Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada

Genus Megarhyssa - Giant Ichneumons

Ichneumonid - Megarhyssa - male Megarhyssa sp - Megarhyssa nortoni - female Giant Ichneumon Wasp - Megarhyssa atrata - male Wasp ID [Megarhyssa? greenei??] Request - Megarhyssa greenei - female Bug found in Ledyard, CT What is this thing?? - Megarhyssa Megarhyssa macrurus? - Megarhyssa macrurus - male Megarhyssa atrata - male Megarhyssa atrata - female
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Hymenoptera (Ants, Bees, Wasps and Sawflies)
No Taxon ("Parasitica" (parasitic Apocrita))
Superfamily Ichneumonoidea (Braconids and Ichneumons)
Family Ichneumonidae (Ichneumon Wasps)
Subfamily Rhyssinae
Genus Megarhyssa (Giant Ichneumons)
Explanation of Names
Megarhyssa Ashmead, 1858
4 spp. in our area(1)
Males 23-38 mm, females 35-75 mm (body), 50-110 mm with ovipositor(2)

M. atrata
    Females:         Males:
M. greenei
    Females:         Males:
M. macrurus
    Females:         Males:
M. nortoni
    Females:         Males:
Forested areas of North America; all 4 spp. widely distributed in the east, two also in w. US(1)(3)
    atrata: Nova Scotia south to Georgia; west to Wyoming and Texas
    greenei: Nova Scotia south to Florida; west to Minnesota, eastern Kansas, and Mississippi
    macrurus icterosticta: Colorado, New Mexico, Utah, and Arizona
    macrurus macrurus: Maine south to Florida; west to western South Dakota and western Texas (also into northern Mexico)
    nortoni: Newfoundland south to northern Georgia, west to Alaska & California (also introduced in Australia and New Zealand)
Coniferous forests of North America
Apr-Oct in NC(4)
The larvae of M. atrata, M. greenei, and M. macrurus are parasitoids of grubs of the horntail Tremex columba.
M. nortoni are recorded on Uroceris albicornis; Xeris morrisoni; and Sirex sp..(3)
All these horntail larvae feed within the wood of conifers.
Life Cycle
Horntail adult females introduce wood-digesting fungi (e.g. Amylostereum) when ovipositing, which helps their grubs extract food value while feeding on the wood. Adult female Megarhyssa are able to detect the odor of these fungi, and once they land on the bark of an infected tree the Megarhyssa will walk along tapping the surface with their antennae (or "antennating") to further pinpoint the location of horntail grubs within the wood.
Once grubs are located, the female Megarhyssa positions herself with back legs extended and ovipositor perpendicular to the bark, and drills into the tree to deposit an egg on or near a horntail grub within its burrow (see videos here). While drilling the female wasp is immobilized and vulnerable to predation by birds.
When the Megarhyssa egg hatches it behaves as an ectoparasitic idiobiont, completely consuming the grub. It pupates within the burrow and emerges in the summer.
Males emerge first, and usually inseminate females within their chambers before they emerge (by inserting their long thin abdomens into the escape tube dug by the female (see Crankshaw & Matthews 1981).
There is a very detailed and fascinating account of the oviposition process in the 1999 paper by Le Lannic & Nénon (see "Print References" below) as captured in the posts below:

Their article focuses on M. atrata...but most of it presumably applies to other Megarhyssa species as well.
The paper Gibbons 1979 explores how the three species M. atrata, M. greenei, and M. macrurus exploit different niches, allowing them to coexist in the same locations and using the same host.
Print References
Gibbons, John R. H. (1979). A Model for Sympatric Speciation in Megarhyssa (Hymenoptera: Ichneumonidae): Competitive Speciation. American Naturalist, 114(5):719-741 (1st page from JSTOR)
Le Lannic, Joseph & J.-P. Nénon (1999). "Functional morphology of the ovipositor in Megarhyssa atrata ( Hymenoptera , Ichneumonidae ) and its penetration into wood", Zoomorphology, Volume: 119, Issue: 2, Pages: 73-79. (First Page)
Pook, V., M. Sharkey, & D. Wahl (2016). Key to the species of Megarhyssa (Hymenoptera, Ichneumonidae, Rhyssinae) in America, north of Mexico(5). Deutsche Entomologische Zeitschrift 63(1): 137-148 (Full Text)
Works Cited
1.Carlson R.W. () Ichneumonidae catalog
2.National Audubon Society Field Guide to Insects and Spiders
Lorus and Margery Milne. 1980. Knopf.
3.Catalog of Hymenoptera in America North of Mexico
Karl V. Krombein, Paul D. Hurd, Jr., David R. Smith, and B. D. Burks. 1979. Smithsonian Institution Press.
4.Insects of North Carolina
C.S. Brimley. 1938. North Carolina Department of Agriculture.
5.Key to the species of Megarhyssa (Hymenoptera, Ichneumonidae, Rhyssinae) in America, north of Mexico
Victoria Pook, Michael Sharkey, David Wahl. 2016. Deutsche Entomologische Zeitschrift 63(1): 137-148.