the world, in all continents but Antarctica. They are known as the “common sawflies.” They can generally be recognized by a cylindrical body and long, segmented antennae. Otherwise, they come in a variety of colors, sizes, and forms (Goulet 1992).

Nematinae is the second-largest subfamily of Tenthredinidae, with over 1,250 species (Prous et al. 2014). They are most diverse in northern Eurasia and North America; only a few species occur in the Southern Hemisphere. Nematinae sawflies have a variety of feeding habits including external leaf feeding, leaf mining, and gall forming, and feed on a variety of hosts (Smith 2003b).

The Nematinae have been subject to numerous revisions in recent years. As of 2021, there are no comprehensive keys to many of the North American species of Nematinae (Prous et al. 2014). Because of changing taxonomy and extreme variability in morphology, identifying genera and species in the Nematinae may be more challenging than in other subfamilies of Tenthredindae. For this reason, knowing the host or behaviors of a specimen can be extremely helpful for identification within this subfamily.

Nematus is a species-rich and widespread genus. In recent years, the genus Craesus was synonymized with Nematus, and many species have been transferred to the genus Euura. As presently defined, Nematus is somewhat difficult to key based on morphology, and will require further species-level revisions in the future (Prous et al. 2014).

Nematus have a variety of colors and forms, but are generally between 3–12 mm in length and mostly black. Five species have distinctly wide hind tarsi (Prous et al. 2014).

Diversity

There are 127 described species worldwide. Twenty species occur in North America (Taeger et al. 2018).

Incomplete keys to groups of Nematus species are included in Smith 1972b (as Croesus) and in Smith 2008a (abbotii and erythrogaster groups).

Diagnostic characteristics

Subfamily characters

Genus characters

There are several forms represented in Nematus, so many characters only apply to some species:‚Äč

May be confused with

Nematus has several shared morphological characters with Euura and Pristiphora, and in some cases cannot be easily distinguished. Together, these three genera can usually be distinguished from other Nematinae by the lack of fore wing vein 2r-rs, incomplete vein 2A+3A, and asymmetrical outer surfaces of the mandibles. Euura and Nematus can usually be distinguished from Pristiphora by the evenly tapering ovipositor sheath, long subapical tooth of the tarsal claw, emarginated clypeus, and fore wing vein C not enlarged apically (Prous et al. 2014). Some species of Nematus are easily recognized by the distinctly widened hind tarsus (Prous et al. 2014)

Exotic pest species of concern

Nematus prunivorous is a bivoltine, serious pest of apricot trees in China (Gangrou 1995).

Host associations

Nematus feeds on a variety of hosts, including but not limited to species of Salix (willow), Populus (poplar, cottonwood), Betula (birch), Corylus (hazel), Castanea (chestnut), Robinia (locust), Ostrya (hophornbeam), Hamamelis (witch hazel), and Alnus (alder) (Smith 1972b, Smith 2003b, Smith 2008a).

Life history

Females generally oviposit along the midribs of leaves (Smith 2008a). Larvae are external feeders on leaves of the host (Smith 2003b), sometimes gregariously, sometimes singly. At maturity, the larvae fall to the ground and spin cocoons in which they pupate or overwinter. The number of generations per year varies among species (Smith 2008a).

Distribution

World: The genus is known from North America, throughout Europe, and throughout most of Asia, including India, Myanmar, Iran, Kyrgyzstan, Korea, China, and Japan (Taeger et al. 2018).

North America: Nematus occurs thoughout the United States and Canada, and in Alaska (Taeger et al. 2018). Two species, N. iridescens and N. mexicanus, occur farther south into Mexico, in Baja California Norte and Sonora, respectively (Smith 2003b).

Map data from: GBIF.org (29 October 2019) GBIF Occurrence Download Nematus

Details about data used for maps can be found here.

Nematus iridescens female lateral habitus; photo by Q. Baine, WSDA

Nematus iridescens female lateral habitus; photo by Q. Baine, WSDA

Nematus alniastri female face; photo by Q. Baine, WSDA

Nematus alniastri female face; photo by Q. Baine, WSDA

Nematus alniastri male lateral habitus; photo by Q. Baine, WSDA

Nematus alniastri male lateral habitus; photo by Q. Baine, WSDA

Nematus alniastri male dorsal habitus; photo by Q. Baine, WSDA

Nematus alniastri male dorsal habitus; photo by Q. Baine, WSDA

Nematus sp. male face; photo by Q. Baine, WSDA

Nematus sp. male face; photo by Q. Baine, WSDA

Nematus sp. wings; photo by P. Jones, WSDA

Nematus sp. wings; photo by P. Jones, WSDA

Nematus alniastri wings; photo by P. Jones, WSDA

Nematus alniastri wings; photo by P. Jones, WSDA