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 antennaeantenna:
the sensory organ emerging from the front of the head, usually between the compound eyes and above the clypeus; includes the flagellum, scape and pedicel
. Otherwise, they come in a variety of colors, sizes, and forms (Goulet 1992Goulet 1992:
Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.).

Nematinae is the second-largest subfamily of Tenthredinidae, with over 1,250 species (Prous et al. 2014Prous et al. 2014:
Prous M, Blank SM, Goulet H, Heibo E, Liston A, Malm T, Nyman T, Schmidt S, Smith DR, Varing;rdal H, Viitasaari M, Vikberg V, and Taeger A. 2014. The genera of Nematinae (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research 40: 1-69. " target="_blank">https://doi.org/10.3897/JHR.40.7442). 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 2003bSmith 2003b:
Smith DR. 2003b. A Synopsis of the sawflies (Hymenoptera: Symphyta) of America south of the United States: Tenthredinidae (Nematinae, Heterarthrinae, Tenthredininae). Transactions of the American Entomological Society 129 (1): 1-45.).

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. 2014Prous et al. 2014:
Prous M, Blank SM, Goulet H, Heibo E, Liston A, Malm T, Nyman T, Schmidt S, Smith DR, Varing;rdal H, Viitasaari M, Vikberg V, and Taeger A. 2014. The genera of Nematinae (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research 40: 1-69. " target="_blank">https://doi.org/10.3897/JHR.40.7442). 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 1975Smith 1975:
Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145).

Diversity

There are 11 described species worldwide. One species occurs in North America (Taeger et al. 2018Taeger et al. 2018:
Taeger A, Liston AD, Prous M, Groll EK, Gehroldt T, and Blank SM. 2018. ECatSymmdash;Electronic World Catalog of Symphyta (Insecta, Hymenoptera). Program version 5.0 (19 Dec 2018), data version 40 (23 Sep 2018). Senckenberg Deutsches Entomologisches Institut (SDEI), Muuml;ncheberg. https://sdei.de/ecatsym/ Accessed: 28 Jan 2020.).

Diagnostic characteristics

Subfamily characters​

• veins M and 1m-cu convergent toward the stigmastigma:
  a section of the fore wing at the apex of vein C that is sclerotized, rounded and often darkened
  (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• fore wing fore wing:
  the anterior wing of each pair of wings; usually the largest wing of the pair
  vein 2m-cu meeting cellcell:
  1. a membranous area of the wing between veins, 2. a small cavity or closed space
  1Rs above (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)

Genus characters

• mandibles symmetrical (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• clypeus clypeus:
  sclerotized area on the front of the head located between the antennal insertions and labrum
  deeply emarginated (Prous et al. 2014Prous et al. 2014:
  Prous M, Blank SM, Goulet H, Heibo E, Liston A, Malm T, Nyman T, Schmidt S, Smith DR, Varing;rdal H, Viitasaari M, Vikberg V, and Taeger A. 2014. The genera of Nematinae (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research 40: 1-69. " target="_blank">https://doi.org/10.3897/JHR.40.7442)
• malar space malar space:
  the minimum distance between the base of the mandible and the ventral margin of the compound eye
  about equal to diameter of front ocellusocellus:
  a simple bead-like eye, often on the dorsum of the head in groups of 1-3
  (Prous et al. 2014Prous et al. 2014:
  Prous M, Blank SM, Goulet H, Heibo E, Liston A, Malm T, Nyman T, Schmidt S, Smith DR, Varing;rdal H, Viitasaari M, Vikberg V, and Taeger A. 2014. The genera of Nematinae (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research 40: 1-69. " target="_blank">https://doi.org/10.3897/JHR.40.7442)
• pedicel pedicel:
  the second antennal segment, between the scape and flagellum
  wider than long (Smith 1975Smith 1975:
  Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145)
• first antennal flagellomereflagellomere:
  A segment of the flagellum of the antenna
  about equal to third (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• antennae long and thin, longer than 2X width of head (Smith 1975Smith 1975:
  Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145)
• fore wing fore wing:
  the anterior wing of each pair of wings; usually the largest wing of the pair
  veins M and Rs+M relatively widely separated on veinvein:
  a tube-like, often darkened, structure on the wings
  R (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• fore wing fore wing:
  the anterior wing of each pair of wings; usually the largest wing of the pair
  vein 2A+3A curved upwards and meeting 1A (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• fore wing fore wing:
  the anterior wing of each pair of wings; usually the largest wing of the pair
  vein 2r-rs present (Prous et al. 2014Prous et al. 2014:
  Prous M, Blank SM, Goulet H, Heibo E, Liston A, Malm T, Nyman T, Schmidt S, Smith DR, Varing;rdal H, Viitasaari M, Vikberg V, and Taeger A. 2014. The genera of Nematinae (Hymenoptera, Tenthredinidae). Journal of Hymenoptera Research 40: 1-69. " target="_blank">https://doi.org/10.3897/JHR.40.7442)
• hind wing hind wing:
  the posterior wing of each pair of wings
  vein 2A meeting 1A; basalbasal:
  towards the base; closest to the body
  anal cellanal cell:
  cell A of either the fore wing or hind wing
  present (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• larger (anterior) fore tibialfore tibia:
  the tibia of the fore leg
  spur with velum (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• pulvilli short; first pulvilluspulvillus:
  soft pads used for surface adhesion, located in sawflies on the first 4 segments of the tarsus
  about 1/3 the length of the tarsomeretarsomere:
  a segment of the tarsus
  (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)
• female cercuscercus:
  structures emerging posterolaterally from the apex of the abdomen
  about 3–6X as long as wide (Goulet 1992Goulet 1992:
  Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.)

