Hemichroa

Taxonomy

Family: Tenthredinidae
Family common name: common sawflies
Subfamily: Nematinae
Tribe: Nematini
Genus: Hemichroa Stephens, 1835
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.

Hemichroa is represented in North America only by a single, introduced pest species. Hemichroa crocea females are bright orange, while the males are mostly black with orange legs (Smith 1975).

Diversity

There are 11 described species worldwide. One species occurs in North America (Taeger et al. 2018).

Diagnostic characteristics

Subfamily characters​

Genus characters

May be confused with

Hemichroa can be distinguished from other genera of Nematinae that have fore wing vein 2A+3A curved upwards and vein 2r-rs present by the long, slender antennae, short pulvilli, and long inner tooth of the tarsal claw (Smith 1975, Goulet 1992).

Exotic pest species of concern

none

Host associations

Hemichroa crocea feeds primarily on Alnus (alder), but may also feed on Betula (birch), Corylus (hazel), and Salix (willow) (Saini and Vasu 2004).

Life history

Hemichroa crocea, known as the striped alder sawfly, is an introduced pest of alder. The larvae feed gregariously on the margin of the leaf. At maturity, the larvae fall to the ground to pupate or overwinter in the soil. This species undergoes up to three generations per year (Smith 1975).

Hemichroa crocea react when disturbed by erratically moving the abdominal portion of the body, and sometimes by hiding on the other side of the leaf. Interestingly, H. crocea also have a unique larval behavior that involves rubbing protuberances on their body against the leaf to make sound. This may work as a vibrational signal to nearby larvae that food is plentiful in their location or may be an additional defensive behavior (Boevé 2015).

Distribution

World: The genus is known from North America, throughout Europe, and in India, Nepal, China, Japan, and Korea (Saini and Vasu 2004, Taeger et al. 2018).

North America: Hemichroa crocea was introduced from Europe and first discovered in British Columbia in 1932 (Hopping 1937). The species is now established in Canada and the northern United States, as far north as Alaska; in the west it occurs farther south in Colorado and New Mexico (Smith 1975).

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

Details about data used for maps can be found here.

Hemichroa crocea female lateral habitus; photo by J. Orr, WSDA

Hemichroa crocea female lateral habitus; photo by J. Orr, WSDA

Hemichroa crocea female dorsal habitus; photo by J. Orr, WSDA

Hemichroa crocea female dorsal habitus; photo by J. Orr, WSDA

Hemichroa crocea female face; photo by Q. Baine, WSDA

Hemichroa crocea female face; photo by Q. Baine, WSDA

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

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