Genus Megarhyssa - Giant Ichneumonid Wasps

Stump stabbers

Stump-stabber wasps

Megarhyssa Ashmead, 1858

4 spp. in our area(1)(2), 37 spp. worldwide(3)

Males 23-38 mm, females 35-75 mm (body), 50-110 mm with ovipositor(4)

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

Key to Giant Ichneumonid Wasps (Megarhyssa) of USA/Canada (Dankowicz & Cepeda Espinosa)

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M. atrata

    Females:         Males:

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M. greenei

    Females:         Males:

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M. macrurus

    Females:         Males:

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M. nortoni

    Females:         Males:

Forested areas of North America; all 4 spp. widely distributed in the east, two also in w. US(2)(5)

    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(6)

The larvae of M. atrata, M. greenei, and M. macrurus are parasitoids of grubs of the pigeon horntail, Tremex columba.

M. nortoni are recorded on Uroceris albicornis; Xeris morrisoni; and Sirex sp..(5)

All these horntail larvae feed within the wood of conifers.

With their very sensitive antennae, prey can be found via odors from the wood-borers fungal partner or movements of the larvae.(7)

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).

Their ovipositor can penetrate as deep as 5"(7)

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.

Abbot, C.E. (1934). How Megarhyssa deposits her egg. Journal of the New York Entomological Society 42: 127–133 (Full Text)

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)

Heatwole, H., Davis, D.M., and Wenner, A.M.. 1962. The behaviour of Megarhyssa, a genus of parasitic hymenopterans (Ichneumonidae: Ephialtinae). Zeitschrift für Tierpsychologie 19: 652–664 (Full Text)

Le Lannic, Joseph & J.-P. Nénon (1999). Functional morphology of the ovipositor in Megarhyssa atrata ( Hymenoptera , Ichneumonidae ) and its penetration into wood. Zoomorphology, Vol. 119(2):73-79. (First Page)

Nenon, J.P., N. Kacem, & J. Le Lannic (1997). Structure, sensory equipment, and secretions of the ovipositor in a giant species of Hymenoptera: Megarhyssa atrata F. (Ichneumonidae, Pimplinae). Canadian Entomologist 129(5): 785-799

Pook, V., M. Sharkey, & D. Wahl (2016). Key to the species of Megarhyssa (Hymenoptera, Ichneumonidae, Rhyssinae) in America, north of Mexico(3). Deutsche Entomologische Zeitschrift 63(1): 137-148 (Full Text)

Subfamily Rhyssinae web page from the American Entomological Institute.

Videos and description of the oviposition process in M. macrurus.

Article on fungi-horntail-Megarhyssa relationship.

Giant Ichneumon Oviposition (Megarhyssa cf nortoni): Photos and discussion of oviposition