May be confused with

Hemichroa can be distinguished from other genera of Nematinae that have fore wingfore wing:
the anterior wing of each pair of wings; usually the largest wing of the pair
vein 2A+3A curved upwards and veinvein:
a tube-like, often darkened, structure on the wings
2r-rs present by the long, slender antennaeantenna:
the sensory organ emerging from the front of the head, usually between the compound eyes and above the clypeus; includes the flagellum, scape and pedicel
, short pulvillipulvillus:
soft pads used for surface adhesion, located in sawflies on the first 4 segments of the tarsus
, and long inner tooth of the tarsal clawtarsal claw:
sharpened appendage emerging from the apex of the tarsus
(Smith 1975Smith 1975:
Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145, Goulet 1992Goulet 1992:
Goulet H. 1992. The genera and subgenera of the sawflies of Canada and Alaska: Hymenoptera. Symphyta. The insects and arachnids of Canada. Part 20. Agriculture Canada Publication.).

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 2004Saini and Vasu 2004:
Saini MS and Vasu V. 2004. Review of the genus Hemichroa Stephens (Hymenoptera: Symphyta: Tenthredinidae: Nematinae) from the Oriental Region. Zoos#39; Print Journal 19 (6): 1487-1491.).

Life history

Hemichroa crocea, known as the striped alder sawfly, is an introduced pest of alder. The larvaelarva:
the immature stage of holometabolous insects
feed gregariously on the margin of the leaf. At maturity, the larvaelarva:
the immature stage of holometabolous insects
fall to the ground to pupate or overwinter in the soil. This species undergoes up to three generations per year (Smith 1975Smith 1975:
Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145).

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 larvaelarva:
the immature stage of holometabolous insects
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 2004Saini and Vasu 2004:
Saini MS and Vasu V. 2004. Review of the genus Hemichroa Stephens (Hymenoptera: Symphyta: Tenthredinidae: Nematinae) from the Oriental Region. Zoos#39; Print Journal 19 (6): 1487-1491., Taeger et al. 2018Taeger et al. 2018:
Taeger A, Liston AD, Prous M, Groll EK, Gehroldt T, and Blank SM. 2018. ECatSymmdash;Electronic World Catalog of Symphyta (Insecta, Hymenoptera). Program version 5.0 (19 Dec 2018), data version 40 (23 Sep 2018). Senckenberg Deutsches Entomologisches Institut (SDEI), Muuml;ncheberg. https://sdei.de/ecatsym/ Accessed: 28 Jan 2020.).

North America: Hemichroa crocea was introduced from Europe and first discovered in British Columbia in 1932 (Hopping 1937Hopping 1937:
Hopping GR. 1937. Sawfly biologies no. 2, Hemichroa crocea Geoffroy. Canadian Entomologist 69 (11): 243-249.). 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 1975Smith 1975:
Smith DR. 1975. The sawfly genus Hemichroa Stephens : A review of species (Hymenoptera: Tenthredinidae). Insect Systematics and Evolution 6 (3-4): 297-302. https://doi.org/10.1163/187631275X00145).

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

Details about data used for maps can be found here.