Key to the species of Megarhyssa ( Hymenoptera , Ichneumonidae , Rhyssinae ) in America , north of Mexico

A dichotomous and an interactive key to the species of Megarhyssa (Hymenoptera: Ichneumonidae) in America, north of Mexico are presented. A diagnosis accompanied by images is provided for male and female wasps of each of the four species, Megarhyssa atrata, Megarhyssa greenei, Megarhyssa macrurus and Megarhyssa nortoni.


Introduction
The subfamily Rhyssinae Morley, of the 'pimpliformes' Ichneumonidae (Order: Hymenoptera), is found worldwide and comprises 234 described species in eight genera (Yu et al. 2012).Though hypothesized to have originated in the northern hemisphere (Wahl and Gauld 1998), the diversity of this subfamily is heavily biased to the tropics with nearly half of the species belonging to the circumtropical genus, Epirhyssa Cresson, 1865 (Yu et al. 2012).Rhyssinae are generally large in size and some of the most impressive specimens occur in the cosmopolitan genus Megarhyssa Ashmead (Hymenoptera: Ichneumonidae: Rhyssinae).The majority of the 37 described species in this genus occur in the Oriental region and the Eastern Palearctic.A handful of species occur in each of the following regions: Western Palearctic, Nearctic and Australasian; with one record for both the Neotropical (Chiapas) and Afrotropical regions (Yu et al. 2012).
Though only four species of Megarhyssa occur in the Nearctic (Yu et al. 2012), they are a common sight in the forests of the United States and Canada.Members of this genus are ectoparasitoid idiobionts of wood-boring siricid larvae (Townes 1969) and one species, M. nortoni (Cresson), is an effective biocontrol agent employed by the National Sirex Coordination Committee to control the invasive woodwasp, Sirex noctilio Fabricius (Hymenoptera: Siricidae).Characteristics such as their bright coloration and large size place these spectacular insects among the 'charismatic megafauna' of the arthropod world, often capturing the attention of hobbyists and nature enthusiasts.Given the frequency with which non-specialists encounter and work with species of Megarhyssa, image-rich dichotomous and interactive keys will increase the accuracy of identifications.
The four species of Megarhyssa found in the United States and Canada are broadly distributed across the region (Townes and Townes 1960;Carlson 1979).The only species native to more southern regions is Megarhys-sa macrurus (Linneaus) which is also found in Mexico (Townes and Townes 1960).In addition, Megarhyssa nortoni now occurs in Australia, New Zealand and South Africa where it was introduced as a biological control agent (Taylor 1976;Bartlett et al. 1978;Taylor 1978;Haugen and Underdown 1990;Tribe and Cillié 2004;Hurley et al. 2007).Siricid wood wasps native to north America are the common hosts of each of the four species (Beaulne 1939;Carlson 1979;Champlain 1922;Heatwole and Davis 1965;Hopkins 1893;Nénon 1995;Stillwell 1967;Townes 1944;Townes and Townes 1960;Treherne 1916) with M. nortoni also parasitizing the invasive wood wasp, Sirex noctilio (Carlson 1979;Nuttall 1980;Valentine and Walker 1991;Vincent and King 1995).
A key to the genera of Nearctic Rhyssinae, illustrated with high quality color images, is available online at http://www.amentinst.org/GIN/Rhyssinae.Here, we present a key to the four Nearctic species, designed for use by non-specialists.We have adopted some characters from previous keys (Merill 1915;Townes and Townes 1960) in addition to creating our own when necessary.This key will be linked to the website of the American Entomological Institute where it will complement its generic key.It will also be advertised on the popular website www.bugguide.netto promote its use by the general public.

Materials and methods
Type specimens for each species were checked for the diagnostic morphological characters assigned by Merill (1915) and Townes and Townes (1960).A broad range of specimens from the hundreds of Megarhyssa housed in the Kentucky and American Entomological Institute collections were examined.Characters included in the keys were chosen based on the ease with which they could be assessed by a non-specialist.

Key to species
Females.

Megarhyssa Ashmead, 1900
Characters diagnostic of Megarhyssa include the presence of a petiolate triangular areolet on the fore wing, a longitudinal ridge on the trochantellus of the middle leg and lateral tubercles on the apical margin of the clypeus.In addition, tergites 3-5 of female Megarhyssa are smooth to punctate and sternites 2-6 possess a pair of tubercles close to the anterior sternal margin.Male Megarhyssa have a strong setiferous groove which is close to and paralleling the apical 0.7 of the ventral interior margin; and tergites 3-6 are strongly concave apically and possess a median apical or subapical longitudinal submembranous area.These male specific characters are not well developed in small specimens and they may key to the genus Rhyssella (Townes & Townes, 1960).206.Synonymized by Carlson (1979) Geographic range.Eastern Nearctic to about longitude 100 °W (Townes and Townes 1960).
Distinguishing characters.Female: Head and antenna mostly yellow, may or may not have a dark spot above the clypeus.Mesosoma and metasoma black.Mesosoma  sometimes with a small yellow spot on posterodorsal corner of pronotum, rarely with yellow markings.Fore wing 15 to 30 mm long; wings infuscate or entirely black.Ovipositor sheath about 3.7 times as long as forewing.
Females may be distinguished from the other species occurring in the USA and Canada by their body color.
Male.Head yellow, may or may not have a dark spot above the clypeus.Mesosoma yellow and blackish brown; metasoma very dark brown to black with a yellow mark on the hind margin of the first tergite; wings hyaline, fore wing 16 to 22 mm long.
Males may be distinguished from M. macrurus by the evenly darkened fore wing and the lack of a brown patch at the base of cell 2R1; from M. nortoni by the additional yellow markings on the mesopleuron; and from M. greenei by the color of the mesosoma.Bul.216 (pt.2): 424.Synonymized by Carlson (1979).

Megarhyssa greenei Viereck, 1911
Geographic range.Most specimens are found in the eastern Nearctic to about longitude 96 °W, with some found as far west as Wyoming (Townes and Townes 1960).
Diagnosis.Female: Head mostly yellow; mesosoma reddish-brown and yellow.Metasoma reddish-brown, each tergite with an angled yellow band; bands on the third and following tergites interrupted on the midline.Fore wing 12 to 27 mm long.Wings hyaline and the fore wing usually with a large brown patch at the base of cell 2R1.Ovipositor sheath about 1.8 times as long as fore wing.Females can be distinguished from M. atrata by their reddish-brown and yellow body color; from M. nortoni by the angled yellow bands on their metasoma; and from M. macrurus by the lack of vertical stripes on the face.
Male: Head mostly yellow; mesosoma reddish-brown and yellow; metasoma reddish-brown with yellow and black markings on first two or three tergites.Wings hyaline; fore wing 10 to 16 mm long.
Males can be distinguished from M. atrata by their mesosoma color; from M. nortoni and M. macrurus by the lack of vertical stripes on the face.Note.The specific epithet macrura is commonly applied to this species, however the name macrurus was interpreted by Townes (1944) and Townes and Townes (1960) as a noun and therefore is not required to match the gender of the genus name.Geographic range.M. macrurus icterosticta is found in Arizona, Colorado, New Mexico, and Utah.M. macrurus lunator ranges across the eastern Nearctic to the Hosts.Tremex columba (Carlson, 1979).
Diagnosis.Female.Head yellow and dark brown, with two vertical stripes on face, one below each antennal socket.Mesosoma and metasoma ranging from dark brown to reddish-brown and yellow.Each tergite with an angled yellow band interrupted on the third to sixth tergites.Wings hyaline with brown patches; fore wing 18 to 29 mm long.Ovipositor sheath about 3.0 times as long as fore wing.
Females can be distinguished from M. atrata by their body color; from M. nortoni by the angled yellow bands on the tergites; and from M. greenei by the vertical stripes on the face.
It should be noted that Carlson (1979) did not separate M. macrurus lunator from M. macrurus macrurus.
Male.Head yellow and dark brown, with two vertical stripes on face, one below each antennal socket.Mesosoma yellow and blackish brown, metasoma brown, first and second tergites with a short yellow band on hind margin.Wings hyaline, fore wing 7 to 21 mm long with a brown patch at the base of cell 2R1.
Males can be distinguished from the other species occurring in the USA and Canada by the brown patch at the base of cell 2R1 of the fore wing and the presence of two vertical stripes on the face.
Geographic range.M. nortoni nortoni is distributed along the Pacific Coast from south-western Canada to California, and extends eastward to central Colorado.M. nortoni quebecensis is found in the north-eastern U.S. and adjacent parts of Canada.In the late 1960s and early 1970s this species was collected across the United States and Canada, encompassing the ranges of both subspecies, for introduction as a biocontrol agent in Australia (Taylor 1976) and New Zealand (Bartlett et al. 1978).The populations established in Australia were then introduced to South Africa (Tribe and Cillié 2004).
Diagnosis.Female.Head black to blackish brown and yellow; mesosoma black or blackish brown, with yellow spots; metasoma brown and usually a little paler than me- sosoma.Subtriangular median subapical spot on first and second tergites, large round yellow spot on side of tergites 3-5, and vertical yellow blotch on side of sixth and seventh tergites.Fore wing 13 to 29 mm long.Ovipositor sheath about 2.7 times as long as fore wing.
Females can be distinguished from M. atrata by the body color; and from M. macrurus and M. greenei by the yellow spots on the tergites.
It should be noted that Carlson (1979) did not separate M. nortoni nortoni from M. nortoni quebecensis.
Male.Head black to blackish brown and yellow; mesosoma black to blackish brown, with yellow spots; metasoma brown, the tergites usually darker medially; the first two tergites with a median subapical yellow spot.Fore wing 9.5 to 20 mm long.
Males can be distinguished from M. macrurus by the lack of a brown patch at the base of cell 2R1 of the fore wing and from M. atrata and M. greenei by the single vertical black band on the face and the color of the mesopleuron.